Ancient Crustal Materials Research Articles

Geochronology and Petrogeochemistry of Metallogenic Intrusions in the Shanmen Large‐scale Silver Deposit in the Siping Area of Jilin Province, NE China

AbstractThe Shanmen deposit, located in the Siping area of Jilin Province, is one of large‐scale silver deposits in Northeast (NE) China. Due to its high Ag grade, associated gold resources and special tectonic location, this deposit has important theoretical and ore‐prospecting significance. To present new data on the ore genesis, mineralization time and tectonic settings, the relationship between silver mineralization and intrusions in this deposit has been studied and some analyses have been carried out, including LA‐ICP‐MS zircon U‐Pb dating, whole‐rock major and trace element analysis and Sr‐Nd‐Pb isotope analysis of the granitoids associated with silver mineralization. Studies on deposit geology indicate that the ore‐hosting granodiorite and monzogranite have intimate genetic relationships with silver mineralization. LA‐ICP‐MS zircon U‐Pb dating results of the both intrusions are 167.6 ± 1.9 Ma and 167.0 ± 1.5 Ma, respectively, implying that the two intrusions and associated silver mineralization in the Shanmen deposit formed during the Middle Jurassic. Major element analytical data suggest that the two intrusions are high‐K calc–alkaline series I‐type granites and belong to metaluminous and peraluminous rocks. Both intrusions have similar REE characteristics with regard to the relative enrichment of LREEs, depletion of HREEs, obvious fractionation of LREEs and HREEs (ΣLREE/ΣHREE = 8.68–14.09, (La/Yb)N = 12.51–21.96), moderately negative Eu anomalies as well as weakly negative Ce anomalies (δEu = 0.56–0.71, δCe = 0.93–1.09). Moreover, the samples are generally enriched in LILEs and depleted in HFSEs. The rock assemblages and geochemical characteristics of granitoids in the Shanmen deposit suggest that they formed in an active continental margin associated with the Paleo‐Pacific plate subduction. The Sr‐Nd‐Pb isotope compositions show that both the granodiorite and monzogranite have low Sr initial ratios and high Sm/Nd initial ratios. The εNd(t) values and young Nd‐model ages are –3.2 to –1.7 and 990 Ma to 1110 Ma, respectively. Considering the Sr‐Nd isotope compositions, in conjunction with the relatively high 206Pb, 207Pb and 208Pb, alongside the relatively low 204Pb, it can be concluded that the Middle Jurassic intermediate‐acidic magma represented by the studied monzogranite and granodiorite in the Shanmen deposit, was derived from the partial melting of juvenile lower crust enriched in mantle‐derived materials and affected by the mixing of ancient crustal materials during the ascension and intrusion processes.

Orogenic belt resulting from ocean-continent collision

Abstract Orogenic belts have been thought to form through plate convergence, involving subduction of oceanic lithosphere at continental margins (accretionary orogens), which may ultimately lead to ocean closure and continent-continent collision (collisional orogens). Intraplate orogens away from plate margins have been known, but the mechanisms controlling their evolution are poorly understood. The South China craton, including the Yangtze and Cathaysia blocks, underwent a Paleozoic orogenesis that formed a >500-km-wide orogenic belt with widespread granitoids that are unconformably overlain by Devonian cover sequences. The pre-Devonian basement rocks were subjected to strong deformation and greenschist-to amphibolite-facies metamorphism at 460–400 Ma. Paleozoic magmatism was characterized by voluminous crustally derived Silurian granitoids associated with incorporation of ancient crustal materials at 450–440 Ma and addition of juvenile mantle-derived melts at 420–410 Ma. Based on the absence of arc-like magmatism and the existence of ophiolites in the West Cathaysia terrane, geochemical evidence that oceanic crust existed beneath the East Cathaysia terrane, and geophysical evidence of contrasting lithospheres on both sides and two discrete slabs beneath their fault boundary, we propose that this Paleozoic orogenic belt was formed by collision between the two terranes that was driven by far-field forces during the assembly of Gondwana, and the East Cathaysia terrane represents oceanic lithosphere that was overthrusted by the continental crustal materials of the West Cathaysia terrane. Numerical modeling shows that this type of collision can explain the dynamics of the Paleozoic orogenesis in the South China craton and may be a mechanism for some orogens in which subduction-related igneous and metamorphic rocks are lacking.

The early–middle Silurian Delite monzogranite and quartz syenite, East Kunlun Orogenic Belt, NW China: petrogenesis and implications for tectonic evolution of the Proto‑Tethys

