Post-collisional Extensional Setting Research Articles

Growth of early Paleozoic continental crust linked to the Proto-Tethys subduction and continental collision in the East Kunlun Orogen, northern Tibetan Plateau

The East Kunlun Orogen (EKO) in the northern Tibetan Plateau records two continental collisional orogenic events and magmatism in early Paleozoic and early Mesozoic. However, possible magmatic additions to the continental crust growth of the EKO in different tectonic stages of early Paleozoic collisional orogeny have been overlooked. Three phases of early Paleozoic plutons from the Xiangride-Kuhai area in the east of the EKO have been chosen for detailed investigation and the results are reported here. The oldest magmatic suite (Stage 1) includes the ca. 471 Ma Qurelong Monzodiorite and ca. 454 Ma granodiorite in the Zhiyu Intrusive Complex. The monzodiorite has a sanukitoid-like composition with high TiO2 and Y contents and is interpreted as being derived from partial melting of metasomatized mantle wedge lherzolite. The granodiorite is typified by its high SiO2 content, high Sr/Y ratio, and depleted Hf isotope, and is interpreted as an adakite-like melt derived from the melting of a subducted Proto-Tethys oceanic crust. The magmatism can be linked to northward subduction of the Proto-Tethys Ocean between 520 and 450 Ma. Stage 2 magmatism is represented by a plutonic suite emplaced during ca. 450−431 Ma with an I-type granitic composition. Of these, the ca. 447 Ma Kengdenongshe Intrusion composed of peraluminous granite with enriched Nd-Hf isotopes is indicative of a Mesoproterozoic igneous source in the orogen. The ca. 450−434 Ma monzogranite and granodiorite in the Walega and Zhiyu intrusive complexes exhibit variable element and isotope compositions. They would have been generated by magma mixing of felsic melts from the old crust and mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion of >30%. The ca. 431 Ma quartz diorite in the Walega Intrusive Complex is formed through crustal assimilation and fractional crystallization of mafic magmas derived from the metasomatized lithospheric mantle, with a mafic melt proportion >60%. Stage 2 suite was emplaced during the closure of Proto-Tethys oceanic branches and subsequent continental collision during 450−426 Ma. Magmatism diminished between ca. 426 and 410 Ma during exhumation of the continental lithosphere as indicated by the presence of retrograde eclogites in the EKO. Stage 3 magmatic suite includes the ca. 408 Ma Langmuri Intrusion and ca. 403 Ma Niantang Syenogranite. These plutons are adakite-like or have an A-type granitic composition and are enriched in Nd-Hf isotopes. They might have been derived from the remelting of old and juvenile continental crust in a post-collisional extensional setting during 410−390 Ma. Identification of partial melts, derived from the subducted Proto-Tethys oceanic crust and metasomatized lithospheric mantle in stage 1 and 2 plutons, show that the subcrustal materials have been significantly transferred to the overlying continental crust. Hence the magmatism in oceanic subduction (Stage 1) and continental collision (Stage 2) settings contributes to the early Paleozoic juvenile continental crust growth of the EKO. The post-collisional extensional setting (Stage 3) is dominated by the reworking of a pre-existing continental crust. The early Paleozoic continental crust growth processes in the EKO are different from the previous view in which the continental collision orogens have no crust growth, and inconsistent with the proposal that crust growth is significant only in a continental collision setting.

Petrogenesis of late Paleoproterozoic post-collisional magmatism in southern north China Craton: Insights from geochemistry and Nd–Hf isotopic compositions of A-type granites

A-type granites are produced commonly in extensional settings and they are important proxies to track breakup of continent. Here we report systematic studies on geochronology and geochemical features of A-type granites in Songxian–Ruyang region, southern margin of the North China Craton (NCC), in order to constrain their petrogenesis and tectonic implications. Zircons from these granites show oscillatory zoning and high Th/U ratios (0.12 to 1.26), indicating a magmatic origin. LA–ICP–MS zircon U–Pb dating shows that they were emplaced at 1813 to 1789 Ma, indicating that they were formed in late Paleoproterozoic. Geochemically, these rocks are high in SiO2 (67.51–74.42 wt%) and total alkalis (Na2O + K2O = 7.76–10.19 wt%), and low in MgO (0.09–1.31 wt%), CaO (0.09–1.11 wt%) and P2O5 (0.12–0.26 wt%) contents, with negative Nb, Ta, Sr, P and Ti anomalies. They are characterized by high FeOT/(MgO + FeOT), 10000 Ga/Al ratios (1.57–3.15) and Zr + Nb + Ce + Y contents (566–835 ppm). They are also characterized by high calculated Zr saturation temperatures (892–918 °C), high Ti-in-zircon temperatures (693–912 °C) and metaluminous to weakly peraluminous natures, which are typical features of aluminous A-type granites, further belonging to the A2 granites. The low zircon εHf(t) values (–13.8 to –8.6), along with the low whole rock εNd(t) values (–8.62 to –6.10) and low apatite εNd(t) values (–8.31 to –6.56), indicate that the Songxian–Ruyang A-type granites were derived from partial melting of the calc-alkaline tonalitic–trondhjemitic–granodioritic gneisses in Taihua Complex, and they were formed in post-collisional extensional setting. Combined with coeval regional granitoids, the south margin of the NCC is in a continuous extensional setting during 1.8–1.6 Ga, and the Songxian–Ruyang A-type granites can represent a response of the breakup of Columbia supercontinent.

