Transition Of Tectonic Regime Research Articles

Jurassic tectonic regime transition of the southwestern Yangtze Block, SW China: evidence from sedimentary provenance analysis

The Lufeng Basin (LFB) in central Yunnan is located on the southwestern margin of the Yangtze Block in the eastern segment of the Tethys tectonic domain, where the Paleo-Tethys and the Paleo-Pacific tectonic domains converge. Jurassic red beds in this basin record the closure of the Paleo-Tethys Ocean and key information of the tectonic transition driven by the spreading of the Paleo-Pacific plate. We present sedimentary provenance analysis on these Jurassic red beds based on heavy mineral assemblages and detrital zircon geochronology. The results indicate that the strata in this basin were deposited after 214 Ma, and the Early Jurassic Lufeng Formation (Lufeng Fm.) mainly sourced from the west of the LFB with minor contributions from the western Yunnan. In contrast, the sedimentary provenance of the Middle Jurassic Chuanjie Formation (Chuanjie Fm.) and Laoluocun Formation (Laoluocun Fm.) shifted to the proximal ancient basement surrounding the basin and the east of the LFB. The regional unconformity between the Early and the Middle Jurassic may have resulted from the subduction of the Paleo-Pacific plate, jointly influenced by two tectonic domains. The sedimentary provenance shift in the Middle Jurassic may be related to the local uplift of the southwestern Yangtze Block caused by the subduction of the Paleo-Pacific plate. The tectonic regime transition associated with this highest peak of local uplift occurred at ∼170 Ma.

Neoproterozoic Buchan-type metamorphism of the northwestern margin of the Yangtze Craton and tectonothermal implications

The tectono-metamorphic mechanism along the margins of the Yangtze Craton in the Neoproterozoic has been a controversy for a long time. The Micangshan terrane is located at the northwestern margin of the Yangtze Craton and is mainly composed of Neoproterozoic metamorphosed volcanic and sedimentary sequences. In this work, two representative garnet-bearing two-mica schist samples and two representative garnet-bearing biotite-plagioclase gneiss samples were performed detailed petrological and geochronological research. Three generations of mineral assemblages include the prograde (M1), peak (M2) and retrograde (M3) stages, respectively. Conventional geothermobarometry and phase equilibria modeling define the respective P–T conditions to be 2.0–3.0 kbar/596–686 °C for the M1, 2.6–4.8 kbar/632–693 °C for the M2 and 1.3–2.7 kbar/500–550 °C for the M3 stages and thus anticlockwise P–T paths were retrieved. The regional peak Buchan-type metamorphism with a higher geothermal gradient of ca. 40–75 °C/km was firstly identified in the northwestern margin of the Yangtze Craton. Secondary ion mass spectrometry (SIMS) U-Pb dating of magmatic zircon yielded the formation ages of the protolith of the rocks to be 838–863 Ma. SIMS U-Pb dating of metamorphic zircon and monazite LA-ICP-MS U-Pb dating yielded the metamorphic ages to be 820–794 Ma and 810–761 Ma, respectively, for the prograde (M1) to peak (M2) stage. Biotite 40Ar/39Ar dating yielded similar plateau ages and normal isochron ages of 750–745 Ma and are interpreted to represent the retrograde (M3) stage. Combined with previous studies, we propose that there was a transition of tectonic regime from compressional continental arc environment to extensional back-arc environment for the Micangshan terrane, which may be related to the southwards subduction of the South Qinling belt beneath the NW margin of the Yangtze Craton in the Neoproterozoic.

Petrogenesis and Tectonic Implications of the Qulihai Pluton in the Northern Margin of the Yili Block, NW China

Abstract Despite extensive research work that has been done, whether the northern margin of the Yili Block (YB) is still an active continental margin during the early Carboniferous period is still in debate. Herein, we conducted zircon U–Pb geochronology, geochemistry, and zircon Lu–Hf isotope studies on the Qulihai pluton in the northern margin of the YB to constrain the petrogenesis and dynamic process. The Qulihai pluton is a granitoid complex that is composed of quartz diorite, quartz monzonite, and syenogranite. The zircon U–Pb dating results revealed that the pluton was formed and emplaced between 346 and 342 Ma. The three different Qulihai pluton rock types had comparable εHf(t) values, ranging from +3 to +8. The corresponding two-stage model ages of 817–1182 Ma indicated their potential derivation from the Meso-Neoproterozoic juvenile crust. The Qulihai pluton typically features medium-to-high SiO2, K2O, and Al2O3 contents and low MgO and Fe2O3T contents. The quartz diorite and quartz monzonite samples had Mg# values of 43–47, indicating the input of mantle-derived melts in the source area. In contrast, the source region of syenogranite was purely crustal material. The Qulihai pluton is mainly characterized as strong metaluminous moderate-to-high-K cal-alkaline rocks of the I-type granite series, which are enriched in large-ion lithophile elements and depleted in high field strength elements while exhibiting active continental margin island arc magmatism. Our findings, combined with the comprehensive analysis of previous studies, suggest that the Qulihai pluton formation resulted from the subduction of the North Tianshan oceanic crust beneath the YB during the early Carboniferous period, contemporary with the tectonic regime transition from subduction advance to subduction retreat.

