Early Paleozoic Igneous Rocks Research Articles

Early Paleozoic tectonic affinity of the Lesser Xing'an–Zhangguangcai Range, NE China: A continental arc along the western margin of the Jiamusi Block

The Lesser Xing'an–Zhangguangcai Range (LXZGCR) in NE China was traditionally interpreted to be the eastern part of the Songliao–Xilinhot Accretionary Terrane, separated from the Jiamusi–Khanka Block to the east by the Heilongjiang Complex. However, recent studies have shown that the Heilongjiang Complex formed in the late Paleozoic–early Mesozoic. Therefore, the tectonic affinity of the LXZGCR during the early Paleozoic remains unclear. To constrain this key issue, we present new zircon UPb ages, Hf isotopic compositions, and whole-rock geochemical data for early Paleozoic igneous rocks in the LXZGCR. The new ages, along with previously published data, suggest that the early Paleozoic magmatism in the LXZGCR can be divided into four stages: Middle–Late Cambrian (523–486 Ma), Early Ordovician (485–469 Ma), Late Ordovician (464–443 Ma), and Late Silurian (433–423 Ma). Middle–Late Cambrian igneous rocks are dominated by calc-alkaline I-type granitoids and high-Mg andesite–dacite–rhyolite association. Early Ordovician igneous rocks consist of a suite of calc-alkaline diorites, granodiorites, monzogranites, alkali-feldspar granites, and rhyolites, with I-type affinities. Late Ordovician igneous rocks consist mainly of I-type granitoids with minor A-type granitoids and rhyolites. Late Silurian igneous rocks are composed of calc-alkaline I-type granitoids with minor dacites. The early Paleozoic granitoids and felsic volcanic rocks originated from partial melting of dominantly Paleoproterozoic–Mesoproterozoic lower crust, whereas the Late Cambrian high-Mg andesite–dacite–rhyolite association was probably derived from partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. The rock associations and geochemical characteristics suggest that these early Paleozoic igneous rocks formed in active continental margin and back-arc extension settings. Comparing the early Paleozoic magmatism, zircon Hf isotopic compositions, Nd model ages, and Precambrian basement features of the LXZGCR and the Jiamusi Block suggest that the LXZGCR was part of the same tectonic unit as the Jiamusi Block during the early Paleozoic. We argue that the formation of the early Paleozoic igneous rocks in the LXZGCR was related to the eastward subduction of a branch ocean of the Paleo-Asian Ocean beneath the Jiamusi–Khanka Block.

Petrography, geochemistry and geochronology of igneous rocks from the Jiangnan Orogen, South China: constraints on the Early Paleozoic tectonic evolution of the South China Block

The Early Paleozoic tectonic evolution of the South China Block (SCB) remains controversial related to intracontinental orogenic and oceanic subduction processes. We present whole-rock major and trace elemental data, LA-ICP-MS zircon U-Pb age and Lu-Hf isotopic data for the Early Paleozoic igneous rocks including granodiorites from the Yuechengling pluton and volcanic breccias from the Damingshan pluton from the western segment of the Jiangnan Orogen in the SCB. LA-ICP-MS zircon U-Pb dating yielded emplacement ages for the Yuechengling S-type granitoids of 438–436 Ma and the deposited age for the Damingshan volcaniclastic rocks to be later than 451 Ma. The Yuechengling granitoids have consistent SiO2, TiO2, Fe2O3T, MgO, and P2O5 contents, higher Al2O3 contents, and Na2O + K2O values, but lower Mg# values, compared with those of the Damingshan volcaniclastic rocks. All the studied samples exhibit enrichment in LREEs and moderate negative Eu anomalies (δEu = 0.63–0.75), with negative Ba, Sr, Nb, Ta, P, and Ti anomalies, and positive Rb, Th, U, Pb, and K anomalies. The granitoids have variable CaO/Na2O ratios of 0.22–1.11, negative εHf(t) values of −11.98 to −0.90, and corresponding TDM2 ages distributed from 2.37 to 1.55 Ga. The petrographic and geochemical characteristics of the Yuechengling granitoids indicate that their parental magma was derived from a crustal meta-greywacke and meta-pelite components in the Paleoproterozoic basement, and have undergone some degree of fractional crystallization. The volcaniclastic rocks have mostly negative εHf(t) values with partially positive (−27.54 to 8.73), and zircons with negative εHf(t) values (−27.54 to −0.14) show TDM2 ages of 3.79 to 1.63 Ga. Combined with petrographic and geochemical data, we suggest that the Damingshan volcaniclastic rocks were derived from Neoarchean-Neoproterozoic crustal materials and the felsic parental magma has undergone some degree of magma mixing with mantle material, and deposited soon after a Late Ordovician volcanic eruption (later than 451 Ma). Integrated with previous studies, our new data support the intracontinental orogenic model to account for the Early Paleozoic tectonic evolution. Thus, we suggest that the Early Paleozoic tectonic setting of the SCB was intracontinental orogeny rather than oceanic subduction-collision.