The tectonic evolution of the Proto‑Tethys in the East Kunlun Orogenic Belt (EKOB) is controversial. Herein, new petrological, whole-rock geochemical, zircon U–Pb geochronological and zircon Lu–Hf isotopic data for early Paleozoic intrusive rocks in the Delite region are reported. LA-ICP-MS zircon U–Pb isotopic data show that the Delite monzogranite and quartz syenite were formed at 437.0 ± 2.9 Ma and 424.8 ± 2.2 Ma, respectively. The Delite monzogranite contains high SiO2, Al2O3 and K2O and A/CNK ratios, but low MgO and Mg# values (33–38). This study considers that the monzogranite is a typical S-type granite and originated from partial melting of ancient crustal clastic material (metagreywacke) in a syn-collision setting. The quartz syenite is K-rich and Mg-poor, with low Mg# values (36–41), exhibiting geochemical signatures of an alkalic syenite derived from partial melting of basaltic lower crust in a post-collision setting. Combining with the previous work, we suggest that in the middle part of the EKOB the Proto-Tethys closed at ca 437 Ma with a post-collisional setting after ca 427 Ma. KEY POINTS Zircons LA-ICP-MS U–Pb isotopic data indicate that the Delite monzogranite and quartz syenite were formed at 437.0 ± 2.9 Ma and 424.8 ± 2.2 Ma, respectively. The Delite monzogranite was formed in a syn-collision setting. The Delite quartz syenite was related to the post-collision setting. It is suggested that the Proto-Tethys closed at ca 437 Ma in the middle part of the EKOB, and the post-collisional setting was after ca 427 Ma in the EKOB.

Chronological framework of Precambrian Dantazi Complex: Implications for the formation and evolution of the northern North China Craton

The Dantazi Complex in the North Hebei Terrane is located in the middle segment of the northern North China Craton, and plays a crucial role for understanding the early Precambrian crustal evolution of the craton. The Dantazi Complex is composed mainly of late Neoarchean tonalites-trondhjemites-granodiorites, potassic granites, sanukitoid granodiorites-monzodiorites and quartz syenites, which were emplaced by Paleoproterozoic gabbroic stocks. SHRIMP and LA–ICP–MS zircon dating analyses have revealed that the Neoarchean diversified granitoids of the Dantazi Complex were formed in ∼2.52–2.47 Ga, and preserved ∼2.60–2.53 Ga inherited/captured zircon ages. All these rocks experienced early metamorphism and deformation at ∼2.40 Ga, later granulite facies metamorphism at ∼1.80 Ga, and alterations at ∼1.73 Ga. Whole-rock chemical characteristics and Sm–Nd–Lu–Hf isotopes indicate that (1) The ∼2.52 Ga tonalites-trondhjemites-granodiorites were derived from partial melting of juvenile mafic crust; (2) the ∼2.51 Ga sanukitoid monzodiorite-granodiorite series came from slab-mantle interaction with different melt/rock ratios; (3) The ∼2.50–2.47 Ga potassic granites were generated by intracrustal recycling of metagreywacks that sourced from the mixture between juvenile and ancient crustal materials; and (4) The ∼2.50 Ga quartz syenites were originated from fractional crystallization of mantle-derived magmas. These petrogenetic processes reveal that the subduction-related crust-mantle interaction is the key reason for the diversity of late Neoarchean granitoids in the Dantazi Complex. Regional comparison indicates that the Precambrian North Hebei Terrane show distinct structure in the chronology and lithological assemblages from other basement terranes in the trans-North China Orogen and Western Block, implying that they were formed in different crust-mantle dynamic systems. Therefore, the Precambrian North Hebei Terrane was probably an independent micro-continental block at least until the early Paleoproterozoic. In the Archean crystalline basement range of the North Hebei–West Liaoning–East Hebei, from NW to SE, the magmatic crystallization ages gradually became older, and the granitoid assemblages transformed from sodium-rich to potassium-rich, the spatio-temporal evolution of chemical composition and lithological assemblages further indicates that the North Hebei Terrane was most likely developed in an arc environment with continuous retreat subduction caused by the slab roll-back.

Tectono-magmatism evolution in the Gaoligong orogen belt during Neoproterozoic to Paleozoic: Significance for assembly of East Gondwana

The precise constraining of Proterozoic to Paleozoic tectono-magmatic processes of the orogen belt is important to understanding the evolution of the Proto-Tethys Ocean and assembly processes of Gondwana. This study presents newly defined Neoproterozoic to Paleozoic granitic gneiss in the Gaoligong orogen belt, western Yunnan, China, by using a combined study of the structure, petrology, zircon U-Pb age and Hf isotopic composition, and whole-rock geochemistry. The magmatic origin zircons yield three main cluster ages of ca. 600–520 Ma, 520–460 Ma, and 460–420 Ma, with decreasing average Th/U ratios. Zircons have εHf(t) values ranging from −8.6 to 2.4, and Hf model ages from 1.23 Ga to 1.78 Ga. The geochemistry of the granitic gneisses shows strongly peraluminous and similar to S-type granites, with high SiO2, Rb/Sr, Rb/Ba and total alkali contents but low Al2O3, MgO, TiO2, FeOt and CaO/Na2O ratios, and are enriched in LILEs (such as Rb, Th, U, K and Pb) and depleted in HFSEs (such as Nb, Sr, P, Eu, and Ti). The geochemical signatures and zircon Hf isotopic results indicate that the granitic gneisses were derived mainly from the anatexis of ancient crustal material during the collision of the Tengchong and Baoshan blocks at 600–520 Ma. Then they together experienced subduction of the Proto-Tethys ocean at 520–460 Ma and the accretion of the micro-continents at 460–420 Ma. Combined with detrital zircon U-Pb ages and the paleogeographic position of the micro-continents that related to East Gondwana. Here, we propose a new evolution model as follows: 1) the Gaoligong orogen belt lies between the Tengchong and Baoshan blocks that collided during the Precambrian, and they together became the passive continental margin of East Gondwana; 2) then the Gaoligong orogen belt and the micro-continents underwent subduction of the Proto-tethys Ocean during the Middle Cambrian to Lower Ordovician; and 3) finally they experienced accretionary orogeny during the Late Paleozoic.