Late Neoarchean and Paleoproterozoic tectonothermal and metamorphic evolution recorded in high-pressure granulites: Evidence from the Xuanhua Complex, northern North China Craton

Archean cratons usually underwent reworked processes during the late Paleoproterozoic orogeny, which may result in multiple tectonothermal records preserved in some high-grade metamorphic rocks; thus, these rocks provide a key opportunity on understanding the tectonic styles during different periods. In this study, we report new petrological, mineral chemical and geochronological data of a suite of newly discovered mafic and felsic metamorphic rocks in the Xuanhua Complex from the northern North China Craton (NCC), revealing the complex metamorphic thermal history and the transition of tectonic regime from the late Neoarchean to the late Paleoproterozoic. The mafic granulite preserves the prograde quartz and clinopyroxene inclusions within garnets (M1), and the peak stage (M2) is recorded by garnet, clinopyroxene, hornblende, plagioclase, quartz and ilmenite. The first retrograde (M3a) and final cooling stages (M3b) are defined by the appearance of orthopyroxene, and the amphibolite-facies overprinting, respectively. The felsic granulite records the similar peak mineral assemblage (M2), but has higher volumes of feldspar and quartz than the mafic granulite. The retrograde stage (M3) is represented by amphibolite-facies assemblage (hornblende ± biotite ± clinopyroxene) and fine-grained intergrowth of hornblende + plagioclase around garnet. Pseudosection modeling and thermobarometry results suggest that both high-pressure (HP) mafic and felsic granulites record clockwise P–T paths, including isothermal decompression and cooling from the peak stage (M2: 10.4–10.8 kbar/830–860 °C and 9.6–11.2 kbar/850–930 °C, respectively) to the retrograde stages (M3a and M3b: 7.5–8.0 kbar/840–870 °C and 7.8–8.2 kbar/650–700 °C, respectively; M3: 7.2–8.6 kbar/670–800 °C). Zircon U–Pb dating shows that the magmatic precursors of these rocks were emplaced at c. 2.5–2.47 Ga. The metamorphic zircons reveal two groups metamorphic ages of c. 2.51–2.45 Ga and c. 1.90–1.85 Ga. The c. 2.51–2.45 Ga ages indicate a late Neoarchean tectonothermal event, and the c. 1.95–1.85 Ga ages are interpreted to represent the Paleoproterozoic HP granulite-facies metamorphism overprinting. The highest positive εHf(t) value of 7.64 and 2.8–2.6 Ga Hf model ages of c. 2.5 Ga magmatic zircons indicate that the c. 2.5 Ga mantle-derived mafic magma and the c. 2.5 Ga partial melting of 2.8–2.6 Ga basaltic juvenile crust generated the protoliths of mafic and felsic granulites, respectively, revealing 2.8–2.6 Ga and 2.5 Ga crustal growth and c. 2.5 Ga crustal reworking in the northern NCC.To sum up, we consider that the late Neoarchean magmatism and coeval metamorphism recorded in our samples are probably resulted from the amalgamation of micro-blocks. The Paleoproterozoic clockwise P–T–t paths with near-isothermal decompression segments and low geothermal gradients (c. 21–26 °C/km) of HP mafic and felsic granulites indicate a rapid post-collisional exhumation process after the significantly crustal thickening, and this long-term orogeny along the TNCO (c. 1.95–1.85 Ga; > 100 Myr) is related to the incorporation of the NCC into the Columbia supercontinent.

Three episodes of Triassic volcanism in the Eastern Kunlun Orogen, NW China: constraints for evolution of the Palaeo-Tethys Ocean

ABSTRACT The Eastern Kunlun Orogenic Belt (EKOB), located north of the Qinghai-Tibetan Plateau, recorded voluminous late Palaeozoic – early Mesozoic intermediate-acid volcanic rocks. The tectonic evolution of the Palaeo-Tethys Ocean from initial subduction and final closure and post-collision extension has been hotly debated. This study examines the petrogenesis of Palaeozoic to Early Mesozoic volcanic rocks at six locations: Xiahe, Elashankou, Jirimai, Harizha, Nagengkangqieer, and Yazigounan. The investigation involves detailed field observations, petrology, zircon U-Pb geochronology, Lu-Hf isotopes, and whole-rock geochemical analysis. The rocks analysed represent three episodic volcanic events. The ca. 257–245 Ma volcanic rocks were derived from magma mixing response to the subduction. The ca. 227–220 Ma highly fractionated I-type volcanics were derived from partial melting of the ancient lower crust, with varying contribution proportions of mantle materials. The ca. 216 Ma A-type volcanic rocks derived from partial melting of the juvenile crust. In this study, using existing regional geological data and spatiotemporal distribution of the magmatic region, combined with our new data, we attempt to propose more rational models to reveal the tectonic evolution of the Palaeo-Tethys Ocean, and we recognize three significant time nodes: (1) Ca. 240 Ma marks the final closure time of the Palaeo-Tethys Ocean. (2) Ca. 230 Ma indicates the vital tectonic regime transition from compression to extension response to slab break-off and continued lithospheric delamination. (3) Ca. 216 Ma mark the beginning of the final stage of orogeny in the EKOB, and mild magmatism lasts to ca. 200 Ma in response to the mountain collapse.