Oldest Basement (ca. 462 Ma) in Indonesian Borneo and its Implication for Early Paleozoic Tectonic Evolution of SE Asia

AbstractThe lack of preserved basement results in uncertain placements of many terranes in Southeast Asia. Here, we flag the first evidence of the oldest basement in Indonesian Borneo, which can help locate terranes in Borneo on the northern margin of Gondwana in the early Paleozoic and explain the regional tectonic setting of the island. Two schist samples from the Embuoi Complex in the Semitau Block, Northwest Kalimantan yielded zircon U‐Pb dates of 453.3 ± 1.9 Ma and 462.4 ± 2.6 Ma, respectively, representing the formation time of the protolith. Petrographic, internal structural and high Th/U ratios of zircons indicate that the protolith of schists is of magmatic origin. The zircons have εHf(t) values of –4.1 to +1.1 and Hf model ages of 1.37–1.69 Ga, indicating they were derived from a mixed source of juvenile crust with old components. By comparison of zircon age distribution, two‐stage Hf model ages and εHf(t) variations of the early Paleozoic igneous rocks from Semitau with those of South China, Tengchong–Baoshan, and Indochina, the Semitau Block was most likely a part of or placed next to the Indochina Block of northern Gondwana during the early Paleozoic. Such a similar tectono‐magmatic pattern on northern Gondwana formed a prolonged early Paleozoic arc‐related belt associated with subduction of the Proto‐Tethyan Ocean.

Early Devonian tectonic conversion from contraction to extension in the Chinese Western Tianshan: A response to slab rollback

Abstract Extensive angular unconformities that occur across the Tianshan Orogen provide insight into the Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt. This study presents new geochronological and geochemical data to unravel the age and origin of a pronounced angular unconformity in the Baluntai domain of the Chinese Western Tianshan. The angular unconformity separates a ductilely deformed metamorphic complex from overlying unmetamorphosed sequences. Detrital zircon age spectra suggest that the sedimentary samples received detritus from both the Precambrian basement and contemporaneous igneous rocks in the Central Tianshan and Yili blocks. The youngest detrital and metamorphic zircons in metasediments from the metamorphic complex reveal that their protoliths were deposited at ca. 419 Ma, and subsequently overprinted by metamorphism and ductile deformation at ca. 409 Ma. Along with ca. 532–407 Ma metavolcanics and foliated intrusions, the metamorphic complex constitutes a diachronous stratigraphic unit deposited during early Cambrian to late Silurian times, rather than a Precambrian basement as previously thought. The youngest detrital zircon age data from siliciclastic samples immediately above the angular unconformity constrain their deposition to ca. 407–397 Ma. In conjunction with a ca. 406 Ma rhyolitic tuff in the lower terrestrial siliciclastic unit and an early Carboniferous paleontological age of carbonates in the upper shallow-marine unit, it is suggested that the overlying unmetamorphosed sequences were accumulated during the early Devonian to the early Carboniferous. Thus, the angular unconformity in the Baluntai domain was formed between ca. 409 and 407 Ma. New and already published geochemical data reveal that the early Paleozoic (ca. 530–410 Ma) and late Paleozoic (ca. 410–320 Ma) igneous rocks are mainly medium- to high-K calc-alkaline I-type granitoids, which exhibit typical subduction-related geochemical signatures despite contrasting structural features. These geochronological and geochemical results are in accordance with those of similar early Devonian angular unconformities and Paleozoic igneous rocks in the southern Yili Block. The early Paleozoic igneous rocks display highly scattered zircon εHf(t) values, while the late Paleozoic rocks show a progressively increasing trend toward more positive values, attributed to slab rollback of the northward subducting South Tianshan Ocean under the Yili and Central Tianshan blocks. Thus, we propose that the formation of the angular unconformity was caused by tectonic conversion from contraction to extension due to slab rollback during the early Devonian. This study highlights the significance of geological and geochronological investigations of angular unconformities in the context of associated episodic magmatism and slab behavior in order to unravel distinct tectonic processes in the long-lived accretionary evolution of the Chinese Western Tianshan.