Geological Significance of Late Permian Magmatic Rocks in the Middle Section of the Ailaoshan Orogenic Belt, SW China: Constraints from Petrology, Geochemistry and Geochronology

The Ailaoshan orogenic belt, located in the SE margin of the Qinghai–Tibet Plateau, is an important Paleo-Tethys suture zone in the eastern margin of the Sanjiang Tethys tectonic domain. The areas of Mojiang and Zhenyuan, located in the middle part of the Ailaoshan orogenic belt, are the key parts of the Ailaoshan Paleo-Tethys Ocean closure and collision orogeny. The rhyolites outcropped in the Mojiang area, and the granite porphyries outcropped in Zhenyuan area, are systematically studied for petrology, isotope geochemistry and geochronology. The Zircon U-Pb geochronology of rhyolites and granite porphyries give weighted average ages of 253.4 ± 4.2 Ma and 253.3 ± 2.0 Ma, respectively, both of which were formed in the late Permian period. The rhyolites belong to potassic calc-alkaline to subalkaline series. The patterns of the rare earth elements (REE) show a right-inclined seagull-type distribution, and the trace elements plot is right-inclined. The granite porphyries are high potassic calc-alkaline to subalkaline. The REE patterns show a right-inclined distribution, and the trace elements plot is right-inclined, which is consistent with the typical patterns observed in the crust. The peraluminous, highly differentiated and high ASI values suggest that rhyolites and granite porphyries are S-type granites. The zircon εHf(t) of the rhyolites range from −7.22 to −0.72, and two-stage Hf zircon model ages are (TDMC) 1771–2352 Ma, indicating that the magma source area is mainly crust-derived. The zircon εHf(t) of the granite porphyries range from −0.97 to 4.08, and two-stage Hf zircon model ages are (TDMC) 1336–1795 Ma, indicating that the magma is derived from a depleted mantle source and the partial melting of ancient crustal materials. The rhyolites and granite porphyries were possibly formed in the syn-collisional tectonic setting during the late Permian, and their ages limited the time of the final closure of the Ailaoshan Ocean and the initiation of collisional orogeny.

Crust-derived felsic magmatism in the Emeishan large igneous Province: New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block, China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The εHf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding TDM1 values are in the range of 919–523 Ma. The εHf(t) values of zircon from the DSC I-type granite are between –1 and 3, with TDM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ18O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.

Early Paleoproterozoic tectono-magmatic and metamorphic evolution of the Yuanmou Complex in the southwestern Yangzte Block

Paleoproterozoic rocks are rare and essential for understanding the early evolution of the Yangtze Block. We present the newly defined Early Paleoproterozoic gneisses from the Yuanmou Complex at the western boundary of the Yangtze Block by a combined study of the structure, petrology, U-Pb age and Hf isotopic composition of zircons, and whole-rock geochemistry. The magmatic origin of inherited and detrital zircons, underlined by high Th/U ratios, of an orthogneiss and a paragneiss yield four main cluster ages of ca. 2500–2300, 2200–1770, 1580–1200, and 1100–1000 Ma. Zircon rims yield two main age groups, 970–840 and 780–750 Ma, which document the timing of overprinting metamorphic/partial melting events. Geochemical data of the orthogneiss show S-type granite characteristics. The Neoarchean to Early Paleoproterozoic ages (2500–2300 Ma) have positive εHf(t) values and two-stage model (TDM2) ages of 2760–2688 Ma, indicating a magmatic source from the depleted mantle with no crustal material involved. The Early to Middle Paleoproterozoic (2200–1770 Ma) zircon cores of mainly negative εHf(t) values and corresponding TDM2 ages (2800–2400 Ma) indicate magmatic materials predominantly from ancient (Neoarchean) crustal material with a small amount of depleted mantle material. The Early to Middle Mesoproterozoic (1580–1200 Ma) zircons indicate a magma originating from ancient crust, with minor input of mantle-derived material. The εHf(t) values of zircon rims indicate reworking of ancient crust based on the Neoproterozoic geological events (two major age groups of ca. 970–840 and 780–750 Ma), which were associated with the post-collision and partial melting events. The 2500–2300 Ma age group corresponds to the Neoarchean magmatism underneath the southwestern Yangtze Block, and the 2200–1770 Ma age group corresponds to the assembly of the Columbia supercontinent with related continental collision. The wide range of zircon cores with the Lu-Hf isotopic data provide strong evidence regarding the assembly and breakup of different stages of the supercontinent, as well as critical information on the evolution of the Yangtze Block in South China.

