Active Continental Margin Setting Research Articles

The Paleomagnetic Pole of the Siberian Paleocontinent at the Late Ediacaran Stage of Evolution of the Active Continental Margin (South Yenisei Ridge)

The results of the first paleomagnetic study of a late Neoproterozoic magmatic complex on the Siberian paleocontinent (Siberia) using U–Pb geochronological data are presented. The igneous rocks of the Zimoveyniy massif (block) of the South Yenisei Ridge formed in an active continental margin setting and recorded the initial thermoremanent magnetization. Our calculations let us determine with reasonable certitude that in the late Ediacaran Siberia was located in the subequatorial latitudes.

Geochemistry, zircon U-Pb and Lu-Hf systematics of high-grade metasedimentary sequences from the South Muya block (northeastern Central Asian Orogenic Belt): Reconnaissance of polymetamorphism and accretion of Neoproterozoic exotic blocks in southern Siberia

High-grade metasedimentary rocks of the South Muya block (Baikal-Muya Foldbelt) have been studied with respect to whole-rock elemental and Nd isotopic compositions, as well as the U-Pb and Lu-Hf systematics of detrital zircon and in-situ grown metamorphic grains in order to reveal the initial source provenance for sedimentation and the age of major metamorphic modification. Based predominantly on trace-elemental compositions, the sedimentary protolith is most likely a poorly sorted and immature greywacke, such as those formed in island arc or active continental margin settings. The island arc-related chemical signature coupled with zircon U-Pb age data supports a Neoproterozoic (>760 Ma) sedimentation age, whereas Nd-Hf isotope data of precursor sediments are linked to a relatively immature Tonian (∼940–780 Ma) continental arc involving Paleoproterozoic crustal basement. The limited provenance information and metamorphic history of the Muya block suggests it was exotic to Siberia and originally related to another continent, e.g. – subduction-accretion complexes of South China. The polyphase metasediments experienced high-grade metamorphism in the Tonian during the 764–754 Ma arc accretion to an enigmatic continental block and then in the Ediacaran during the 622–608 Ma accretion of the Baikal-Muya belt to the arc situated at the southern Siberia margin. Metamorphic overprinting during the introduction of supracrustal rocks to deeper-crustal levels led to protracted recrystallization of primary igneous zircon population and growth of new granulite-facies grains that did not involve notable juvenile input. Tonian sedimentary provenance and two metamorphic events suggest a protracted evolution for the Baikal-Muya belt associated with the formation of precursor complexes beyond the southern margin of Siberia and their later accretion to Late Cryogenian – Ediacarian arc complexes of the southern Siberian, which proceeded only after the opening of the Paleoasian Ocean (670–630 Ma).

Neoarchean magmatic arc in the Western Liaoning Province, northern North China Craton: Geochemical and isotopic constraints from sanukitoids and associated granitoids

The Neoarchean Western Liaoning basement terranes in the northern part of North China Craton chiefly consist of tholeiitic basalts in the north, TTG gneisses and metavolcanic rocks in the northwest, and K-rich granitoids in the southeast. In the southeastern zone, porphyritic monzodioritic-quartz monzodioritic-granodioritic-monzogranitic (MQGM) gneisses are the major lithological assemblage, and locally contain minor xenoliths of TTG gneisses. Based on the major mafic mineral phases, the porphyritic MQGM gneisses can be divided into amphibole-dominated and biotite-dominated groups. Zircon UPb isotopic dating indicates that the amphibole-dominated MQGM gneisses, biotite-dominated MQGM gneisses, and tonalitic gneiss xenolith were emplaced during 2546–2529 Ma, 2546–2531 Ma, and at 2563 Ma, respectively. The 2563 Ma tonalitic gneiss xenolith displays geochemical affinities to high-SiO2 adakites, and may be formed by the partial melting of descending oceanic slabs, with the melt contaminated by mantle peridotite. The amphibole-dominated MQGM gneisses are geochemically analogous to sanukitoids, and their magmatic precursors were formed by partial melting of lithospheric mantle, which was metasomatized by dehydration fluids and melts derived from subducted oceanic sediments and slabs. The magmatic precursors of the biotite-dominated MQGM gneisses were generated by the partial melting of medium-K to high-K mafic rocks at medium pressures. Integrated with previous studies on supracrustal sequences and granitoids in Western Liaoning Province, the Neoarchean lithological assemblages in the southeastern zone were most likely formed in an active continental margin setting.