Emplacement and evolution of zoned plutons: Multiproxy isotopic and geochemical evidence from the peraluminous Laojunshan leucogranite suite, southwestern China, and implications on the regional geodynamic and metallogenic history

The petrogenesis and geodynamic setting of peraluminous leucogranites are important topics for reconstructing regional geodynamic and metallogenic models. Here we present the results of an integrated study of zircon U-Pb ages and Hf-O isotopic compositions, monazite U-Th-Pb-Nd isotopes, tourmaline boron isotope, and bulk-rock elemental and Sr-Nd isotope geochemistry of an unusual suite of leucogranite, the Laojunshan leucogranite (LJSL), from the southwestern margin of the Yangtze Block in southwest China. The LJSL samples are strongly peraluminous with their zircon δ18O (8.8–10.2 ‰) and tourmaline δ11B (−14.32 to −12.75 ‰) values suggesting S-type origin. The LJSL rocks show high SiO2, Al2O3, K2O and Rb/Sr, low P2O5, CaO, FeOt, MgO, low REE abundances with moderate LREE enrichment and strong negative Eu anomalies, high (87Sr/86Sr)i (0.7127–0.7421) and low εNd(t) (−10.36 to −12.67). These features suggest protracted fractional crystallization from S-type parental granitoid magmas with Ti-in-zircon thermometry giving a mean temperature of ∼760 °C. The bulk-rock Sr-Nd, monazite Nd and zircon Hf-O isotope data suggest that the S-type magmas parental to the LJSL formed through partial melting involving muscovite-breakdown in the Paleoproterozoic middle-upper crustal metasedimentary rocks. Zircon and monazite U-(Th)-Pb dating reveals three intrusive phases for the LJSL (91–89 Ma, 87–85 Ma and 83–82 Ma), correlating with the compression-to-extension tectonic regime transition in response to the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the Neotethys oceanic lithosphere. Our findings contribute to the understanding of Mesozoic plate reconstruction associated with the India-Asia collision in the Cenozoic.

Anatexis and resultant magmatism of the Ama Drime Massif: Implications for Himalayan mid-Miocene tectonic regime transition

In the middle Miocene, the Himalayan Orogen underwent a remarkable tectonic regime transition from a north-south extension to an east-west extension, but the underlying mechanisms of the Himalayan mid-Miocene tectonic regime transition are still debated. In this study, we conducted an integrated petrological, geochronological, and geochemical study on the high-grade metamorphic rocks and leucogranites from the Ama Drime Massif (ADM) to detect the anatexis and magmatism associated with the east-west extension. According to geochronological results, the structurally higher ADM paragneisses and the structurally lower ADM orthogneisses underwent partial melting in 22–12.5 Ma and 14–13 Ma, respectively, accompanied by the formation of 12 Ma tourmaline-muscovite leucogranites and 12–11 Ma two-mica leucogranites. By comparison, the tourmaline-muscovite leucogranites and the ADM paragneisses have compatative protolith ages and Sr-Nd isotopic compositions. Likewise, the two-mica leucogranites and the ADM orthogneisses are comparable in protolith ages and Sr-Nd isotopic compositions. For whole-rock geochemistry, the two-mica leucogranites are characterized by higher TiO 2 , TFeO, MgO, CaO, Sr, Ba, LREE, Th, Zr/Hf, but lower Rb, Rb/Sr, and weak Eu anomalies compared to the tourmaline-muscovite leucogranites. Based on the spatial, temporal, and protolith relationships between the gneisses and leucogranites in the ADM, the tourmaline-muscovite leucogranites originated from muscovite dehydration melting of the paragneisses and then experienced strong fractional crystallization, while the two-mica leucogranites resulted from biotite dehydration melting of the orthogneisses. Anatexis and resultant magmatism in the ADM reveal that the mid-Miocene tectonic regime transition changed not only the style of the Himalayan extensional structures but also the crustal thermal structure of the Himalayan Orogen. In this contribution, a potential lateral delamination model is proposed to accommodate the heating and coeval rapid east-west extension in the ADM region and to explain the Himalayan mid-Miocene tectonic regime transition- • The ADM underwent various anatexis and magmatism during 14–11 Ma. • The anatexis between the paragneisses and orthogneisses was decoupled in the ADM and produced various leucogranites. • The Himalayan east-west extension was accompanied by the change of crustal thermal state. • A new model of lateral delamination is proposed to explain the Himalayan mid-Miocene tectonic regime transition.

Tectonic regime transition of the western South China Block in early Cambrian: Evidence from the Meishucun volcanic ash beds

The tectonic evolution of the South China Block, especially its western part, is ambiguous in the early Paleozoic owing to the lack of contemporary tectono-magmatic records. Thick ash beds preserved in the early Cambrian successions in the eastern Yunnan Province of the western South China Block provide new insights into their magmatic origins and tectonic evolution. We report here integrated in-situ zircon U-Pb data, Hf-O isotopes, and whole-rock geochemistry of two early Cambrian ash beds (Bed 5 and Bed 9) from the Meishucun section in the eastern Yunnan Province. SIMS zircon U-Pb data indicate the two ash layers were deposited at 533.2 ± 3.8 Ma and 526.2 ± 4.1 Ma, respectively. Zircon Hf-O isotopes and whole-rock geochemistry of the two ash beds demonstrate crustal origins of their parental magma and their close affinities to A-type granites. These features suggest that the ash beds were formed in extensional settings and the tectonic regime of the study area switched from compression to extension in the early Cambrian. The Meishucun volcanic records, in combination with other late Ediacaran–early Paleozoic magmatism along the northern margin of East Gondwana, may represent the initial opening phase of the Paleo-Tethys Ocean.