Formation of the Late Cretaceous-Paleogene Sn-rich granites in the Tengchong block: The magma origin and differentiation conditions

腾冲地块锡成矿作用主要与晚白垩世-古近纪岩浆活动相关，但仅矿区花岗岩体具有较高的锡含量（平均25×10^-6），区域上同时代的非成矿花岗岩体并未发生锡的富集。本文通过搜集分析现有研究数据，总结了富锡成矿岩体和区域非成矿岩体在岩浆源区、演化条件和结晶分异程度的异同：区域非成矿岩体锆石ε_Hf（t）值（-15.1~+3.39）表明，自东北向西南幔源物质加入有升高的趋势，但区域玄武岩中低含量的Sn （1.61×10^-6）表明幔源物质混入不利于岩浆中锡的富集。晚白垩世部分非成矿岩体与成矿岩体具有相同ε_Hf（t）（-9.7）值，表明其皆源于古老地壳物质的部分熔融，但岩体均表现为准铝质-弱过铝质特征，且锆石Hf同位素（t_DM2=1724Ma）和全岩Nd同位素（t_DM2=1836Ma）二阶段模式年龄基本一致，因此其源区可能并非富锡的高黎贡山群变质沉积岩，而可能是其中未经风化的变质花岗岩。根据腾冲地块地层厚度（28km）和莫霍面深度（47~35km）推断岩浆源区至少位于地下30km （8.4kbar），由于仅靠地温梯度（25℃/km）无法达到初始熔融温度（>1066℃），源区部分熔融过程很可能受地幔热的影响。根据Fe₂O₃/FeO比值，非成矿岩体（0.59）与成矿岩体（0.48）均具有较低的氧逸度，属钛铁矿系列，但成矿岩体的结晶分异程度明显高于非成矿岩体，且成矿岩体富含挥发分，高含量的挥发分降低了岩浆固结温度（650~550℃），延长了结晶分异时间，促进了锡在晚期岩浆中的富集。因此腾冲地块富锡花岗岩主要是普通岩浆在低氧逸度环境下发生高度结晶分异的结果。;Tin metallogenesis in Tengchong block is associated with the Late Cretaceous and Paleocene magmatism. However, only the ore-forming granitoids are characterized by high Sn content (25×10^-6 on average). The regional contemporary unmineralized granitoids hardly show Sn enrichment. By collecting and analyzing available research material, the differences between the Sn-rich ore-forming rocks and regional unmineralized rocks are summarized in terms of their magma origins, evolution conditions and differentiation degrees. Zircon ε_Hf(t) values (-15.1~+3.39) of the regional unmineralized rocks indicate that the magmas in this area are added with increasing mantle material from northeast to southwest, while the addition of the mantel material is disadvantageous to tin enrichment in the mixing magma according to the Sn content (1.61×10^-6) of the regional basalt. Part of the regional unmineralized granitoids formed in Late Cretaceous have the same ε_Hf(t) values of -9.7 with the Sn-rich ore-forming rocks, suggesting both of them were derived from partial melting of ancient crustal material. Considering these rocks are all metaluminous or weak peraluminous, and the crustal model age (t_DM2) calculated from zircon Hf isotopes (1724Ma) are nearly identical to that from the whole-rock Nd isotopes (1836Ma), the magma may not be derived from the Sn-rich Gaoligongshan Group consisting of metasedimentary rocks, but probably the inner metamorphic granitoids. The metamorphic granitoids are at least 30km underground (corresponding to 8.4kbar) based on the regional strata thickness (28km) and the Moho depth (47~35km). The initial melting temperature (1066℃) is unable to be reached according to geothermal gradient (25℃/km), therefore, the partial melting process is likely influenced by mantle heat. According to Fe₂O₃/FeO values, both the unmineralized granitoids (0.59) and the tin mineralized granitoids (0.48) have low oxygen fugacity, and belong to ilmenite series. While the fractionation of Sn-rich granitoids is much higher than that of unmineralized granitoids, moreover, the Sn-rich granitoids contain abundant volatiles, which decrease the magma consolidation temperature (650~550℃), prolong magma differentiation process, and promote Sn gathering in the late magma. In conclusion, the Sn-rich granitoids in the Tengchong block is mainly resulted from the high fractionation of normal magma under a low oxygen fugacity.

Ca. 110 Ma adakite-like magmatism along the Bangong–Nujiang suture zone: Implications for crustal thickening and early uplift in the central Tibetan Plateau

The suggested Early Cretaceous timing of early formation of the Tibetan Plateau remains controversial. This study investigates this uncertainty by examining newly identified Early Cretaceous adakite-like magmatic rocks in the Rutog and Nyima areas of the northern–central Tibetan Plateau. Zircon U Pb dating indicates that these rocks formed at ca. 110 Ma and have negative to positive zircon ε Hf (t) values. They contain high concentrations of Sr and have high Sr/Y values, but contain low concentrations of Y and Yb and have low (La/Yb) N , suggesting they have an adakite affinity. They also have ε Nd (t) values that range from −4.5 to +1.3. These geochemical and isotopic data indicate that the adakite-like rocks probably formed from magmas generated by the partial melting of garnet-bearing amphibolite within a thickened region of juvenile and ancient crustal material. This magmatism also most likely occurred during the Lhasa–Qiangtang collisional event. Crustal thicknesses along the Bangong–Nujiang suture zone (BNSZ) were > 40 km in places at ca. 110 Ma, and most likely had a significant impact on the initial uplift of the Tibetan Plateau. In summary, the Early Cretaceous adakite-like magmatic rocks in the Rutog and Nyima areas suggest that the Tibetan Plateau started to form during the Early Cretaceous. • Early Cretaceous adakite-like rocks occur in the north-central Tibet. • Adakite-like rocks were derived from the thickened crust. • The Lhasa–Qiangtang collision happened before ca. 110 Ma. • The significant crustal thickening led to the early uplift in the Early Cretaceous.