LateCarboniferous–earlyPermian arc magmatism in the south‐westernAlxaTectonicBelt (NW China):Constraints on the latePalaeozoic subduction history of thePalaeo‐AsianOcean

The tectonic setting of the Alxa Tectonic Belt (ATB) during the late Palaeozoic is highly controversial. The nature and tectonic origin of the late Palaeozoic magmatism in the ATB are key to resolving the current controversy. This paper provides field, petrographic, geochemical, and zircon U‐Pb‐Hf isotopic data for the late Palaeozoic granitoids and volcanic rocks from the south‐western ATB. The granitoids display a wide range of SiO2contents from diorite, granodiorite, to granites, with widely distributed hydrous minerals such as hornblende and biotite. They are calc‐alkaline to high‐potassium calc‐alkaline, metaluminous with an enrichment of light rare earth elements and large‐ion lithophile elements (LILE), and a depletion of high‐field‐strength elements (HFSE). The presence of mafic‐intermediate enclaves in the plutons suggests the role of crust–mantle interaction in generating the granitoids. The volcanic rocks show “block‐in‐matrix” structures in the field. They are dacite‐porphyry and rhyolite and show calc‐alkaline characteristics with an enrichment of LILE and a depletion of HFSE. Laser ablation inductively coupled plasma mass spectrometry zircon U‐Pb dating shows that the granitoids and enclaves crystallized during ~317–287 Ma and the dacite porphyry formed at ~295 Ma. Hf‐in‐zircon isotopic compositions reveal predominately positive εHf(t) values and Neoproterozoic TDMCages for the magmatic rocks, indicating mixing between mantle‐derived magma and Precambrian basement during their genesis. The diagnostic field, geochemical data, and isotopic data imply these rocks were generated in a subduction‐related active continental margin setting. The presence of A2‐type granite indicates an extensional environment resulted from slab rollback in a retreating accretionary context can best interpret the large‐scale late Carboniferous–early Permian magmatism in the ATB. Our new data, combined with published data, imply that a large active continental margin existed in the Beishan, Alxa, and the northern margin of the North China Craton, due to the south‐dipping subduction of the Palaeo‐Asian Ocean (PAO) in the late Palaeozoic. Therefore, our data suggest that the PAO did not close until after the early Permian.

Early Silurian adakitic high‐Mg diorite from the Longshan area: Implication for melting of mantle lithosphere in the south‐eastern Qilian Orogenic Belt

The Longshan area is located in the junction zone between the Qinling and eastern Qilian orogenic belts. Early Palaeozoic igneous rocks in this area can provide important insights into the evolution of Shangdan Ocean. This paper reports new petrology, chronology, major and trace element geochemistry, Sr–Nd isotope, and zircon Lu–Hf isotopic compositions for the Early Silurian pyroxene diorite in the Longshan area. The pyroxene diorite has a zircon U–Pb age of 432 ± 1 Ma (MSWD = 0.25, n = 27). The pyroxene diorite intrusion displays low SiO2 (53.34 to 56.32 wt.%), high Na2O (3.11% to 3.79 wt.%) and Sr (1112 to 1234 ppm) contents, and moderate Mg# (52.70 to 55.00) values, high Sr/Y (67 to 84) ratios, depleted in Y and HREE; these features are identical with the low‐Si adakite or sanukitoids that derived from melting of metasomatized mantle wedge. The pyroxene diorite intrusion has evolved Sr–Nd isotopic compositions, (87Sr/86Sr) i = 0.705286 to 0.705310, εNd(t) = −3.4 to −3.9. Zircons from the pyroxene diorite display variable εHf(t) values of −3.6 to +0.1. These features suggest that the pyroxene diorite intrusion was derived from a hydrous melting metasomatized mantle wedge which has been modified by slab melts. The occurrence of pyroxene diorite indicates an Early Silurian active continental margin setting in the south‐eastern Qilian Orogenic Belt.

Petrogenesis and tectonic implications of Late Carboniferous continental arc high-K granites in the Dongwuqi area, central Inner Mongolia, North China

Central Inner Mongolia is located in the southeastern segment of the Central Asian Orogenic Belt (CAOB), and controversy has long surrounded its late Paleozoic tectonic evolution. Here, we present an integrated study of zircon U-Pb ages, whole rock major and trace elements, and zircon Hf isotopes for five less studied alkali feldspar granitic plutons in the Dongwuqi area to the north of the Erenhot-Hegenshan-Heihe suture zone. Zircon U–Pb dating by laser ablation-inductively coupled plasma–mass spectrometry (LA–ICP–MS) indicates that these granitic plutons were emplaced in the Late Carboniferous (315–300 Ma). The Late Carboniferous alkali feldspar granites belong to high-K calc-alkaline series and are evolved I-type granites. They are high in SiO2 (73.22–77.36%), and low in MgO (0.07–0.49%) with enrichments in light rare earth elements (LREEs), Zr, Hf, Rb, Th, and U, and depletions in Ba, Nb, Ta, Sr, and Eu. The Dongwuqi granitic rocks are characterized by high positive zircon εHf(t) values (+7.6 to +15.1), and relatively young two-stage Hf model (TDM2) ages (364–835 Ma). These geochemical data, together the common miarolitic texture, indicate that they were derived from partial melting of juvenile arc-related mafic to intermediate rocks in the lower crust, and eventually emplaced in low-pressure conditions or at shallow crustal levels, with involvement of extensive fractional crystallization during magma evolution. Our new results, along with the published regional data, delineate a NE-trending Carboniferous continental arc belt (more than 1700 km) in the northern accretionary zone of the Erenhot-Hegenshan-Heihe suture. Accordingly, we invoke that the Dongwuqi Late Carboniferous high-K granites were emplaced in an active continental margin setting, which was probably related to the northwestward subduction of the Hegenshan oceanic plate beneath the Xing'an-Uliastai-Nuhetdavaa block.