Late Indosinian granitoids in the Zhacanggou, Guide Basin, NW China: Whole‐rock geochemistry, zircon U–Pb geochronology, Lu–Hf isotopes, and implications for magma mixing

The Guide Basin, located in the West Qinling orogen of central China, is a typical geothermal field. However, the granodiorites occurring in the Zhacanggou area have been poorly restricted. Herein, we report new isotopic compositions of whole‐rock major‐trace elements, zircon U–Pb, and Lu–Hf, on the host granodiorites and their microgranular enclaves (MEs) in the Zhacanggou area. Zircon U–Pb dating for the host rocks (porphyritic granodiorite and granodiorite) revealed ages of 230.5 ± 1.4 Ma and 229.0 ± 1.6 Ma, similar to those (230.5 ± 1.5 Ma and 231.4 ± 1.4 Ma) of the MEs. Zircon Hf isotope analyses for the granodiorite and MEs yielded εHf(t) values from −11.0 to −1.5 and from −7.8 to −0.7, with corresponding TDM2 ages of 1,355–1,959 and 1,304–1,755, respectively. The MEs possess plagioclase phenocrysts, acicular apatites, and chilled borders. They were characterized by relatively low SiO2 (58.39–65.24 wt%) and high Cr, Ni, and MgO contents, leading to relatively high Mg# values (44–59) and low Sr/Y (14.23–19.84) ratios. The host granodioritic rocks show similar adakitic geochemical characteristics with SiO2 = 64.23–68.57 wt%, MgO = 1.29–2.22 wt%, Mg# = 48–53, Sr = 340–596 ppm, Y = 11.3–13.7 ppm, Sr/Y = 28.39–47.89, and no obvious Eu anomalies. These geochemical and isotopic data, combined with the mineralogical features, indicate that the granodiorites in the Zhacanggou area were produced by the mixing of high‐Mg# melts originating from an enriched lithospheric mantle source, and felsic magmas deriving by the partial melting of the normal lower continental crust with the tectonic regime transition from a syn‐collision to a post‐collision setting.

Siderian mafic-intermediate magmatism in the SW Yangtze Block, South China: Implications for global ‘tectono-magmatic lull’ during the early Paleoproterozoic

It is debated whether global-scale tectono-magmatic quiescence prevails on the Earth during the early Paleoproterozoic. The early Paleoproterozoic meta-mafic and intermediate rocks in the southwestern Yangtze Block of South China could provide an opportunity to constrain not only the tectonic evolution of the Yangtze Block itself but also augment our understanding of the global tectono-magmatic lull during the early Paleoproterozoic. Zircon UPb geochronological study indicates that the meta-mafic rocks were emplaced at ca. 2.32 Ga, whereas the intermediate rocks were emplaced at ca. 2.35 Ga. These two lithologies are characterized by subduction-related whole-rock geochemical (such as depleted Nb, Ta and enriched Rb, K, Pb) and unradiogenic NdHf isotopic signatures (εHf(t) < 0, εNd(t) < 0), implying a derivation from a geochemically enriched lithospheric mantle modified by ancient subduction. Thermodynamic modeling further suggests that the intermediate rocks are products of the AFC process between mantle-derived basaltic magma and TTG-like basement rocks at shallow crustal levels. We interpret this as occurring within a post-orogenic extensional setting. Integrated with the published work, we further posit that the Yangtze Block experienced a tectonic regime transition from compressive orogenesis to post-orogenic extension during the early Paleoproterozoic, and the magmatic activities in several crustal blocks are still active until ~2.3 Ga and therefore places important constraints on the beginning of the tectono-magmatic lull.

Early Paleozoic tectono–magmatic evolution in the South Altun orogenic belt, northwest China: Insights from zircon U–Pb geochronology, Hf isotope and geochemistry of the granitoids

ABSTRACT Precise timing of granitoids and constraints of their magma sources are critical to understanding the subduction–collision tectonic evolution of the Altun orogenic belt (AOB), a vital part of the Qinghai-Tibet plateau in northwestern China. We provide in-situ zircon U–Pb age, Hf isotopic composition and whole-rock geochemistry of the Yusupualeke granitic pluton in the South Altun orogenic belt (SAOB), an integral unit of the AOB, to determine the tectono-magmatic evolution of the SAOB at early Palaeozoic. The Yusupualeke granitic pluton comprises the medium-coarse grained porphyritic monzogranite and medium-fine grained granodiorite. The monzogranite sample yielded a weighted mean 206Pb/238U age of 476.8 ± 3.6 Ma (MSWD = 0.59), while the granodiorite yielded a weighted mean 206Pb/238U age of 453.2 ± 4.7 Ma (MSWD = 0.013). Zircon U–Pb ages suggest that the subduction–collision process of the SAOB remained from the middle stage of the early Palaeozoic to the end. Both of the studied granitoids belong to metaluminous to weakly peraluminous series and show typical I-type granite characteristics with depletions in Ba, Nb, Sr, P, and Ti, and enrichments in light rare earth elements (LREEs), Rb, Th, K, and slightly negative Eu anomalies. Based on the geochronological data and regional geological background, we believe that the porphyritic monzogranites were formed during the northward subduction process of the South Altun ocean (SAO), while the granodiorites were formed during the tectonic regime transition stage. Besides, combing the previous research achievements with our newly obtained data, we put forward a new division and geodynamic model of the early Palaeozoic tectono-magmatic evolution for the South Altun orogenic belt.