Volatile evolution of magmas associated with the Bairong deposit, Tibet, and implications for porphyry Cu-Mo mineralization

Volatiles are considered to play an important role in porphyry Cu mineralization, and high apatite S and Cl contents are often used to distinguish fertile magma systems from barren ones in the subduction zone. However, magmatic volatile evolution and its implications in post-collisional porphyry deposits remain enigmatic. To address this issue, we conducted zircon U-Pb dating and Hf isotope, and zircon and apatite chemical compositions of multi-stage magmatic rocks from the Bairong post-collisional porphyry Cu-Mo deposit in the Gangdese belt, Tibet. The magmatic rocks included pre-mineralized porphyritic monzogranite and biotite monzogranite, syn-mineralized monzogranite porphyry, and post-mineralized dacite porphyry that formed in 14.1–12.7 Ma, ca. 12.1 Ma, and 11.8–11.6 Ma, respectively. All these intrusions have depleted zircon Hf isotope (εHf(t) = 1.7–8.0, 3.7–9.0, 3.3–9.7 for pre-, syn-, and post-mineralized intrusions, respectively) together with the research results of contemporaneous magmatic rocks in the adjacent area, suggesting that the multi-stage intrusions at Bairong are derived from a cogenetic magma chamber in post-collisional extension setting. All the intrusions have similar zircon CeN/CeN*, EuN/EuN* ratios with average ΔFMQ values of 2.1 – 2.6, together with the high apatite EuN/EuN* ratios and low Ga concentrations, implying that their magmas are all oxidized. These intrusions have zircons (10,000 × EuN/EuN*) /Y all > 1 and Ce/Nd/Y mostly > 0.01, and obtain low average titanium-in-zircon temperatures of 659 to 714 °C, which indicate their magmas are hydrous. Apatite Cl contents decrease, combined with the drop of apatite XCl/XOH and the surge of XF/XCl ratios from pre- to syn-mineralized intrusions, possibly indicating that fluid exsolved from magma at the syn-mineralization stage. The apatite SO3 contents in these different-stage intrusions, however, are almost unchanged. Combined with previous studies, the buffering effect of magmatic anhydrite maintains the stability of magma S content, which together with fluid exsolution leads to the S-Cl decoupling. Consequently, we conclude that the oxidized and hydrous magma with effective fluid exsolution is a critical control on the formation of Bairong porphyry Cu-Mo mineralization. Considering the wide variations of apatite Cl contents and the occurrences of S-Cl decoupling, simple application of high apatite SO3 and Cl contents to distinguish the ore-forming and barren systems in the post-collisional porphyry system should be cautious.

Zircon U-Pb ages and Hf isotope characteristics of granitic rocks from the Shapinggou molybdenum deposit, Dabie Shan, China

The giant Shapinggou porphyry molybdenum deposit in Jinzhai county, Anhui province is located in eastern Dabie Shan, but its genesis, genetic type and tectonic setting are still debated. New zircon U − Pb age data presented in this contribution show that the granitic rocks at Shapinggou area have been episodically formed at ∼ 135 Ma, ∼127 Ma, ∼117 Ma and ∼ 113 Ma in Early Cretaceous, and can be classified into the early-stage (135–127 Ma) granodiorite and monzogranite plutons, and the late-stage (117–113 Ma) quartz-orthophyre stock and cryptoexplosive breccia pipes. Mo mineralization is closely associated with the late-stage shallow-seated granitic stock or pipes. Granitic rocks of both stages have high contents of SiO2, K2O + Na2O and Al2O3, low contents of TiO2, CaO and MgO, showing a metaluminous to peraluminous, high-K calc-alkaline to shoshonite affinity. They are generally featured by enrichments of LILE (Rb, U, Th) and LREE, and depletions of HFSE (Nb, Ta, Ti) and HREE, with high (La/Yb)N ratios. The early-stage granitic plutons show trace element geochemical signatures of partial melting of over-thickened continental crust (>50 km), whereas element geochemistry of the late-stage small-sized granitic intrusions suggests a shallow source depth (ca. 35 km) under post-collisional extension setting. The Hf isotope data indicate that the Shapinggou granitic rocks are originated from a crustal source predominated by the Kuanping Group mixed with other components such as the mantle. The southward subduction and partial melting of the southern North China Block (including Kuanping Group) beneath the Dabie Shan lead to the generation of the Shapinggou porphyry Mo system in the post-collisional extension setting. Hence the Shapinggou Mo system distinguishes for its crustal source and post-collisional tectonic setting from the porphyry Mo deposits of Climax- or Endako-types, and should belong to collision-type or Dabie-type porphyry Mo deposit.

Early Permian magmatism in northern Inner Mongolia, southeastern Central Asian Orogenic Belt: Implications on lithospheric extension in a post-collisional setting

Identifying the petrogenesis and geodynamic mechanism of the Early Permian igneous rocks widely distributed in the Inner Mongolia–Daxing'an Orogenic Belt (IMDOB) is crucial for understanding the tectonic evolution of the southeastern Central Asian Orogenic Belt. In this study, a combined study of petrology, geochronology, and geochemistry of Early Permian volcanic–plutonic rocks from the Uliastai continental margin, northern IMDOB, was conducted aiming to constrain their petrogenesis and tectonic setting. The UPb zircon data for the magmatic zircons indicate that the Baoligaomiao Formation (BG Fm.) volcanic rocks, including felsic and mafic rocks, erupted at approximately 298–285 Ma, and quartz monzonite porphyries that intruded the BG Fm. were emplaced at approximately 292 Ma. The mafic volcanic rocks exhibit intraplate nature with basaltic compositions transitional between the calc-alkaline and shoshonite series. The geochemical behavior and isotopic data of these volcanic rocks reflect a depleted lithospheric mantle source metasomatized by fluids released from the stagnant slab. Felsic volcanics and quartz monzonite porphyries show affinities with A-type granites and are derived from the partial melting of juvenile crustal materials. Moreover, we suggest that the Early Permian magmatism formed in a post-collisional extensional setting, which is the continuation of the Late Carboniferous extension and was induced by the lithospheric collapse after the slab break-off of the Hegenshan Ocean. Notably, the felsic rocks formed after 290 Ma, with affinity changing from A2-type to A1-type granite, and approximately 288 Ma mafic volcanic rocks exhibited intraplate characteristics. These changes, combined with the development of coeval bimodal volcanic rocks and dike swarms, suggest that the tectonic setting at the end of the Early Permian in the northern IMDOB may have gradually transferred to an intraplate environment.