Neoproterozoic and Early Paleozoic magmatic records from the Chalukou ore district, northern Great Xing’an Range, NE China: Implications for tectonic evolution and Mesozoic Mo mineralization

To constrain the pre-mineralization tectonic evolutionary history and the nature of the magma source of the Chalukou Mo deposit, which is located on the western margin of the Xing’an block, we recognized and studied a period of Neoproterozoic magmatism and a suite of Early Paleozoic volcanic rocks in this ore district. The magmatic zircons of quartz chlorite schist collected from the Wolegen Group yielded a concordant 206Pb/238U age of 685.1 ± 4.5 Ma (MSWD = 0.21), and they show very high εHf(t) values (+15.5 to +17.3) and young Hf modal ages (573–664 Ma) close to the formation age, indicating that the protolith was generated during the Neoproterozoic, and was most likely derived from a depleted mantle region. This Neoproterozoic magmatism might represent an important crustal growth event within the Xing’an Block. The host wallrocks in the Chalukou deposit mainly consist of rhyolite, dacite, andesite and volcanic tuff which have undergone slight deformation and metamorphism. The results of zircon LA-ICP-MS U-Pb dating indicated that they were generated during 472–475 Ma. The results of whole-rock geochemical analysis indicate that they have arc-affinity features characterized by LREE enrichment with moderate to strong fractionation between LREE and HREE (LaN/YbN = 7.7–13.6), intense negative Eu anomalies (δEu = 0.36–0.71), pronounced positive Ba, Th, U, Pb and La anomalies and negative Nb-Ta, P and Ti anomalies. Hf isotopes show various εHf (t) values ranging from −2.0 to +5.9 and slightly old two-stage model ages ranging from 1070 Ma to 1573 Ma. Combined with discrimination diagrams, these data indicate that they were derived from the partial melting of a slightly older lower crust with the addition of a juvenile crust component in an active continental margin setting related to the subduction of the Paleo-Asian Ocean between the Xing’an and Songliao blocks. The existence of Neoproterozoic magmatic activity and Mesoproterozoic lower crustal materials obtained from the Hf model ages of the Paleozoic volcanic rocks in the Chalukou ore district demonstrated that Precambrian basement exists in the Xing’an Block, but its distribution area may be limited. As the largest porphyry Mo deposit in China, the ore-forming materials might have multiple sources, considering the long-term and multiple magmatic events that occurred under different tectonic settings in the ore district. The previous modification and refertilization of the lower crust and/or lithospheric mantle by the subduction of the Paleo-Asian Ocean might be important in the generation of the fertile lower crust, which is the direct source of the ore-related porphyries.

Palaeoproterozoic meta-rhyolite and meta-dacite of the Liaohe Group, Jiao-Liao-Ji Belt, North China Craton: Petrogenesis and implications for tectonic setting

Whole-rock major and trace element geochemistry, along with zircon U–Pb ages and Lu–Hf isotope data, are reported for Palaeoproterozoic meta-rhyolites and meta-dacites of the Liaohe Group in the northern segment of the Jiao-Liao-Ji Belt, North China Craton. These data are used to constrain the petrogenesis of the rocks and the Palaeoproterozoic tectonic evolution of the Jiao-Liao-Ji Belt. Zircons from these rocks are euhedral–subhedral, display oscillatory growth zoning, and have high Th/U ratios (0.26–1.21), implying a magmatic origin. LA–ICP–MS zircon U–Pb age data indicate that the silicic metavolcanic rocks formed during the Palaeoproterozoic (2184–2127 Ma). The zircons yield εHf(t) values of −9.7 to +8.9 and TDM2 model ages of 2205–3313 Ma. Geochemically, the silicic metavolcanic rocks are metaluminous to weakly peraluminous and belong to the calc-alkaline I-type series. The rocks are characterized by enrichment in large-ion lithophile elements and depletion in high-field-strength elements (e.g., Nb, Ta, P, and Ti), and have affinities with igneous rocks that form in an active continental margin setting. The geochemical and isotopic data suggest that the rocks were derived mainly from partial melting of Meso-Neoarchaean juvenile lower crust, with a possible contribution from older materials. The present results, combined with the regional geology, further demonstrate that an oceanic plate was subducted northward beneath the Longgang Block during the Palaeoproterozoic.