Late Neoarchean to Paleoproterozoic arc magmatism in the Shandong Peninsula, North China Craton and its tectonic implications

The transition in tectonic regime of our planet during the late Archean with the initiation broadly modern style subduction system is one of the major topics of interest in earth science. The North China Craton (NCC), one of the oldest cratonic nuclei in the world, preserves important records of Archean and Paleoproterozoic crustal evolution, cratonization and related geodynamic history that are important in understanding the continental growth of the early Earth. Here we investigate a suite of Archean to Paleoproterozoic rocks including hornblendite, amphibolite, gabbro, diorite, basalt, plagiogranite, granite porphyry and greenschist from Shandong Peninsula within the southwestern Jiaoliao block of the eastern NCC using zircon U-Pb geochronology, Lu-Hf isotopes, and whole-rock geochemistry. We aim to understand the timing and petrogenesis of the rock suite and its implications on the geodynamic history of the NCC. Zircon U-Pb data show 207Pb/206Pb ages in the range of 2.68–1.83 Ga, and is divided into three groups of 2.68–2.43, 2.38–1.96 and 1.95–1.83 Ga. Among these, the 2.68–2.43 Ga group confirms with the major magmatic phase during Neoarchean-Paleoproterozoic transition in the eastern NCC, whilst the other groups of 2.38–1.96 and 1.95–1.83 Ga ages might represent the Paleoproterozoic orogenic events in the NCC. Zircon Lu-Hf isotopic data display εHf(t) values ranging from −0.4 to 10.0 and TDMC of 3074–2080 Ma, which are subdivided into one group with U-Pb ages of 2672–2445 Ma characterized by εHf(t) values from −0.4 to 7.0 and TDMC of 3074–2588 Ma and another group (2134–2009 Ma) with εHf(t) values in the range of 2.8–10.0 and TDMC ranging from 2458 to 2080 Ma. These results indicate that the magmas (2672–2445 Ma group) were sourced mainly from Late Mesoarchean to Neoarchean juvenile components with minor input of reworked Mesoarchean crustal components, whereas the 2134–2009 Ma group was derived from Paleoproterozoic juvenile components. Whole-rock geochemical data of the granite porphyry and plagiogranites show typical subduction-related arc signature with the enrichment of LREEs and LILEs and depletion of HREEs and HFSEs (e.g. Nb, Ta, Ti), and mainly belong to I-type granites generated at arc-related setting. The ultramafic–mafic magmatic suite corresponds with tholeiitic to calc-alkaline series which were formed by partial melting of enriched mantle within an island arc setting in an active continental margin. In conjunction with the results from previous studies in the NCC, we correlate the 2.68–2.43 Ga ages with Late Neoarchean amalgamation of microblocks and cratonization of the NCC driven by late Neoarchean oceanic plate subduction, and the 2.38–1.83 Ga ages represent Paleoproterozoic orogenic events including the assembly of major crustal blocks in the NCC within the Paleoproterozoic supercontinent Columbia, and the final collision of these crustal blocks and cratonization.

The Geological Significance of the Deformation and Geochronology of the Xiaotian–Mozitan Shear Zone in the Dabie Orogenic Belt (East‐Central China)

AbstractThe Xiaotian–Mozitan Shear Zone (XMSZ) is the boundary of the Dabie High‐grade Metamorphic Complex (DHMC) and the North Huaiyang Tectonic Belt. It was deformed in ductile conditions with a top‐to‐NW/WNW movement. Geothermometers applied to mineral parageneses in mylonites of the shear zone give a temperature range of 623–691°C for the predeformation and 515–568°C for the syndeformation, respectively, which indicates a retrograde process of evolution. A few groups of zircon U‐Pb ages were obtained from undeformed granitic veins and different types of deformed rocks in the zone. Zircons from the felsic ultramylonites are all magmatic, producing a weighted mean 206Pb/238U age of 754 ± 8.1 Ma, which indicates the time of magmatic activities caused by rifting in the Neoproterozoic. Zircons from the granitic veins, cutting into the mylonites, are also of magmatic origin, producing a weighted mean 206Pb/238U age of 130 ± 2.5 Ma, which represents the time of regional magmatic activity in the Cretaceous. Zircons from the mylonitic gneisses are of anatectic‐metamorphic origins and are characterized by a core‐mantle interior texture, which yielded several populations of ages including the Neoproterozoic ages with a weighted mean 206Pb/238U age of 762 ± 18 Ma, similar to that of the felsic ultramylonites and the Early Cretaceous ages with a weighted mean 206Pb/238U age of 143 ± 1.8 Ma, indicating the anatectic metamorphism in the Dabie Orogenic Belt (DOB). Based on integrated analysis of the structure, thermal conditions of ductile deformation and the contact relations of the dated rocks, the activation time of the Xiaotian–Mozitan Shear Zone is constrained between ∼143 Ma and 130 Ma, during which the DOB was undergoing a transition in tectonic regime from compression to extension. Therefore, the deformation and evolution of this shear zone plays an instrumental role in fully understanding this process. This research also inclines us to the interpretation of it as an extensional detachment, with regard to the tectonic properties of the shear zone. It may also be part of a continental scale extension in the background of the North China Block's cratonic destruction, dominated by the subduction and roll‐back of the Paleo‐Pacific plate, but more detailed work is needed in order to unravel its complicated development.