Timing and origin of orogenic gold mineralization in the Kanggurtag area, NW China

The Kanggur, Kangxi and Matoutan deposits are three important orogenic gold deposits located in the Kanggurtag gold mineralization belt of Eastern Tianshan, NW China. These deposits are hosted in low-grade metamorphic (greenschist-facies) volcaniclastic rocks of the Early Carboniferous Yamansu Formation. The orebodies are distributed in approximately E–W-striking shear zones and show evidence of hydrothermal alteration and four deformation events. The first deformation (D1) is associated with progressive regional N–S shortening without growth of gold mineralization. The second deformation (D2) is manifested by right-lateral shear zones under ductile conditions, which triggered the initial enrichment of gold in the stage I pyrite. The third deformation (D3) is characterized by a transpressional event under ductile–brittle conditions and formed stage II–Ⅳ pyrites with dominant Au mineralization; the native gold is mainly distributed in the interstices or interface of the pyrites. The fourth deformation (D4) is archived by post-collision extensional structures accompanied by magma emplacement and numerous limonite and comb-like quartz veins.Six pyrite samples (stage III) separated from the Kanggur deposit yield a Re–Os isochron age of 321 ± 6.7 Ma (MSWD = 3.0, initial 187Os/188Os = 0.35 ± 0.22). Three pyrite samples (stage III) separated from the Kangxi deposit yield a Re–Os isochron age of 327 ± 4.5 Ma (MSWD = 1.2, initial 187Os/188Os = 0.94 ± 0.12). The δ34S value significantly increases from stage I pyrite (−37.3‰ to −25.5‰, average = −32.2‰) to stage II−Ⅳ pyrite (−0.90‰ to 6.30‰, average = 1.96‰), indicating that sulfur in stage I pyrite migrated from sedimentary sources whereas in stage II–Ⅳ pyrites it was derived from the pyroclastic rocks in the country rocks. The H–O isotopic compositions of quartz show that the fluid in stage I (δ18Ow = 2.25‰ to 5.15‰, δ2D = −66.0‰ to −42.5‰) was metamorphic water and the fluid in stage Ⅴ was mixed with a large proportion of meteoric water. The fluids in stages II–Ⅳ maybe a “second hand” magmatic origin related to the andesite in the country rocks. Therefore, we propose that Au mineralization in the Kanggur, Kangxi and Matoutan deposits derived mainly from a metamorphic fluid source associated with the pyroclastic rocks in the country rocks. In addition, we conclude that these deposits are orogenic gold deposits that formed during the Late Carboniferous collisional tectonism (ca. 320 Ma) and the epithermal Au deposits (ca. 280 Ma) maybe associated with the post-collision extensional setting.

Late Triassic granitic magmatism and tungsten mineralization in NE China: Geochronological and geochemical constraints from the Tantoushan quartz-wolframite vein-type deposit

NE China, located at the eastern Central Asian Orogenic Belt, experienced extensive magmatism during the Mesozoic and hosts multistage granitic plutons and accompanying W mineralization. However, due to the limited number of studies on Triassic W deposits and spatially related granitoids, the petrogenesis of these granitoids and their relation to W mineralization remain enigmatic. The Tantoushan quartz-wolframite vein-type deposit is located on the southern margin of NE China. Tungsten mineralization occurs mainly in the veins and veinlets within monzogranites. A lower intercept 206Pb/238U age of 234.3 ± 6.2 Ma (1σ, MSWD = 0.41) was obtained for wolframite, which is identical within uncertainties to the zircon weighted mean 206Pb/238U age of 233.1 ± 1.8 Ma (1σ, MSWD = 0.41) from the W-bearing monzogranites. The monzogranites have the petrological, mineralogical, and geochemical characteristics of highly fractionated I-type granitoids. The rocks are enriched in Rb, Th, U, K, and Pb, and depleted in Ba, Sr, P, and Ti. They have higher W concentrations and Rb/Sr ratios, and lower Nb/Ta, Zr/Hf, and K/Rb ratios than the contemporary W-barren granitoids in NE China. These geochemical characteristics and negative zircon εHf(t) values (−17.7 to −8.6), as well as old two-stage model ages (TDM2 = 1807–2378 Ma), suggest that the monzogranites were derived as a product of the partial melting of the Paleoproterozoic lower crust and subsequently underwent extreme fractional crystallization. Geochronological and geochemical evidence collectively suggest that the W mineralization in the Tantoushan deposit is genetically related to the W-bearing monzogranites, and extreme fractional crystallization was essential for W enrichment in the granitic magma. In contrast, Triassic W-barren granitoids did not induce W mineralization, probably because of their low fractionated signatures. We preliminarily demonstrate that an isovalent substitution mechanism of 4A(Fe, Mn)2+ + 8BW6+ + B□ ↔ 3AM3+ + AN4+ + 7B(Nb, Ta)5+ + 2BN4+ played a critical role in the formation of hydrothermal wolframite in the Tantoushan deposit, and the trace elements compositions of wolframite were controlled by both the crystallochemical parameters and composition of the initial hydrothermal fluids. In the context of the regional geology, we propose that the Tantoushan monzogranites and corresponding W mineralization were formed in a post-collision extensional setting controlled by the closure of the Paleo-Asian Ocean during the Late Triassic. In combination with previous studies, we suggest that NE China may have enormous potential for Triassic W mineralization and the Triassic highly fractionated granitoids distributed on both sides of the Solonker-Xar Moron-Changchun Fault represent potential targets for future exploration of additional W resources.