Petrology, geochemistry, and zircon U–Pb–Hf isotopes of Late Triassic enclaves and host granitoids at the southeastern margin of the Songnen–Zhangguangcai Range Massif, Northeast China: Evidence for magma mixing during subduction of the Mudanjiang oceanic plate

We report zircon U–Pb ages and geochemistry (major and trace elements, mineral compositions, and Hf isotopic compositions) for the Hailin pluton in the southeastern Songnen–Zhangguangcai Range Massif, northeastern China, to constrain the role of magma mixing in its petrogenesis and to reconstruct the tectonic evolution of the Mudanjiang Ocean. Zircon U–Pb dating yields ages of 215 ± 1 Ma, 217 ± 1 Ma, and 219 ± 1 Ma for the host granitoids of the Hailin pluton, and 221 ± 1 Ma for mafic microgranular enclaves (MMEs) within the granitoids, indicating they were emplaced coevally. The host granitoids are characterized by 63.18–70.44 wt% SiO2 and 0.69–1.94 wt% MgO, and are depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti) and heavy rare earth elements (HREEs), and enriched in large ion lithophile elements (LILEs; e.g., Rb, Sr, and Ba) and light rare earth elements (LREEs). The zircons in the host granitoids yield two-stage model ages of 1058 Ma to 1770 Ma, together with εHf(t) values of −6.6 to +3.1. These results indicate that the host granitoid magmas were generated by partial melting of Mesoproterozoic lower crust contaminated by ancient crustal material. The MMEs have a restricted SiO2 range (47.61–57.4 wt%), high contents of Ni, Co, and Cr, and high Mg# values (42–54). They have higher Fe2O3T, MgO, Cr and Co contents than the host granitoids, as well as higher total rare earth element (REE) concentrations. Their REE patterns are sub-parallel to those of the host granitoids, characterized by enrichments in LILEs and LREEs, and depletions in HFSEs and HREEs. Zircons from two samples of the MMEs yield single-stage Hf model ages of 666–1077 Ma and εHf(t) values of −5.4 to +5.2. The geochemical characteristics of the MMEs are similar to those of the host granitoids, indicating crust-mantle interaction during their formation. The mafic magmas were derived from melting of depleted lithospheric mantle that had been metasomatized by slab-derived fluids. MMEs are common in the host granitoids and are characterized by irregular spheroidal shapes, with sharp, rounded, or irregular transitional contacts, acicular apatites, and plagioclase reverse zoning, suggesting the importance of magma mixing during petrogenesis. Our geochemical and isotopic data suggest that the Late Triassic Hailin pluton formed by the mixing of felsic and mafic magmas in an active continental margin setting, closely related to the westward subduction of the Mudanjiang oceanic plate beneath the Songnen–Zhangguangcai Range Massif.

Neoarchean microblock amalgamation in southern India: Evidence from the Nallamalai Suture Zone

A collage of crustal blocks ranging in age from Mesoarchean to Neoarchean accreted to the southern margin of the Dharwar Craton in Peninsular India preserve distinct evolutionary history, but share a common metamorphic record during Archean-Proterozoic transition. Here we investigate the tectonic boundary between two of these microblocks, the Shevaroy Block to the west and the Madras Block to east, termed as the Nallamalai Suture Zone (NLSZ). We present integrated field, petrological, geochemical and zircon U-Pb and Lu-Hf data from a suite of meta-igneous and metasedimentary units along the NLSZ and its flanks. Zircon grains from the meta-monzogranite, hornblende-biotite gneiss, amphibolite, granodiorite, diorite, charnockite, BIF (banded iron formation) and BMQ from this area show magmatic emplacement ages clustering around 2.50 to 2.56 Ga, except for the meta-monzogranite showing ages up to 3.2 Ga, correlating with long-lived convergent margin magmatism through multiple slab melting episodes. All the rocks show tightly constrained early Paleoproterozoic (ca. 2.46–2.48 Ga) metamorphic ages, including the metamorphic zircon in the BIFs, marking the timing of collision of the two continental blocks with consumption of the intervening oceanic lithosphere. The εHf(t) values of magmatic zircon grains from the different rock types range from −4.1 to +5.7, and together with TDMC Hf model ages of 2672–3247 Ma, a dominantly juvenile crust growth is indicated, which initiated around 3.3 Ga and continued to 2.7 Ga, followed by crustal reworking in a continental arc towards the end of Neoarchean.The geochemical data suggest that tholeiitic to calc-alkaline parental melts for the mafic-intermediate-felsic suite were generated by low degree partial melting of a peridotitic mantle wedge metasomatized by subduction-derived fluids and sediments and continuous slab melting ensued by intracrustal fractional crystallization of melts with reworking of older continental crust. The BIF and BMQ samples display positive Eu anomalies, negative to negligible Ce anomalies, and superchondritic Y/Ho ratios suggesting their formation in an oceanic realm proximal to an active continental margin setting. The magmatic and tectonic attributes are consistent with eastward oceanic subduction and ocean closure along the NLSZ which is defined here as the trace of a suture welding the Shevaroy and Madras Blocks. We envisage that multiple subduction and amalgamation of several microblocks occurred in the Dharwar Craton and its southern domains, amalgamating several microblocks during the Archean – Paleoproterozoic transition.