Permian-triassic magmatic and thermal events in the Dunhuang orogenic belt: implications for subduction records of the Paleo-Asian Ocean

ABSTRACT The final subduction and suture of the Paleo-Asia Ocean (PAO) are controversial, and the key to this question is to locate the latest subduction/final suture of the accretionary type orogenic belt. Previous studies mainly focused on the Tianshan, Beishan, Alxa, and Solonker Orogens for this issue. As a Phanerozoic orogenic belt located further south of the Beishan-Alxa Tectonic Belts, the Dunhuang Orogenic Belt (DOB) is becoming a research hotspot for revealing the evolutionary history of the southernmost Central Asia Orogenic Belt (CAOB). The youngest subduction-related arc magmatism is pivotal for constraining the lower limit of subduction of a lost ocean. This study presents integrated investigations of field observation, whole-rock geochemistry, and geochronology, targeting diabase dykes, intermediate-acid magmatic rocks, and garnet-bearing metabasite samples in the middle part of the DOB. LA-ICP-MS U-Pb dating of zircon yielded episodic magmatism of ca. 274–273 Ma for Xiaohongshan granite and Dongbatu syenogranite, ca. 250 Ma for Keziletage syenite, ca. 243–239 Ma for diabase and granitic dykes in the Kalatashitage area and Mogutai syenogranite, and maximum deposition age of ca. 262 Ma for pyroclastic rock in the Kalatashitage area. Most of those Permian-Triassic magmatic rocks show continental arc-related geochemical features during the subduction stage. The negative εHf(t) values and ancient TDM2 model ages of zircon indicate they were mainly derived from the partial melting of ancient crustal materials. The hornblende 40Ar/39Ar ages of ca. 258–250 Ma of the garnet-bearing metabasite samples also reflect a high heat flow setting during this period possibly triggered by arc activities. These Permian-Triassic magmatic and thermal records in the DOB are similar to contemporaneous records in surrounding South Tianshan-Alxa-Beishan orogens. We suggest that the final subduction of the PAO in the DOB possibly at least lasted to the Middle Triassic.

Late Paleozoic–Early Mesozoic granitoids in the Khangay-Khentey basin, Central Mongolia: Implication for the tectonic evolution of the Mongol-Okhotsk Ocean margin

The Mongol-Okhotsk Belt is the youngest segment of the Central Asian Orogenic Belt, which is the venue of the massive juvenile crust emplacement, and its formation and evolutions are still pending problems. This paper presents the first up-to-date U–Pb zircon ages, Hf-in-zircon isotope, geochemical and whole-rock Nd isotope data from igneous rocks of the Khangay-Khentey basin, Central Mongolia. The U–Pb zircon ages indicate three groups of magmatism at ~296 Ma, ~280 Ma, and ~230 Ma. The ~296 Ma magmatic rocks are characterized by negative εHf(t) and εNd(t) values and old Hf and Nd model ages suggesting their derivation by the melting of the crustal source. The ~280 Ma rocks are A2-type monzonites, granitoids, and rhyolites show positive εHf(t) and εNd(t) values and Neoproterozoic Hf and Nd model ages. The geochemical and isotope data suggest that ~280 Ma magmatism derived by the melting of a crustal source, induced by mantle upwelling. The ~230 Ma rock assemblage includes granitoids and volcanic rocks. The I-type calc-alkaline granitoids are enriched in K, Rb, U, and Th. The geochemical characteristics suggest that they have formed by the melting of a hornblende-bearing crustal source with the participation of fluids separated from the subducting slab. The positive εHf(t) and εNd(t) ~230 Ma rocks suggest partial melting of a depleted lower crustal material with the contribution of ancient crustal material. The ~296 Ma granitoids possess coherent/coupled Nd–Hf isotopic compositions supporting their origin from the ancient crust. Although the number of ~296 Ma samples are small, we suggest that they were probably emplaced at an active continental setting, ~280 Ma samples could have formed in a setting of local extension environment, ~230 Ma granitoids were also formed at an active continental margin. These magmatic rocks formed during the subduction of the Mongol-Okhotsk oceanic plate beneath the Central Mongolia-Erguna Block.