Petrogenesis of early–middle paleozoic granitoids in the qilianblock, northwest China: Insight into the transition from adakitic to non-adakitic magmatism in a post-collisional extensional setting

To better understand geodynamic implications for the transition from adakitic to non-adakitic magmatism in a continental collision setting, an integrated study including zircon U–Pb ages and whole-rock geochemistry was performed for the Gahai and Erhai granitoid intrusions within the Qilian Block. The ca. 450–428 Ma Gahai biotite granite and Erhai muscovite-bearing granite are peraluminous S-type granites with adakitic geochemical characteristics, and have low Mg# (41.2–31.7) and εNd(t) (−10.8 to −6.5) values with two–stage Nd model ages of 2.05–1.72 Ga, implying their derivation almost entirely from an ancient crustal source under a thickened lower crust condition. The geochemical characteristics of the granites indicate that the Gahai and Erhai S-type granites were derived from partial melting of meta-greywacke and meta-pelite sources, respectively. In comparison, the ca. 415 Ma Gahai granodiorite belongs to metaluminous I-type granitoid and has low Sr/Y ratios as well as high Y and heavy rare Earth element (REE) concentrations, with nearly flat heavy REE patterns, implying a shallow source region. The granodiorites were most likely generated by partial melting of a K-rich basaltic magma source, with some contributions from mantle-derived melts. Synthesized data from this and previous studies suggest that the ca. 450–415 Ma Gahai and Erhai granitoids within the Qilian Block were generated in a post-collisional extensional regime triggered by the break-off of the northward subducting South Qilian Ocean slab beneath the Qilian Block. The geochemical transition from adakitic to non-adakitic intermediate-acidic magmas indicated that the thickened continental crust of the Qilian Block had experienced significant extension and thinning after ca. 420 Ma.

Petrogenesis and tectonic implications of 1.86–1.80 Ga A-type granites in the Daqingshan Complex, Khondalite Belt, North China Craton

The Khondalite Belt is regarded as a Paleoproterozoic continent–continent collisional belt along which the Yinshan Block and the Ordos Block amalgamated to form the Western Block in the North China Craton. Associated with the collision of the Yinshan and Ordos Blocks was granitic magmatism represented by quartz monzogranites in the Khondalite Belt, whose petrology, geochemistry and petrogenesis will provide important insights into understanding of the crustal evolution of the Khondalite Belt in the late Paleoproterozoic. In this paper, we carried out a detailed study on the petrography, geochronology, whole-rock geochemistry, and zircon Lu-Hf isotope of quartz monzogranites exposed in the Daqingshan Complex of the Khondalite Belt. Zircon LA-ICP-MS U-Pb dating results show that the Daqingshan quartz monzogranites were formed at 1.86–1.80 Ga, at some time posterior to the ∼ 1.95 Ga collision of the Yinshan and Ordos Blocks. Geochemically, the Daqingshan quartz monzogranites belong to ferroan, metaluminous to peraluminous, calc-alkalic to alkalic-calc series, exhibiting high SiO2, Na2O, K2O and FeOT / (FeOT + MgO), low Al2O3, MgO and CaO, and high Zr, Nb, Ce and Y, low Cr and Ni contents, with an affinity of A-type granites. In terms of REE and trace elements, the Daqingshan quartz monzogranites have right dipping patterns and negative Eu anomalies, enriched large ion lithophile elements (LILEs) (e.g. Rb, Zr and Hf), and depleted high field strength elements (HFSEs) (e.g. Ta, P and Ti). Zircon Lu-Hf isotopic characteristics (εHf(t) = -9.63 – −4.31, TDMC = 2744–3050 Ma) indicate that the Daqingshan quartz monzogranites were most likely originated from the partial melting of a Neoarchean ancient crust. Combining new results presented in this paper with previous data, we propose that the quartz monzogranites in the Daqingshan Complex were derived from the partial melting of Neoarchean TTG rocks under high temperatures caused by the upwelling of mantle magmas under a post-collision extensional setting. Meanwhile, it indicates that the Khondalite Belt completed collision at 1.86–1.80 Ga to form the Western Block.

Zircon U-Pb-Hf Isotopes, Biotite 40Ar/39Ar Geochronology, and Whole-Rock Geochemistry of the Baogeqi Gabbro in the Northern Alxa, Southernmost Central Asian Orogenic Belt

The final closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic settings in the northern Alxa are very important but controversial tectonic issues. The geochronology and petrogenesis of mafic igneous rocks are superior in clarifying regional tectonic settings. Here, we report on zircon U-Pb-Hf isotopes, biotite 40Ar/39Ar geochronology and whole-rock geochemical data of the hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa. The LA-ICP MS U-Pb analysis of zircon grains from the hornblende gabbro yield a weighted mean age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74), manifesting that the Baogeqi gabbro pluton emplacement was during the late Middle Permian (Capitanian). The 40Ar/39Ar dating of biotite grains from the hornblende gabbro yields a plateau age of 231.3 ± 1.6 Ma (2σ, MSWD = 0.55), indicating that the Baogeqi gabbro pluton cooled to below 300 ℃ in the Triassic. The hornblende gabbro samples are calc-alkaline with metaluminous character, and show enrichment in large ion lithophile elements (e.g., Rb, Ba, Sr, and K) but depletion in Nb, Ta, P, Th, and Ti relative to primitive mantle. Combined with the positive zircon εHf(t) values (+4.9–+9.4), we suggest that the magmas formed from the partial melting of depleted mantle were metasomatized by slab-derived fluids. Together with regional geology, these geochemical data suggest that the Baogeqi gabbro pluton was formed in an intracontinental extension setting, further indicating that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region was in a post-collision extensional setting during the Capitanian-Late Permian. In addition, the Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.