The Early Paleozoic Active Margin of the Khangai Segment of the Mongol–Okhotsk Ocean

Within the northern fringe of the western (Khangai) flank of the Mongol–Okhotsk fold belt, magmatic complexes of intermediate to moderately acidic rocks occur. They comprise widely distributed gabbro–diorites, diorites, tonalites, and granodiorites. Geochronological studies have demonstrated that these rocks were formed in the time span of 437 to 375 Ma. The geochemical affinities of the rocks suggest their formation in subduction tectonic settings; hence, their paleotectonic position corresponds to the continental margin of the Mongol–Okhotsk paleoocean. It has been concluded that this Middle Paleozoic igneous activity occurred in the active continental margin settings, formed by subduction of the paleooceanic plate under the Siberian continent.

Review on Age of Magmatism and Crust Formation in Sri Lanka: U–Pb and Lu–Hf Isotopic Perspectives

Recent studies on zircons at high resolution separated from metamorphosed rocks of igneous origin of the four tectonic domains of Sri Lanka by Lu–Hf and U–Pb isotope systematics have shed light upon its tectono-magmatic history. While providing better insights, the new dataset is not contradictory with Nd-model ages and U–Pb zircon dates discovered from Sri Lanka in early 90s. These new data suggest that both Wanni (WC) and Vijayan (VC) Complexes were magmatic arcs with the former being relatively older than the latter, and the Kadugannawa Complex (KC) is a marginal arc magmatic suite in the vicinity of WC. The oldest Highland Complex (HC) has been derived of Mesoproterozoic to Archean crustal and subducted sedimentary components. The Lu–Hf systematics of rocks from the HC record the oldest Hf-crustal model age of ~ 3.5 Ga and highly negative eHf(t)(t) values (up to − 30) inferring contribution of older subducted sediments and/or crustal components. In the WC and KC tectonic units, variable eHf(t) of zircon from negative to positive values from gneisses indicate the involvement of both juvenile mantle components and older continental materials in the generation of the arc-related magma with Hf-model ages from ~ 700 to 2800 Ma. On the other hand, the meta igneous rocks of the VC have distinct positive eHf(t) data with Hf-modal ages in the range of ~ 700–1600 Ma supporting entirely a juvenile origin. During the Neoproterozoic to Cambrian (ca. 700–500 Ma), the HC has predominantly served as a suture zone for the collision of the WC and VC arcs. This suggests that the Sri Lankan terrains were juxtaposed at an active continental margin setting associated with two-staged subduction during the Gondwana amalgamation. The metamorphism took place during the Neoproterozoic in the entire basement up to the granulite facies conditions reaching intermittently ultrahigh temperature (UHT) conditions.

Petrography and geochemistry of Oligocene to Lower Miocene sandstones in the Baiyun Sag, Pearl River Mouth Basin, South China Sea: Provenance, source area weathering, and tectonic setting

There is a controversy regarding the source area and weathering intensity of Oligocene to Lower Miocene sediments in the Baiyun Sag of the Pearl River Mouth Basin. Detrital modes, geochemistry, heavy mineral data, and detrital zircon U–Pb ages were analysed to understand the provenance, source area weathering, and tectonic setting of Oligocene to Lower Miocene sandstones in the Baiyun Sag. Petrographic data reveal that the sandstones were derived from a recycled orogen. With increasing transport distance, sediment maturity increased correspondingly. The Oligocene to Lower Miocene sandstones in the Baiyun Sag have low compositional maturity and have experienced low to moderate weathering. The sediments of the Zhuhai Formation have experienced relatively strong weathering intensity compared with those of the Enping and lower Zhujiang formations. Due to low degree of illitization and differences in the geochemical compositions of the sediments, the continental island arc data from different boreholes in the Baiyun Sag are more irregularly distributed in the A‐CN‐K plot. The modal composition (e.g., quartz, feldspar, and lithic fragments) and geochemical indices (Al2O3/TiO2 ratio; Al2O3‐TiO2 diagram; discriminant F1‐F2 diagram; Eu/Eu*, La/Sc, La/Co, Th/Sc, and Th/Co ratios; Hf‐La/Th and La/Sc‐Co/Th diagrams; and rare earth element patterns) indicate that the sediments in the Baiyun Sag are mainly derived from felsic source rocks. Although the uplift around the Baiyun Sag includes multistage magmatism and basic rocks, it has little influence on the sediment in the depression, and no sediments were transported by the Han River to the Baiyun Sag. Geochemical data and detrital zircon U–Pb ages suggest that the Pearl River was formed in the Early Oligocene and became the main source for the sediments in this area. The river carried felsic material from the South China Coast forming a large delta on the northern slope of the Baiyun Sag. The ratios of Eu/Eu*, La/Sc, La/Co, Th/Sc and Th/Co, and heavy mineral data indicate that the sediments transported by the Pearl River to the Baiyun Sag were changed from the arc‐dominated coastal zone to the interior of the South China Block during the Late Oligocene to Early Miocene and even reached the south‐eastern margin of the Tibetan Plateau (West Yunnan Plateau). The La‐Th‐Sc and Th‐Sc‐Zr/10 diagrams suggest that the provenance of these sandstones is evolved continental island arc or an active continental margin setting.