Himalayan Miocene adakitic rocks, a case study of the Mayum pluton: Insights into geodynamic processes within the subducted Indian continental lithosphere and Himalayan mid-Miocene tectonic regime transition

AbstractThe Himalayan Orogen is a typical continental collision orogenic belt that formed during India-Asia collision. The orogeny involved a transition in tectonic regime, which led to E-W–trending extension during the middle Miocene. At the same time, widespread post-collisional adakitic magmatism occurred in southern Tibet, particularly in the Lhasa Terrane, and these rocks provide a valuable record of the collisional geodynamic processes within the lithosphere. Few studies have focused on the middle Miocene adakitic rocks of the Himalayan Orogen, so further research is required to constrain their origin and geodynamics. This study presents new geochronological and geochemical data, including the whole rock Sr-Nd and zircon Hf isotopic compositions from the Mayum pluton in the Mayum-Gong Tso area, Northern Himalaya. Zircon U-Pb ages show that the Mayum granodiorite porphyries crystallized at 16.67 ± 0.14 Ma (mean square weighted deviation [MSWD] = 0.82, n = 29) and 16.68 ± 0.16 Ma (MSWD = 1.13, n = 28), indicating that they formed during the middle Miocene. The major and trace element characteristics are as follows: SiO2 = 65.79–67.31 wt%, Al2O3 = 15.28–16.00 wt%, MgO = 1.77–1.89 wt%, Y = 12.0–13.5 ppm, Yb = 0.11–0.99 ppm, Sr = 719–822 ppm, (La/Yb)N = 21.89–27.02, Sr/Y = 56.1–65.5, and the rocks have weak negative Eu anomalies (Eu/Eu* = 0.76–0.85), indicating that they are adakitic rocks and show high Sr-Ba granite affinity. The Mayum granodiorite porphyries have high K2O contents (3.42–3.65 wt%), Na2O (3.76–4.04 wt%), and K2O/Na2O ratios of 0.91–0.95, indicating a high-K calc-alkaline affinity. All samples are enriched in large-ion lithophile elements and depleted in high field strength elements. The initial 87Sr/86Sr ratios are 0.709262–0.709327 and εNd(t) values are −6.36 to −7.07, which correspond to two-stage Nd model ages [TDM2(Nd)] of 1405–1348 Ma. In situ zircon Lu-Hf isotopic compositions are variable, with 176Hf/177Hf ratios of 0.2823845–0.282824, εHf(t) values of −13.37 to +2.17, and two-stage Hf model ages [TDM2(Hf)] of 1704–841 Ma. These geochemical and Sr-Nd-Hf isotopic characteristics indicate that the Mayum granodiorite porphyries may be derived from partial melting of the subducted Indian thickened ancient mafic lower crust (∼92%) mixed with depleted mid-ocean ridge basalt mantle material input (∼8%) slightly. Crust-mantle interaction was induced by asthenospheric upwelling followed by the underplating of the Himalayan lower crust beneath Southern Tibet during the middle Miocene in response to significant changes in the geodynamics of the India-Asia collisional orogen. These deep geodynamic processes reflect the break-off or rollback of the subducted Indian continental slab, which caused asthenospheric upwelling, the input of juvenile heat, and the addition of depleted mantle material. This study indicates that the middle Miocene Himalayan adakitic rocks, which include the Miocene Yardoi two-mica granite, Gyangzê granite porphyry dike, Bendui two-mica granite, Langkazi enclave, Kuday dacitic dike, Lasa pluton, and Mayum pluton, form a belt of adakitic rocks in the Northern Himalaya to the south of the Indus-Tsangpo Suture Zone. These adakitic rocks have similar temporal and spatial distributions, geochemical features, and Sr-Nd isotopic compositions, indicating that their petrogenesis and geodynamic settings were similar. At that time there was widespread initiation of N-S–trending rifting, exhumation of central Himalayan eclogites, and large changes in the compositions of Himalayan leucogranites in the Himalayan Orogen. These can be attributed to significant changes in the characteristics of the subducted Indian continental lithosphere during the middle Miocene. The middle Miocene Himalayan adakitic rocks provide valuable insights into this transition in the tectonic regime and deep geodynamic processes.

Transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean: constraints from the Jurassic adakitic and I-type granites, and calc-alkaline diorites at the northern margin of the North China Craton

ABSTRACT There is a broad consensus that subduction of the Paleo-Pacific oceanic plate has caused extensive magmatism, lithospheric deformation, and metal mineralization in the eastern margin of the Eurasian continent during the Late Mesozoic. However, the onset and extent of subduction of the Paleo-Pacific Ocean, especially the timing of the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific, and extent of the subduction of the Paleo-Pacific Ocean remain poorly constrained. To address these key issues, we present regional geological observations, zircon U–Pb dating, whole-rock geochemistry, and Hf isotopic compositions of the Jurassic intrusions at the northern margin of the North China Craton, with the aim of constraining their petrogenesis and the tectono-magmatic events related to subduction of the Paleo-Pacific Ocean. The geochronological and geochemical results indicate that these intrusions were emplaced between 177 and 155 Ma, with distinct geochemical features. The Huajianggou pluton (HJG), Tongjiatun pluton (TJT), and Haoguantun pluton (HGT) are characterized by high Al2O3 and Sr contents, and low MgO, TFe2O3, Y, Yb, and heavy rare earth elements (HREEs) contents, coupled with high Sr/Y values (34.10–94.36), showing adakitic affinities. Based on these geochemical characteristics, we suggest that these adakitic magmas (i.e. the HJG, TJT, and HGT plutons) were produced by partial melting of thickened lower crust in response to earlier subduction of the Paleo-Pacific oceanic plate beneath the northern margin of the North China Craton. In contrast, the Baijiagou pluton (BJG) and Sheshangou pluton (SSG) are high-K calc-alkaline rocks with low Sr/Y ratios (0.80–17.77), and they exhibit petrological and geochemical characteristics of I-type granites, indicating an origin from the lower crust, with variable degrees of fractional crystallization. In addition, the Bakeshu pluton (BKS) is mainly comprised of diorite that displays high SiO2, low MgO, Cr, and Ni contents, with Mg# of 39.8–42.9, indicating a lower crustal origin. Despite the range in ages of these intrusions, all the Jurassic calc-alkaline intrusions show typical geochemical characteristics of active continental margins, such as enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs). Combined with spatial-temporal variations of coeval magmatic rocks along the northern margin of the North China Craton, we suggest that the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean was initiated in the Early Jurassic. These Jurassic intrusions formed in an active continental margin setting, related to westward subduction of the Paleo-Pacific oceanic plate. Importantly, the Early Jurassic subduction-related calc-alkaline magmatic rocks in northeast (NE) China constitute a NE-trending magmatic arc belt, which is best explained as continental magmatism triggered by the westward subduction of the Paleo-Pacific oceanic plate.