Geochemical and Geochronological Constraints of Permian-Triassic Magmatism on Oceanic Subduction and Continental Collision during the Eastern Paleo-Tethyan Evolution

The Jinshajiang–Ailaoshan–Song Ma orogenic belt (JASB), as a vital segment of the eastern Paleo-Tethyan tectonic zone, is one of the most important zones in which to study the Paleo-Tethyan tectonic evolution. We have undertaken an integrated geochronological, petrological, and geochemical study of mafic rocks from the JASB to reveal the subduction and closure processes of the eastern Paleo-Tethyan Ocean during the Permian to Triassic. In conjunction with previous magmatic and metamorphic records in the JASB, three important tectonic stages are identified: (1) Early Permian to Early Triassic (ca. 288–248 Ma). Most of the Early Permian to Early Triassic mafic rocks have normal mid-ocean ridge basalt (N-MORB)- or enriched MORB (E-MORB)-like rare earth elements (REE) and trace element-normalized patterns with positive εNd(t) and εHf(t) values and negative Nb and Ta anomalies. Their La/Nb ratios and εNd(t) values show that approximately 3%–15% of slab-derived fluid accounts for the generation of these rocks. These characteristics suggest that the mafic rocks formed in an arc/back-arc basin setting at this stage. Additionally, the Early Permian mafic rocks are mainly exposed in the Jomda–Weixi–Yaxuanqiao–Truong Son magmatic rock belt (JYTB) on the western side of the JASB, indicating that the westward subduction of the Jinshajiang–Ailaoshan–Song Ma Paleo-Tethys Ocean (JASO) began in the Early Permian. Middle Permian mafic rocks are exposed in the Ailaoshan-Day Nui Con Voi metamorphic complex belt and the JYTB on both sides of the JASB. We propose that the bipolar subduction of the JASO occurred in the Middle Permian and ended in the Early Triassic. (2) Middle Triassic (ca. 248–237 Ma). The mafic rocks at this stage have LREE- and LILE-enriched patterns, negative Nb and Ta anomalies and negative εNd(t) values. Their variable εHf(t), εNd(t) values and La/Nb ratios show that these mafic rocks were highly affected by crustal material (ca. 16%). Considering the Middle Triassic high-pressure (HP) metamorphism and massive Al-enriched felsic magmatism in the JASB, these rocks may have formed in a collisional setting between the South China Block (SCB) and the North Qiangtang–Simao–Indochina Block (QSIB) during the Middle Triassic. (3) Late Triassic (ca. 235–202 Ma). The mafic rocks at this stage have negative εNd(t) and εHf(t) values and show terrestrial array characteristics. The εNd(t) values and La/Nb ratios show that approximately 30% of crustal components account for the generation of these rocks. Combined with the contemporaneous bimodal magma and metamorphism during the Late Triassic, we suggest that these rocks may have formed in a postcollisional extensional setting associated with magma diapir.

Geology, C-H-O isotopes, and muscovite 40Ar-39Ar dating of the Qingbaishan gold deposit: Implications for tectonism and metallogenesis of early Devonian gold deposits in the Beishan orogen, NW China

The Beishan orogen is an economically important gold mineralization belt that hosts more than 20 gold deposits. Most Au orebodies occur within large-scale fault zones, indicating a correlation between structural evolution and gold mineralization. Previous studies have demonstrated that Au mineralization generally developed in the Permian–Triassic; however, pre-Permian gold mineralization and its temporal relationship with tectonic evolution remain poorly understood. The Qingbaishan deposit, located in the Beishan orogen, NW China, is hosted by the Sanjiashan shear zone. A detailed field-based structural analysis revealed that the Sanjiashan shear zone has thrust and sinistral strike-slip characteristics, such as asymmetrical folds and S-C fabrics. The muscovite 40Ar/39Ar age from auriferous quartz shows that the Qingbaishan gold system formed around 399.7 ± 1.6 Ma, consistent with the time of Sanjiashan sinistral ductile shearing in the Beishan orogen. The carbon and oxygen isotope compositions of auriferous quartz suggest that the host rocks within the Precambrian Gudongjin Group would have been a significant source of ore metals and that the low δ18O values in quartz resulted from fluid reactions with 18O-poor host rocks. The Qingbaishan Au deposit is concluded to be a strike-slip-dominated orogenic-type system related to continental collision between the Tarim and Kazakhstan–Ili plates.By integrating the age data obtained from this study with previous geochronological results, two episodes of gold mineralization in Beishan are distinguished: an early episode around 399 Ma, when sinistral strike-slip zone-related orogenic gold deposits formed in a syncollisional setting, and a late episode around 298–231 Ma, when dextral strike-slip deformation produced deposits in a postcollisional extensional setting.