Petrogenesis of Jurassic granitoids at the northeastern margin of the North China Craton: New geochemical and geochronological constraints on subduction of the Paleo-Pacific Plate

At the junction between the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB), northern Liaoning province, NE China, there are widespread Jurassic igneous rocks. The tectonic setting and petrogenesis of these rocks are unresolved. Zircon U–Pb dating, whole-rock geochemistry, and Hf isotopic compositions of Jurassic granitoids were investigated to constrain their ages and petrogenesis in order to understand the tectonic evolution of the Paleo-Pacific Ocean along the northeastern margin of the NCC. Geochronological data indicate that magmatism occurred between the early and late Jurassic (180–156 Ma). Despite the wide range in ages of the intrusions, Jurassic granitoids were likely derived from a similar or common source, as inferred from their geochemical and Hf isotopic characteristics. Compared to the island arc andesite-dacite-rhyolite series, the Jurassic granitoids are characterized by higher SiO2, Al2O3, and Sr contents, and lower MgO, FeOT, Y, and Yb contents, indicating that the primary magmas show typical characteristics of adakitic magmas derived from partial melting of thickened lower crust. These findings, combined with their εHf(t) values (+1.4 to +5.4) and two-stage model ages (1515–1165 Ma), indicate the primary magmas originated from partial melting of juvenile crustal material accreted during the Mesoproterozoic. They are enriched in large-ion lithophile elements (e.g., Rb, K, Th, Ba, and U) and light rare-earth elements (REE), and depleted in high-field-strength elements (e.g., Nb, Ta, Ti, and P) and heavy REE. Based on these findings and previous studies, we suggest that the Jurassic adakitic granitoids (180–156 Ma) were formed in an active continental margin and compressive tectonic setting, related to subduction of the Paleo-Pacific Plate.

Late Paleocene–early Eocene granitoids in the Jiamusi Massif, NE China: Zircon U–Pb ages, geochemistry, and tectonic implications

ABSTRACTNortheast (NE) China is characterized by large areas of Mesozoic and Paleozoic granitoids, whereas Cenozoic granitoids are scarce. This paper reports LA-ICP-MS zircon U–Pb ages and whole-rock geochemical data for late Paleocene–early Eocene granitoids from the Jiamusi Massif, NE China, in order to determine their petrogenesis and tectonic implications. Geochronological data indicate that the granodiorite and dioritic porphyry from the Wudingshan pluton formed at 51.5 ± 0.3 Ma and 56.3 ± 0.8 Ma, respectively. The biotite–quartz diorite, biotite granodiorite, and dioritic porphyry have high SiO2 (68.38–70.06 wt.%), Al2O3 (15.34–15.79 wt.%), and Na2O (3.96–4.49 wt.%) contents, low MgO contents (1.10–1.26 wt.%), A/CNK ratios of 0.99–1.11, and are classified as medium- to high-K calc-alkaline and weakly peraluminous I-type granitoids. They are enriched in LREEs and LILEs, and depleted in HFSEs, with Nb/Ta ratios of 10.4–15.0. Moreover, they have negative Nb–Ta–Ti anomalies, indicating that they were derived from continental crust. Combining with the previously published isotopic data and regional geological results, we suggest that the late Paleocene–early Eocene granitoids (56–52 Ma) were probably derived from partial melting of juvenile lower crust, and formed in an active continental margin setting, possibly related to subduction slab rollback of the Paleo-Pacific Plate.