Geochemical constraints on Eocene–Miocene geodynamic and magmatic evolution of the Varan-Naragh area, Urumieh-Dokhtar Magmatic Arc, Iran

Two different types of igneous rock formed during separate Cenozoic magmatic phases in the Varan-Naragh area in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA) of Iran as a part of the Alpine-Himalayan system. The first phase comprises late Eocene – early Oligocene Naragh gabbroic rocks (Ns), and the second phase is characterized by the emplacement of both volcanic and plutonic rocks of the early Miocene. Both phases display moderate enrichment of large rare earth elements and depletion of high field strength elements coupled with negative Nb, Ti, and P anomalies, indicative of subduction-related magmatic events within an active continental margin. Initial values of 87Sr/86Sr and εNdT are 0.70684 and +0.15 and 0.70560–0.70654 and +2.55 to +3.49 for Ns and early Miocene intrusive and volcanic rocks, respectively. Comparisons of rare earth element patterns and mantle-like isotopic ratios suggest that Ns mafic and early Miocene magmatic rocks were derived from partial melting of a common subcontinental lithospheric mantle. Geochemical and isotopic ratios of Ns gabbroic rocks, in combination with the data related to other coeval and proximal mafic-intermediate intrusions (such as Nashalj), suggest enrichment of the lithospheric mantle by slab-derived fluids with a minor subducted sediment melt. The low εNdT of Ns gabbroic rocks can reflect involvement of slab-derived components. The geochemical similarity and the close spatial and temporal association of Varan intrusive and volcanic rocks suggest a common petrogenetic relationship. Geochemical, isotopic, and geochronological evidence from the region indicate three major phases of igneous activity in the Kashan magmatic segment of the central UDMA during late Eocene to Miocene, resulting in complex tectonic regime transition from compressional subduction to extensional post-collisional settings. Integrated with published studies, the new results support a model suggesting that subduction-related magmatic activity was still influencing the central UDMA in the early Miocene time and are also consistent with the notion of oblique and diachronous collision along the northeast margin of the Arabia plate.

Petrogenesis of Archean TTG-series rocks from the Jiaodong complex, eastern China: Implications for crustal evolution in the North China Craton

Rocks of the tonalite-trondhjemite-granodiorite (TTG) series with associated supracrustal rocks are dominant components of many Archean terranes, but their petrogenesis remain controversial. Here, we present whole-rock major and trace elements, zircon U-Pb ages, Sm-Nd and zircon Hf isotopes of the three phases of TTG gneisses (2.9, 2.7 and 2.5 Ga) and amphibolites in the Jiaobei terrane, China, to investigate their petrogenesis and implications for crustal evolution. The 2.5 Ga TTGs are characterized by high Sr, Sr/Y and (La/Yb)N values, enrichment in LILE and depletion in HFSE, positive or negligible Eu anomalies and similarity to medium- to high-pressure (MP-HP) TTGs. In combination with their geochemical modeling results and high contents of MgO, Mg#, Cr and Ni, it is considered that the 2.5 Ga TTGs were subduction-related and mainly from melting of rutile-bearing eclogite and garnet amphibolite. In contrast, the 2.7 and 2.9 Ga TTGs show similarity to low- to medium-pressure (LP-MP) TTGs and have lower Sr contents and negligible Eu anomalies. Combining their Y/Yb and (Ho/Yb)N ratios, the geochemical modeling results and their MgO, Mg#, Cr and Ni contents, it can be inferred that they were mostly produced by melting of garnet amphibolite and amphibolite at the base of thickened lower crust. The available geochronological and zircon Hf isotopic data suggest that the major crustal growth period was around 2.7 Ga with the subordinate growth peroid at ~2.9–3.0 Ga. Synthesizing all lines of evidence, we propose that the Jiaodong complex possibly witnessed a transition of tectonic regime during the Meso-Neoarchean.