Late Paleoproterozoic magmatism in North Hengshan: Final collapse of the Trans-North China Orogen

Post-collisional magmatism carries important information for understanding when and how an ancient continental collisional orogen unrooted and collapsed. However, the post-collisional magmatic response to the final collapse of the Trans-North China Orogen, one of the 2.1–1.8 Ga collisional orogens that formed in coincidence with the assembly of the Paleo–Mesoproterozoic Columbia supercontinent, remains controversial. Here we present the results of a combined geochronological and geochemical investigation on a suite of Late Paleoproterozoic potassic granites in the North Hengshan terrane of the Trans-North China Orogen, the North China Craton. In situ zircon U-Pb dating shows that these potassic granites, as the last phase of Precambrian magmatism in this terrane, were emplaced between 1790 Ma and 1774 Ma. Their enriched bulk-rock Nd and zircon Hf isotopic compositions point to an ancient crustal source, and their zircon O isotopic variation indicates the involvement of weathered supracrustal rocks in their sources. Considering their bulk-rock major and trace elements, the Late Paleoproterozoic potassic granites in North Hengshan are best explained to be produced by melting of immature metagraywacke that belonged to the Neoarchean–Early Paleoproterozoic sedimentary sequences, at middle–lower crustal depths. The 1.79–1.77 Ga potassic granites in North Hengshan postdated the main phase of the Late Paleoproterozoic collisional orogeny, and were a magmatic response to the final collapse of the Trans-North China Orogen. These 1.79–1.77 Ga potassic granites, along with other 1.80–1.75 Ga magmatism in the Trans-North China Orogen, were most likely generated in a post-collision extensional setting, possibly superimposed by an Andean-type continental arc at its southern margin. The 1.80–1.75 Ga magmatism marked the end of collision between the Western and Eastern Blocks that gave rise to the final consolidation of the North China Craton and the assembly of the Columbia supercontinent, and recorded the outgrowth of this supercontinent through accretion at its continental margins.

Ridge subduction and episodes of crustal growth in accretionary belts: Evidence from late Paleozoic felsic igneous rocks in the southeastern Central Asian Orogenic Belt, Inner Mongolia, China

Abstract The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens on Earth and preserves evidence that more than 50% of this orogen represents juvenile crustal growth over an extended period of some 750 million years from ca. 1000 Ma to ca. 250 Ma. However, the mechanism of crustal growth is controversial, as implied by a variety of proposed models ranging from contributions of mantle-derived basaltic underplating in a post-collisional extensional setting to subduction-related processes in an island arc setting. To distinguish among these models, we report petrological, geochemical, whole-rock Sr-Nd isotope and zircon U-Pb and Hf isotope analyses of late Paleozoic felsic igneous rocks from the northern Inner Mongolia region, southeastern CAOB. New zircon U-Pb analyses of three plutonic and extrusive magmatic suites yield Late Carboniferous to Early Permian ages of 319–279 Ma. The Xi Ujimqin granodiorites have low-K tholeiitic to calc-alkaline, metaluminous to weakly peraluminous compositions, and are magnesian I-type granitoids. These granitoids are also characterized by relatively high MgO and Sr contents, high Mg# values, mid-ocean ridge basalt (MORB)-like Nd-Hf isotope compositions and young Nd-Hf model ages of 600–298 Ma. These features indicate that the parental magmas of the Xi Ujimqin I-type granitoids originated from a depleted lithospheric mantle that had been metasomatized by fluids released from a subducting slab. By contrast, the Xilinhot alkali-feldspar granites and Dashizhai rhyolites display geochemical signatures of ferroan granites. These, together with their moderately depleted Nd-Hf isotope compositions and young Nd-Hf model ages of 809–277 Ma, suggest that they were produced by re-melting of juvenile lower crust via underplating of mantle-derived magmas. The coexistence of an association of boninite-adakite-high-Mg andesite-Nb-enriched basalt, and MORB to ocean island basalt-type mafic rocks in the northern Inner Mongolia region implies a dramatic change in composition from fluid-related calc-alkaline arc magmatism to melt-related and mantle-derived magmatism initiated by upwelling asthenosphere. We infer that ridge subduction may have occurred in this region during Late Carboniferous to Early Permian. Combined with previous studies, there are two ridge subduction events during the early and late Paleozoic in Inner Mongolia within the southeastern CAOB, which coincide with two large-volume magmatic flare-ups (300 ± 20 and 450 ± 20 Ma). Furthermore, our model calculations suggest that most of the juvenile crust in the southeastern CAOB was generated in a relatively short interval (~20–40 m.y.) during each long-lived tectonic cycle (>140 m.y. spanning oceanic rifting, subduction, and collision) of magmatic activity. Ridge subduction may have played a significant role in the rhythmic growth of juvenile crust in the CAOB during the Phanerozoic. Repeated cycles of punctuated, rapid growth of juvenile crust associated with long-lived subduction systems represent a pattern that may be generalized to other Phanerozoic accretionary belts on Earth.