Geochronology and geochemistry of Mesozoic intrusive rocks in the Xing'an Massif of NE China: Implications for the evolution and spatial extent of the Mongol–Okhotsk tectonic regime

This study presents new zircon U–Pb–Hf and whole-rock geochemical data for intrusive rocks in the Xing'an Massif of NE China, with the aim of furthering our understanding of the evolution and spatial influence of the Mongol–Okhotsk tectonic regime. Zircon U–Pb dating indicates that five stages of Mesozoic magmatism are recorded in the Xing'an Massif, namely during the Middle Triassic (~237 Ma), the Late Triassic (~225 Ma), the Early Jurassic (~178 Ma), the Middle Jurassic (~168 Ma), and the late Early Cretaceous (~130 Ma). The Middle Triassic–Early Jurassic intrusive rocks in the Xing'an Massif are dominantly granodiorites, monzogranites, and syenogranites that formed from magma generated by partial melting of newly accreted continental crust. Geochemistry of the Middle Triassic–Early Jurassic granitoid suites of the Xing'an Massif indicates their formation at an active continental margin setting, related to the southwards subduction of the Mongol–Okhotsk oceanic plate. The Middle Jurassic monzogranites in the Xing'an Massif are geochemically similar to adakites and have εHf(t) values (+3.8 to +5.8) and Hf two-stage model ages (TDM2; 979–850 Ma) that are indicative of derivation from magma generated by partial melting of thickened juvenile lower crust. The Middle Jurassic monzogranites formed in a compressional setting related to the closure of the Mongol–Okhotsk Ocean. The late Early Cretaceous intrusive rocks in the Xing'an Massif are dominated by A-type granitoids that are associated with bimodal volcanic rocks, suggesting their formation in an extensional environment related to either (i) delamination of a previously thickened region of the crust, associated with the Mongol–Okhotsk tectonic regime; (ii) the subduction of the Paleo-Pacific Plate; or (iii) the combined influence of these two tectonic regimes.

The Totumo mud volcano and its near-shore marine sedimentological setting (North Colombia) — From sedimentary volcanism to epithermal mineralization

The Holocene mud volcano exposed at Totumo (younger than 4150 ± 50 yr BP) lines up together with some other landforms of its kind along the Caribbean Coast in northern Colombia. It currently vents a mud of the silicate-phosphate-bearing sulfur-sodium chloride type. The mud volcanoes evolved in an active continental margin setting of the South American Cordillera with high seismicity and affected by pervasive neotectonic structural disturbances. During the Neogene and Quaternary linear terrigenous shoreline sediments alternating with delta deposits evolved on this mobile crustal segment between the Andes and ancient Precambrian cratons. Meso- to microtidal sedimentary settings during transgression and progradation created meta- to instable sedimentary and petrophysical conditions (e.g. overpressure and gas-bearing bubble sands), favorable for the formation of mud volcanoes, whose lithofacies is subdivided into (1) footwall facies (detritus from metabasic, -pelitic source rocks), (2) mud volcano plus lateral facies (material from deep-seated hydrothermal sources, hydrocarbon plays, and brine reflux from the sea), (3) hanging wall facies, sand characterized by a strong longshore drift. The sedimentary volcanism in the area is characterized by different temperatures of formation: (1) pre-stage (<100 °C) and (2) recent stage (≈25 °C). Heavy (pyroxene, amphibole, epidote-clinozoisite, Fe-Ti silicates and oxides, garnet, alumosilicates, tourmaline, zircon, barite, Fe sulfides and -sulfates), light (Ca sulfates, calcite, quartz, feldspar) and clay minerals (kaolinite, mica, pyrophyllite, chlorite, vermiculite) are efficient tools to determine the source of mud, to subdivide the mud volcano system as to its facies and describe its physical-chemical regime as to the temperature of formation, pH and Eh values. The mud volcano system of Totumo bridges the gap between sedimentary “volcanism” and epithermal hot spring deposits of intermediate to high sulfidation and forms a useful “guide” to hydrocarbon accumulation.

The transition from a passive to an active continental margin in the Jiamusi Block: Constraints from Late Paleozoic sedimentary rocks

The eastern margin of the Jiamusi Block records the tectonic transition from a passive to an active continental margin during the Late Paleozoic- Early Mesozoic. Late Paleozoic sedimentary rocks from the Heitai and Zhenzishan formations along the eastern margin of the Jiamusi Block are a key to resolving these events. Major and trace element data (including REE) for 18 samples from late Paleozoic sedimentary rocks, with the characteristics of high SiO2, Al2O3, Na2O and K2O, low CaO, MgO and TiO2, enrichment in LREE and LILE, depletion in HFSE, and negative Eu anomalies, were plotted on various discrimination diagrams, indicating that the Heitai Formation formed in a passive continental margin setting, whereas the Zhenzishan Formation formed in an active continental margin setting. LA-ICPMS U-Pb dating of detrital zircon from two samples of the Heitai Formation and two samples from the Zhenzishan Formation indicates a range in age from 2735 ± 19 to 308 ± 3 Ma, showing similar populations to the Jiamusi Block. These detrital zircons not only indicate that the sedimentary material was derived from the Jiamusi Block, but also limit the depositional ages of the Heitai and Zhenzishan formations. The youngest zircon population of 374–384 Ma, with a peak at 382 Ma, from the Heitai Formation, together with evidence from paleontological studies, indicates the depositionol age of the Heitai Formation is Devonian (Givetian- Famennian), whereas the 308–318 Ma population, with a peak at 315 Ma, from the Zhenzishan Formation indicates that the depositionol age was late Carboniferous (post- Moscovian). This indicates that the transition from a passive to an active continental margin in the Jiamusi Block occurred between the Late Devonian and the late Carboniferous (380-310Ma) and was more likely related to the Mongol-Okhotsk oceanic subduction-accretion or Panthalassa Ocean, and not to the Paleo-Pacific Ocean or to the Paleo-Asian Ocean as previous considered.