Late Triassic dacites from Well NK-1 in the Nansha Block: Constraints on the Mesozoic tectonic evolution of the southern South China Sea margin

Identifying whether and when the southern continental margin of the South China Sea (SCS) was part of the South China Block (SCB) is important for Mesozoic–Cenozoic paleogeographic reconstructions of Southeast Asia. In this paper, we present geochronological and geochemical data for dacites from Well NK-1 in the Nansha Block (Dangerous Grounds) of the southern SCS margin to investigate their petrogenesis and tectonic affinity. Zircon UPb and 40Ar/39Ar datings show that these dacites formed during the Late Triassic (218–217 Ma). The dacites have high total alkali (K2O + Na2O = 7.39–8.42 wt%) and relatively low CaO contents (1.87–3.38 wt%), and show geochemical affinities with A-type granites, with high Zr saturation temperatures (806–855 °C), FeOTot/MgO ratios (5.22–7.48), Zr + Nb + Ce + Y contents (530–680 ppm) and 10,000 × Ga/Al values (2.40–2.94), and relatively low A/CNK values (0.86–1.04). The primitive magma for the dacites was likely generated by partial melting of ancient crustal materials. Late Triassic dacites in the Nansha Block and Triassic A-type granites in the SCB formed in an extensional setting at the continental margin of the SCB. This setting was probably related to the Paleo-Pacific plate subduction that restarted prior to the Early Triassic. Therefore, the southern SCS margin was part of the SCB continental margin during the early Mesozoic, which provides more detailed insights into the paleogeography of SE Asia since the Mesozoic.

Geochronology and petrogenesis of Eocene gabbros and granitic rocks of the eastern Gangdese belt, southern Tibet: Implications for the timing of India-Asia collision

The Palaeocene-Eocene (66–41 Ma) hosts the most intense stage of extensive magmatism in the Gangdese belt of southern Tibet. However, the magma sources and tectonic setting of this magmatic belt have been a matter of discussion for decades. In this study, we conducted zircon geochronological and geochemical studies on Eocene gabbroic rocks and granitic rocks from the Jiacha area in the eastern Gangdese belt. Laser ablation-ICP-MS zircon U–Pb dating suggests almost synchronous crystallization ages of 50–47 Ma for both rock types. Geochemically, the gabbroic rocks have low total rare earth elements and high positive zircon εHf(t) values (from +8.4 to +15.0), indicating that the parental magma was derived from an isotopically depleted mantle. The granitic rocks are magnesian calc-alkalic to calcic granites that are characterized by variable zircon εHf(t) values (from +3.0 to +10.8), indicating that the magmas were generated by differentiation of a mantle-derived magma with variable contributions from ancient crustal materials. Furthermore, an enlarged database of 66–41 Ma plutonic rocks and coexisting Linzizong volcanic rocks reveals an evolutionary trend from mafic to silicic rocks. The zircon Hf and Nd isotopic compositions become more evolved with increasing whole-rock SiO2. The whole-rock and isotope geochemistry of the 66–41 Ma plutonic rocks and the Linzizong volcanic rocks indicate an origin and evolution of these rocks in an active continental margin setting. Therefore, we argue that the Gangdese belt was associated with the subduction of the Neo-Tethyan Oceanic plate throughout the Early Eocene, thereby implying that the initial India-Asia collision occurred after ~ 41 Ma.

An Early Paleoproterozoic back-arc system along the southern margin of the Yinshan Block: Evidence from a newly-defined bimodal volcanic sequence in the Daqingshan Complex, Khondalite Belt

As one of the 2.1 to 1.9 Ga orogenic belts that welded the Columbia supercontinent, the Khondalite Belt in the North China Craton is a typical continent-continent collisional orogen that formed through the collision between the Yinshan and Ordos Blocks. Previous studies mostly focused on the collisional event in the Khondalite Belt but paid little attention to how the subduction system operated before the final closure of the ocean. To address this issue, we identified a series of interlayered meta-mafic and felsic rock assemblages in the Daqingshan Complex and implemented geochemical and geochronological analyses. Petrological and geochemical studies revealed that these rocks are bimodal and include plagioclase amphibolite (Group 1) and biotite plagiogneiss (Group 2). Geochemically, Group 1 samples show tholeiitic affinity, whereas Group 2 samples belong to the high-K calc-alkaline series. Geochemical data indicate that the protolith magma of Group 1 was most likely derived from the partial melting of lithospheric mantle with minor crustal contamination, whereas Group 2 rocks represent highly differentiated magma derived from the partial melting of ancient crustal materials. All the samples show depletion of HFSEs and enrichment of LILEs, indicative of a subduction-related magmatic arc environment. Zircon U-Pb dating results show that the protoliths of Group 1 samples yield crystallization ages of ∼2.47 Ga and metamorphic ages of 1.95 to 1.85 Ga, whereas the protoliths of Group 2 samples yield crystallization ages of ∼2.40 Ga and metamorphic ages of ∼1.85 Ga. Our new results and available geochemical, petrological, and isotopic data demonstrate that the bimodal volcanic sequence of the Daqingshan Complex was developed in a 2.47 to 2.40 Ga back-arc system along the southern margin of Yinshan Block. Subsequent collision between the Ordos and Yinshan Blocks resulted in the formation of the Khondalite Belt and final amalgamation of the Western Block between 1.95 and 1.85 Ga.