The multiple granitic magmatism in the giant Huayangchuan uranium polymetallic ore district: Implications for tectonic evolution of the southern margin of North China Craton in the Qinling Orogen

The giant Huayangchuan U–Nb–Pb deposit in the Qinling Orogen is a newly verified carbonatite-hosted deposit in the southern margin of the North China Craton (NCC), central China. Granitic magmatism is extensively developed in the Huayangchuan deposit area, but their ages and petrogenesis are not well constrained. The exposed granitic rocks are mainly biotite monzogranite porphyry, granite pegmatite, granodiorite, and biotite monzogranite with zircon U–Pb ages of 1808 ± 10 Ma, 1807 ± 14 Ma, 233 ± 1.4 Ma, and 132 ± 0.6 Ma, respectively. The Paleoproterozoic biotite monzogranite porphyry belongs to shoshonite and metaluminous series, showing enrichment of LREE and LILE and depletion of HREE and HFSE, with high Zr + Y + Nb + Ce values and Ga/Al ratios, which are consistent with A-type granite. Whereas, the contemporaneous granite pegmatite dykes with weak mineralization are also cala-alkaline to shoshonite and peraluminous series, enriched in Rb, Ba, and LREE, and depleted in Nb, Ta, Ti and HREE. The shoshonite and weakly peraluminous Triassic granodiorite is slightly enriched in LREE with flat HREE patterns, enriched in Ba and Sr and depleted in Nb, Ta, P, and Ti, with similar geochemical characteristics to adakite-like rocks. The Early Cretaceous biotite monzogranite is characterized by LREE enrichment and flat HREE patterns, belonging to shoshonite and metaluminous to weakly peraluminous I-type granite, with U and LILE enrichment, and HFSE-depleted. The high initial 87Sr/86Sr ratios and enriched Nd (εNd(t) = −17.5 to −17.1) and Hf (εHf(t) = −33.2 to −14.6) isotopes reveal that the Huayangchuan granitic rocks are obviously sourced from crustal-derived magmas. Combined with regional geology and this study, we proposed that: (1) the Paleoproterozoic biotite monzogranite porphyry and granite pegmatite were generated from ancient lower crust during post-collisional extension setting; (2) the granodiorite was likely sourced from partial melting of thickened lower crust with pelagic sediments materials addition during the Triassic although the mineralization-related carbonatite having similar age with granodiorite may derive from mantle; and (3) the Early Cretaceous biotite monzogranite was mainly sourced from the partial melting of lower crust induced by the underplating of the mafic magma. We suggest that the first phase of magma occurred in the Huayangchuan district during the Paleoproterozoic following the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen. Since the Mesozoic, the ongoing northward subduction of the Yangtze Craton (YZC) resulted in the crust thickening, developing the Triassic granitic magmatism and major mineralization associated with carbonatite. The widespread Cretaceous granite and deposit in the southern margin of NCC indicate that intracontinental extension and lithospheric thinning occurred in response to the tectonic regime transition from NS-trending to EW-trending subduction.

Paleoproterozoic transition in tectonic regime recorded by the Eastern Block of the North China Craton: evidence from detrital zircons of the Langzishan Formation, Jiao-Liao-Ji Belt

ABSTRACTThe Eastern Block of the North China Craton records Paleoproterozoic oceanic plate subduction, the opening and subsequent closure of a back-arc basin, and a following collisional orogenic event to generate the Jiao-Liao-Ji Belt (JLJB). However, the processes involved in the initiation of subduction remain unclear. This study presents U–Pb age and trace elemental data for detrital zircons from the Langzishan Formation, the lowermost sedimentary unit within the JLJB, to constrain the provenance, depositional age, and depositional environment of this formation as well as the processes involved in the Paleoproterozoic transition in tectonic regime recorded by the Eastern Block. The U–Pb ages of these detrital zircons yield one major age peak at ca. 2529, and three minor age peaks at ca. 2700, ca. 2347, and ca. 2198 Ma. Provenance analysis indicates that the Langzishan Formation was derived from basement rocks in the Longgang Block with minor contributions from early magmatic rocks within the JLJB. Detrital zircon age contribution further suggests that the deposition of the Langzishan Formation was in a passive continental margin and commenced no earlier than 2205 Ma. Combining with previous research on the widespread distribution of magmatic and sedimentary rocks that formed in convergent settings in this area, this study suggests that the Eastern Block records a transition in tectonic regime at ~2205–2170 Ma, from passive continental margin to active continental margin settings, most likely as a result of the re-initiation of global plate tectonic cycling during the early history of the Earth.

Inversion of the Erlian Basin (NE China) in the early Late Cretaceous: Implications for the collision of the Okhotomorsk Block with East Asia

A significant transition in tectonic regime from extension to compression occurred throughout East Asia during the mid-Cretaceous and has stimulated much attention. However, the timing and driving mechanisms of the transition remain disputed. The Erlian Basin, a giant late Mesozoic intracontinental petroliferous basin located in the Inner Mongolia, Northeast China, contains important sedimentary and structural records related to the mid-Cretaceous compressional event. The stratigraphical, sedimentological and structural analyses reveal that a NW–SE compressional inversion occurred in the Erlian Basin between the depositions of the Lower Cretaceous Saihan and Upper Cretaceous Erlian formations, causing intense folding of the Saihan Formation and underlying strata, and the northwestward migration of the depocenters of the Erlian Formation. Based on the newly obtained detrital zircon U-Pb data and previously published paleomagnetism- and fossil-based ages, the Saihan and Erlian formations are suggested as latest Aptian–Albian and post-early Cenomanian in age, respectively, implying that the inversion in the Erlian Basin occurred in the early Late Cretaceous (Cenomanian time). Apatite fission-track thermochronological data record an early Late Cretaceous cooling/exhuming event in the basin, corresponding well with the aforementioned sedimentary, structural and chronological analyses. Combining with the tectono-sedimentary evolutions of the neighboring basins of the Erlian Basin, we suggest that the early Late Cretaceous inversional event in the Erlian Basin and the large scale tectonic transition in East Asia shared the common driving mechanism, probably resulting from the Okhotomorsk Block–East Asia collisional event at about 100–89 Ma.