Neoproterozoic gabbro–granite association from the Micangshan area, northern Yangtze Block: Implication for crustal growth in an active continental margin

Neoproterozoic igneous rocks along the northern Yangtze Block are closely related to the evolution of the Rodinia supercontinent. This paper presents zircon U–Pb dating, whole‐rock geochemistry and Sr‐Nd isotopes of the gabbro–granite association from the Micangshan area, northern Yangtze Block. Zircon LA‐ICP MS U–Pb dating indicates that the tonalite and monzogranite from the Pinghe display identical ages of 869 ± 4 Ma (MSWD = 1.3, 2σ) and 859 ± 8 Ma (MSWD = 1.9, 2σ), respectively. According to the regional geology, the gabbro may be younger. The monzogranite and tonalite display almost identical geochemical features, both of them have high Na2O/K2O ratios, with insignificant negative Eu anomalies, relatively juvenile Sr‐Nd isotopic compositions. The monzogranite has initial (87Sr/86Sr)i ratios of 0.703403 to 0.705218, and positive ɛNd(t) values of +1.23 to +2.98. The tonalite have (87Sr/86Sr)i ratios of 0.700387 to 0.706222, positive ɛNd(t) values of +1.98 to +7.79, indicating that they were derived from juvenile source. The 860 Ma monzogranite–tonalite association suggests crustal growth event in an active continental margin. The gabbro has low (La/Yb)N ratios (1.80 to 7.61) and total rare earth element contents (∑REE = 44.95 to 83.92 ppm), they have initial (87Sr/86Sr)i ratios of 0.701913 to 0.704815, positive ɛNd(t) values of +0.65 to +5.73, indicating an island‐arc affinity. In combination with the other Neoproterozoic igneous rocks along the western and northern margin of the Yangtze Block, we propose that the gabbro and granitoid association from the Micangshan area represent two stage crustal growth events in the active continental margin setting, and these results imply that the Neoproterozoic crustal growth events in the Yangtze Block may be controlled by subduction.

Geochronology and geochemistry of early Paleozoic intrusive rocks from the Khanka Massif in the Russian Far East: Petrogenesis and tectonic implications

This paper presents new geochronological and geochemical data for early Paleozoic intrusive rocks from the Khanka Massif in the Russian Far East, with the aim of elucidating the Paleozoic evolution and tectonic attributes of the Khanka Massif. New U–Pb zircon data indicate that early Paleozoic magmatism within the Khanka Massif can be subdivided into at least four stages: ~502, ~492, 462–445, and ~430Ma. The ~502Ma pyroxene diorites contain 58.28–59.64wt% SiO2, 2.84–3.69wt% MgO, and relatively high Cr and Ni contents. Negative εHf(t) values (−1.8 to −0.4), along with other geochemical data, indicate that the primary magma was derived from partial melting of mafic lower crust with the addition of mantle material. The ~492Ma syenogranites have high SiO2 and K2O contents, and show positive Eu anomalies, indicating the primary magma was generated by partial melting of lower crust at relatively low pressure. The ~445Ma Na-rich trondhjemites display high Sr/Y ratios and positive εHf(t) values (+1.8 to +3.9), indicating the primary magma was generated by partial melting of thickened hydrous mafic crust. The ~430Ma granitoids have high SiO2 and K2O contents, zircon εHf(t) values of −5.4 to +5.8, and two-stage model ages of 1757–1045Ma, suggesting the primary magma was produced by partial melting of heterogeneous Proterozoic lower crustal material. The geochemistry of these early Paleozoic intrusive assemblages indicates their formation in an active continental margin setting associated with the subduction of a paleo-oceanic plate beneath the Khanka Massif. The εHf(t) values show an increasingly negative trend with increasing latitude, revealing a lateral heterogeneity of the lower crust beneath the Khanka Massif. Regional comparisons of the magmatic events indicate that the Khanka Massif in the Russian Far East has a tectonic affinity to the Songnen–Zhangguangcai Range Massif rather than the adjacent Jiamusi Massif.