Petrogenesis of the Late Jurassic Cu-Mo mineralization-related Xianyang granite porphyry in the Dehua district, Southeast China: Response to the subduction of paleo-Pacific slab and implication for regional exploration

The Dehua Au-polymetallic ore cluster, as one of the most important area in the Wuyi Au-Cu-Mo metallogenic belt, consists of many large to middle sized epithermal Au deposits and recently discovered porphyry Cu-Mo deposits. We first study the geochronology and geochemistry characteristics of the Cu-Mo mineralization-related Xianyang granite porphyry to constrain its petrogenesis, tectonic setting and metal endowment. LA-ICP-MS zircon U-Pb dating yield a precise age of 157.2 ± 0.8 Ma for the Xianyang granite porphyry, indicating that the granite porphyry was emplaced during the Late Jurassic. The Xianyang granite porphyry was suggested to be generated under continental arc setting, associated with subduction of the paleo-Pacific slab beneath the South China Block. The Xianyang granite porphyry belongs to the high-K calc-alkaline series, with SiO2 of 71.16–72.02 wt% and K2O of 4.28–4.69 wt%. It shows light rare earth element-enriched distribution patterns with (La/Yb)N of = 9.74–13.93, enrichment in large ion lithophile elements, but depletion of high field strength elements, with negative Nb, Ta and Ti anomalies, suggesting an arc magma geochemical affinity. It has lower initial 87Sr/86Sr ratios of 0.7079–0.7086 and relatively higher εNd(t) and εHf(t) values of −8.03 to −7.69 and − 8.86 to −5.48, respectively, than those of basement rocks of the Cathaysia block, suggesting that it was most likely generated by partial melting of the regional ancient crustal materials, with the involvement of asthenosphere mantle-derived materials. The Xianyang granite porphyry has remarkably high zircon Ce4+/Ce3+ values of 119–404 (mean = 245) and EuN/EuN* values of 0.37–0.60 (mean = 0.51), indicating a high oxygen fugacity, which is favor of the formation of the Xianyang porphyry Cu-Mo mineralization. Combining with published research results for regional subvolcanic rocks-related epithermal Au deposits, the porphyry Cu-Mo deposits and epithermal Au deposits in the Dehua ore cluster share similar petrogenetic-metallogenic ages and magma sources in the continental arc setting during the Late Jurassic period. The identification of the coeval porphyry-epithermal Cu-Mo-Au metallogenic system in the Dehua ore cluster highlights the possibility of finding the hidden porphyry Cu-Mo orebodies beneath the discovered epithermal Au deposits.

Statherian (ca. 1714–1680 Ma) Extension-Related Magmatism and Deformation in the Southwestern Korean Peninsula and Its Geological Significance: Constraints from the Petrological, Structural, Geochemical and Geochronological Studies of Newly Identified Granitoids

In this study, petrological, structural, geochemical, and geochronological analyses of the Statherian alkali feldspar granite and porphyritic alkali feldspar granite in the southwestern part of the Korean Peninsula were conducted to examine petrogenesis of the granitoids and their tectonic setting. Zircon U-Pb dating revealed that the two granites formed around 1.71 Ga and 1.70–1.68 Ga, respectively. The results of the geochemical analyses showed that both of the granites have a high content of K2O, Nb, Ta, and Y, as well as high FeOt/MgO and Ga/Al ratios. Both granites have alkali-calcic characteristics with a ferroan composition, indicating an A-type affinity. Zircon Lu-Hf isotopic compositions yielded negative εHf(t) values (−3.5 to −10.6), indicating a derivation from ancient crustal materials. Both granite types underwent ductile deformation and exhibited a dextral sense of shear with a minor extension component. Based on field relationships and zircon U-Pb dating, it was considered that the deformation event postdated the emplacement of the alkali feldspar granite and terminated soon after the emplacement of the porphyritic alkali feldspar granite in an extensional setting. These data indicated that there were extension-related magmatic activities accompanying ductile deformation in the southwestern part of the Korean Peninsula during 1.71–1.68 Ga. The Statherian extension-related events are well correlated with those in the midwestern part of the Korean and eastern parts of the North China Craton.

Geochronology and geochemistry of the Xiaoqinling Taihua Complex in the southern Trans-North China Orogen: Implications for magmatism during the early Paleoproterozoic global tectono-magmatic shutdown