Genetic link between the Late Mesozoic granitic magmatism and W mineralization in NE China: Constraints from in-situ U-Pb geochronology and geochemistry of wolframite, and whole-rocks geochemistry analyses of W-bearing granites from the Sansheng W-Mo deposit

The Sansheng quartz-vein type W-Mo deposit is located on the southern margin of NE China. Tungsten mineralization occurs mainly in veins and veinlets within the Early Cretaceous granitic pluton. In-situ U-Pb dating of hydrothermal wolframite yields a W mineralization age of 137.3 ± 2.0 Ma (1σ, MSWD = 4.4), which provides evidence for an Early Cretaceous W mineralization event in the Sansheng area. W-bearing granites show petrological, mineralogical, and geochemical characteristics of highly fractionated A-type granites. The rocks belong to the high-K calc-alkaline series and are characterized by the enrichment in large ion lithophile elements (e.g., Rb, Th, U, and K) and significant negative Eu, Ba, Sr, P, and Ti anomalies. They also have higher W concentrations, Rb/Sr and U/Th ratios, and lower (La/Yb)N, LREE/HREE, Eu/Eu*, K/Rb, Zr/Hf, Nb/Ta, and Y/Ho ratios than the contemporary W-barren granitoids in NE China. The geochemical signatures of the W-bearing granites indicate that the primary magmas were derived from partial melting of the lower crust and subsequently underwent extensive fractional crystallization. The W mineralization in the Sansheng is genetically related to the W-bearing granites, and extreme fractional crystallization was critical for W enrichment in granitic magma. The trace element compositions of the wolframite show similar enrichments in HREEs, Nb, Ta, and U, as well as depletion in LREEs, Rb, Ba, Th, Pb, Sr, and Ti, compared to those of the upper continental crust. We demonstrate that isovalent substitution mechanisms could have played critical roles in the formation of hydrothermal wolframite in the Sansheng deposit, and the chemical composition of wolframite is controlled by both the crystallochemical parameters and composition of the primary hydrothermal fluids. In the context of the regional geology, the new data suggest that the Sansheng W-bearing granites and corresponding mineralization formed in a post-collisional extensional setting controlled by the closure of the Mongol-Okhotsk Ocean and the rollback of the Paleo-Pacific Ocean plate during the Early Cretaceous.

Zircon Th/U ratios suggest a post-collision extensional setting for the Permian Ni-Cu sulfide deposits in the Eastern Tianshan, NW China

Magmatic Ni-Cu sulfide deposits have been discovered in orogenic belts worldwide. However, the optimal period for extensive basaltic magmatism generating Ni-Cu sulfide mineralization during the development of orogenic belts has not been well addressed. Zircon Th/U may be correlated with end-member tectonic stress regimes (continental extension vs. compression) that correlate with distinct magmatic conditions. Here, zircon Th/U ratios from the Permian Huangshan-Jingerquan Ni-Cu metallogenic belt in the Eastern Tianshan are revisited, including magmatic zircons from mafic and felsic rocks and detrital zircons from sedimentary rocks. These zircons are filtered to minimize the effect of amagmatic conditions (e.g., metamorphism, alteration, inclusion and metamictization). After filtration, zircon Th/U ratios in the Huangshan-Jingerquan belt are likely linked to the silica content of the host melt and crystallization temperature under zircon growth with fractionating melts. They display high Th/U ratios (up to 2.5) and δEu values (up to 1.35) during the early Permian. In contrast, the others with low Th/U (most < 1.0) ratios and δEu (<0.8) appear in the pre-Permian except the late Devonian. This indicates that the crust in the region was thickened induced by collision at the end of late Carboniferous and then extended during the early Permian. Conclusively, we suggest that the post-collision extensional setting is the preferred stage for the Permian short-lived (<5 Ma) Ni-Cu sulfide deposits during the development of the Tianshan Orogenic Belt.

Bastnäsite U-Th-Pb age, sulfur isotope and trace elements of the Huangshui’an deposit: Implications for carbonatite-hosted Mo-Pb-REE mineralization in the Qinling Orogenic Belt, China

The Huangshui’an deposit is hosted in carbonatite dykes and located in the southern margin of the North China Craton which forms part of the Qinling Orogenic Belt. The genesis of Mo-Pb-REE mineralization associated with carbonatitic magmatism and hydrothermal activity remain poorly understood. Here we present results from field investigation, petrography, in situ sulfur isotope and trace elements of pyrite, and bastnäsite U-Th-Pb age with a view to understand the age and genesis of the Huangshui’an deposit. Three types of pyrite are identified in the Huangshui’an deposit as follows: (i) Py1a is composed of irregular pyrite in stage Ia carbonatite dykes; (ii) the euhedral Py1b coexists with sphalerite and galena in the margin of stage Ia carbonatite dykes; and (iii) Py2 occurs as irregular and anhedral grains in the fluorite + quartz veins. The highly variable Co (Py1a = 0.032–3230 ppm; Py1b = 0.053–779 ppm; Py2 = 0.98–164 ppm) and Ni values (Py1a = 2.1–418 ppm; Py1b = 0.78–28.8 ppm; Py2 = 8.6–373 ppm) indicate heterogeneous material sources. The δ34S values of the various pyrite types range between −9.6‰ and −3.5‰, and the obviously negative values resulted from sulfur isotope fractionation between sulfide and sulfate minerals. The total δ34S value calculated from co-genetic galena and barite is −0.11‰, in combination with previous Sr-Nd-Pb-He-Ar isotopes, indicate a material source dominantly from enriched mantle (EMI) with an additional contribution through partial melting of the thickened lower crust. In situ bastnäsite U-Th-Pb dating results yield mean ages of 211.7 ± 3.1 Ma and 209.6 ± 2.1 Ma, representing the timing of Mo-Pb-REE mineralization in the Huangshui’an deposit. The Late Triassic carbonatite magmatism and associated mineralization were controlled by the fertile source from the enriched mantle and the thickened lower crust, and the post-collisional extension setting facilitated the partial melting and fast propagation of carbonatite magma from the mantle to shallow crustal level.