The Taihua Complex in the Xiaoqinling area of the southern Trans-North China Orogen (TNCO) records extensive magmatism during the early Paleoproterozoic. This contrasts with the tectono-magmatic quiescence recorded globally at this time. Based on zircon U−Pb geochronological and whole-rock geochemical data, the 2.36–2.30 Ga granitic gneisses in the Taihua Complex can be subdivided into three groups. Group-1 gneisses (2.36–2.33 Ga), mainly in the Dumutai area, are high-K2O granitoid rocks, which record the earliest magmatism. These rocks have fractionated rare earth elements (REE) patterns, marked negative Eu−Sr anomalies, and variable whole-rock εNd(t) (−1.67 to +2.41) and εHf(t) values (−0.89 to +3.41), and were likely derived by partial melting of a mixture of pre-existing tonalite–trondhjemite–granodiorite (TTG) and K-rich rocks with residual plagioclase in the source at high-temperature and low-pressure conditions. Group-2 gneisses (2.33–2.31 Ga), mainly in the Jialu and Xitongyu areas, are granitoid rocks that formed after the Group-1 gneisses. These rocks have less fractionated REE patterns, low La/Yb and Sr/Y ratios, and slightly positive to negative whole-rock εNd(t) (−3.91 to +2.00) and εHf(t) values (−7.99 to +0.87), which resulted from high-degree partial melting of ancient crust in an extensional setting. Group-3 gneisses (~2.30 Ga) are TTG rocks from the Bayuan area, which represent the youngest magmatism. These rocks have high La/Yb and Sr/Y ratios, high MgO, Cr, and Ni contents, and negative whole-rock εNd(t) (−1.71 to −1.67) and εHf(t) values (−0.57 to −0.10), which indicate derivation by partial melting of delaminated lower crust and interaction with mantle materials. Samples from all groups have variable zircon εHf(t7/6) (−6.38 to +7.70) and δ18O values (3.97‰ − 6.79‰), indicative of a heterogeneous magmatic source. Most zircons have slightly positive to negative εHf(t7/6) values, indicating significant reworking of ancient crustal materials and limited crustal growth during this period. The age, geochemistry, and petrogenesis of the studied granitoid rocks record orogenic collapse and a transition from post-collisional extension to crustal delamination in the early Paleoproterozoic. This followed late Archean subduction–collision–accretion processes in the southern NCC.

Prototethyan Accretionary Orogenesis Along the East Gondwana Periphery: New Insights From the Early Paleozoic Igneous and Sedimentary Rocks in the Sibumasu

AbstractNew geochronological and geochemical data document a new and widely distributed succession of igneous and sedimentary rocks within the Sibumasu that extend from SW Yunnan through SE Asia as far as Sumatra. Our data revealed the presence of an Ordovician‐Silurian (mainly ∼490–430 Ma) igneous zone. The early Paleozoic granites in the zone display similar elemental and Sr–Nd–Pb–Hf isotopic compositions. They were derived from a common mixed source of ancient crustal materials with a juvenile component and formed in a subduction‐related setting. Detrital‐zircon U‐Pb apparent ages from the previously defined Precambrian Lancang and Ximeng units in the SW Yunnan portion of the Sibumasu are in the range of 3,306–428 Ma and 3,316–512 Ma, with the youngest age‐peaks of 505 Ma and 447 Ma, respectively, indicating the lower Paleozoic (Cambrian‐Silurian) sedimentary sequences. The Tarutao Unit and its equivalents in the SE Asia portion of the Sibumasu yield the U‐Pb apparent ages of 3,177–510 Ma, suggesting the maximum deposited time being Cambrian‐Ordovician. All these detrital zircon U‐Pb ages define two main peaks of ∼554–528 Ma and 964–905 Ma with εHf(t) values similar to those in the Qiangtang, Tethyan Himalaya, and eastern Indochina. The lower Paleozoic detritus in the Sibumasu were likely derived from the India‐Antarctica region of East Gondwana. We propose that the late Cambrian‐Silurian units formed in an outward‐propagating accretionary orogen in response to the consumption of the Prototethyan Ocean in SW Yunnan and SE Asia. Sibumasu might represent a part of the easterly migrating accretionary orogen along the northern margin of East Gondwana in the late Cambrian‐late Silurian.

Early Cenozoic partial melting of meta-sedimentary rocks of the eastern Gangdese arc, southern Tibet, and its contribution to syn-collisional magmatism

AbstractContinental magmatic arcs are characterized by the accretion of voluminous mantle-derived magmatic rocks and the growth of juvenile crust. However, significant volumes of meta-sedimentary rocks occur in the middle and lower arc crust, and the contributions of these rocks to the evolution of arc crust remain unclear. In this paper, we conduct a systematic study of petrology, geochronology, and geochemistry of migmatitic paragneisses from the eastern Gangdese magmatic arc, southern Tibet. The results show that the paragneisses were derived from late Carboniferous greywacke, and underwent an early Cenozoic (69–41 Ma) upper amphibolite-facies metamorphism and partial melting at pressure-temperature conditions of ~11 kbar and ~740 °C, and generated granitic melts with enriched Hf isotopic compositions (anatectic zircon εHf(t) = −10.57 to +0.78). Combined with the existing results, we conclude that the widely distributed meta-sedimentary rocks in the eastern Gangdese arc deep crust have the same protolith ages of late Carboniferous, and record northwestward-decreasing metamorphic conditions. We consider that the deeply buried sedimentary rocks resulted in the compositional change of juvenile lower crust from mafic to felsic and the formation of syn-collisional S-type granitoids. The mixing of melts derived from mantle, juvenile lower crust, and ancient crustal materials resulted in the isotopic enrichment of the syn-collisional arc-type magmatic rocks of the Gangdese arc. We suggest that crustal shortening and underthrusting, and the accretion of mantle-derived magma during the Indo-Asian collision transported the supracrustal rocks to the deep crust of the Gangdese arc.