Trace element geochemistry of Bitlis ignimbrites sourced by Quaternary Nemrut Volcano: inferences for A2-type magma generation in Eastern Anatolia post-collisional extensional setting

In the study presented here, we investigated the K-Ar age determination and petrological significance of trace element compositions of Bitlis (Eastern Anatolia) ignimbrites associated with explosive activities of the well-known Nemrut volcano. K-Ar geochronological dating of Bitlis ignimbrite flow deposits gave the age of 790 ka which corresponds to the first products of pre-caldera stages of Nemrut stratovolcano. Bitlis trachydacitic ignimbrites with shoshonitic affinity are characterized by the typical negative anomalies of Eu, Nb-Ta, and Ti in chondrite-normalized spider and multi-element variation diagrams, and plot in the fields of within-plate and post-collisional tectonic setting in Rb vs Y+Nb discrimination diagram. Accordingly, similar to that of Nemrut volcanic products, trace element signatures (e.g., high Nb, Ce, Y, and Zr contents) of Bitlis ignimbrites show coincidence with geochemical characteristics of A2-type silicic magmas. Relative to the upper crust, higher La/Ba and Nb/La, and lower Ba/Ta and La/Ta ratios of Bitlis ignimbrites, which are close to that of Nemrut basalts, suggest an interaction between OIB-like basaltic magma and upper crust. Our results point to A2-type silicic magma generation in Eastern Anatolia post-collisional extensional setting, and Nemrut volcanic and pyroclastic rocks (e.g., Bitlis ignimbrites) formed by partial melting of upper crustal rocks at shallow level via interaction with OIB-like basaltic magma derived from a common mantle source.

Campanian transformation from post-collisional to intraplate tectonic regime: Evidence from ferroan granites in the Southern Qiangtang, central Tibet

Ferroan granitoids are typically generated in the extensional tectonic settings, and thus record important geodynamic processes related to crustal and mantle evolution. However, their petrogenesis remains controversial. In this study, we investigated newly identified ferroan granites in the Lake Cona region of the southern Qiangtang subterrane (SQT), central Tibet. We present zircon U–Pb age and Hf–O isotope, whole-rock major and trace element and Sr–Nd isotope, and mineral major element data for these rocks. The Lake Cona granites consist of biotite monzogranite and granite porphyry. Zircon U–Pb dating indicates these rocks were generated in the late Late Cretaceous (ca. 75 Ma). The biotite monzogranites are metaluminous to weakly peraluminous, and characterized by relatively high SiO2 (67.1–68.6 wt%), total alkali (10.4–10.8 wt%), Th (31–46 ppm), light rare earth element (LREE), and Zr + Nb + Ce + Y (421–730 ppm) contents, and high Fe-numbers (0.83–0.91; FeOt/[FeOt + MgO]). These rocks can be classified as alkalic ferroan granites. However, compared with the typical ferroan granitoids generated under relatively anhydrous conditions, the biotite monzogranites have higher Al2O3 (15.9–17.0 wt%) contents, lower 10000 × Ga/Al (2.1–2.5) ratios, and small negative Eu anomalies, suggesting that they were generated by brine-assisted (a fluid with low water activity) crustal anatexis. The biotite monzogranites have (87Sr/86Sr)i = 0.7065–0.7069, εNd(t) = −2.3 to −1.9, δ18O = 6.0–6.6‰, and εHf(t) = +2.1 to +9.3, indicative of a magma source that was a mixture of juvenile and ancient crustal components. The granite porphyries also have high contents of total alkalis (9.9–10.2 wt%) and Fe-numbers (0.87–0.90), and similar Sr–Nd–Hf–O isotopic compositions as the biotite monzogranites, but have higher SiO2 and lower TiO2, MgO, TFe2O3, P2O5, CaO, and REE contents, and larger negative Eu anomalies. These features suggest that the granite porphyries represent the highly evolved biotite monzogranitic magmas that underwent differentiation in the shallow crust. The Lake Cona ferroan granites formed in a post-collisional extensional setting that had additional heat input from the deep mantle. Unlike the early Late Cretaceous that was dominated by calc-alkaline magmatism in the SQT, the occurrence of alkali ferroan granites in the late Late Cretaceous may record the transformation from a post-collisional to an intraplate tectonic regime in the central Tibet.

Petrogenesis and tectonic setting of Late Devonian high-Mg andesites in the Beishan orogen, NW China

AbstractHigh-Mg andesites (HMAs) are crucial for the reconstruction of plate tectonics, continental margin formation and lithospheric evolution. In this study, we present new fossil age, whole-rock geochemical and Sr–Nd isotope data on the newly discovered Dundunshan Group HMAs in the Dundunshan area of the Beishan orogen (central-southern Central Asian Orogenic Belt). The Dundunshan HMA samples are characterized by high MgO (6.47–7.02 wt%) contents and high Mg# values (67.27–68.77), with SiO2 (58.57–62.13 wt%), Al2O3 (14.49–16.07 wt%) and CaO (5.05–6.24 wt%) resembling typical HMAs. The Dundunshan HMA samples are calc-alkaline and strongly enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), with slightly negative Eu anomaly and high-field-strength element (HFSE) depletions. Their (87Sr/86Sr)i ratios (0.7041–0.7057) and ϵNd(t) (3.73–5.59) indicate that the Dundunshan HMAs were mainly formed by the interactions between subducted oceanic sediment-derived melts and mantle peridotites. Fossil evidence and published radiometric age data constrain the formation of the Dundunshan HMAs to early Late Devonian time. Sedimentological features of the Middle Devonian Sangejing Formation and regional tectonic correlation suggest that the Hongliuhe–Niujuanzi–Xichangjing Ocean in the Dundunshan area was likely closed during late Middle Devonian time, and that the Dundunshan HMAs were formed in a post-collision extensional setting.