Early Paleozoic magmatic “flare-ups” in western Qinling orogeny, China: New insights into the convergence history of the North and South China Blocks at the northern margin of Gondwana

The geodynamic evolution of an accretionary orogeny commonly includes multiple magmatic episodes, which can be used to trace orogenic geodynamic processes using the geochronological and petrogenetic records preserved by igneous rocks. Paleomagnetic data indicate that the juxtaposed North China Craton (NCC) and South China Block (SCB) were located on the northern margin of the Gondwana supercontinent, but the geodynamic history of the two related blocks is still unclear. To address this issue, this paper integrates literature data with new geochronological, elemental, and isotopic data from early Paleozoic intermediate and silicic intrusions of the western North Qinling Terrane. We recognize three early Paleozoic flare-ups of intermediate–silicic intrusive magmatism in this area for the first time. Despite their similar lithology, the early Paleozoic igneous rocks formed during the three pulses have distinct geochemical compositions and petrogenesis, and record the evolution from subduction to continental collision. Geochronological data record: (1) An early Silurian (c. 441–434 Ma) pulse of diorite–quartz diorite–granite magmatism. The diorites and quartz diorites probably formed by fractionation of partial melts of mantle wedge modified by slab-derived hydrous fluids, while the granites formed by the partial melting of juvenile mafic lower crust. (2) A middle Silurian high-flux pulse of diorites, quartz diorites, and leucogranites (two-mica and muscovite granites) at c. 430–423 Ma. The diorites and quartz diorites were generated by crustal assimilation and fractional crystallization of basaltic partial melts of a pre-existing mantle source that had been altered by slab-derived hydrous fluids, and that was subsequently modified by slab-melts. The two-mica and muscovite granites formed by water-present melting of early–middle Silurian metamorphosed igneous rocks derived from enriched mantle and (meta-)graywackes, respectively. (3) A late Silurian (c. 415–409 Ma) magmatic pulse formed quartz diorites, A-type granites, and leucogranites (muscovite granites). The quartz diorites formed by differentiation of basaltic partial melts of enriched mantle, with a large contribution from depleted asthenospheric mantle. The A-type granites were generated by anatexis of juvenile mafic lower crust and subsequent fractional crystallization. The muscovite granites formed by biotite dehydration melting of (meta-)graywackes. The origins of these three early Paleozoic magmatic pulses were integrated with published regional paleomagnetic data and descriptions of sedimentary, metamorphic and igneous rocks, to provide a unique record of the geotectonic evolution of the Qinling Orogenic Belt that included: (1) slab rollback at c. 448–434 Ma; (2) initial continental collision at c. 433 Ma; (3) slab break-off at c. 430–423 Ma; (4) small-scale lithospheric foundering in a post-collisional setting at c. 415–409 Ma. Literature-derived geochronological records of collision-related magmatism and paleomagnetic data indicate that docking of the NCC and SCB with the northern Gondwana margin probably ended during the early Silurian (c. 433 Ma).

Discovery of Paleozoic rocks at northern margin of Sambagawa terrane, eastern Kyushu, Japan: Petrogenesis, U–Pb geochronology and its tectonic implication

Ordovician diorite-quartz diorite mylonite (Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula, Kyushu Island, Japan. The LA-ICP-MS zircon U–Pb geochronology revealed that the intrusion age of Saganoseki quartz diorite was 473.3 ​± ​3.6 ​Ma. These rocks show the volcanic arc affinity based on the trace element composition. On the basis of geochronological and geochemical results, Saganoseki quartz diorite is considered to be a member of the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone. Saganoseki quartz diorite is located just south of the Median Tectonic Line (MTL) and is in close contact with the pelitic and psammitic schist without any brittle shear zone. U–Pb ages of detrital zircon grains from two psammitic schists show the estimated sedimentation age of early Late Cretaceous, indicate that these psammitic schists are the member of Sambagawa metamorphic rocks. Together with these results and the mode of occurrence in the field, we argue that the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone existed as an upper structural unit of the Sambagawa terrane. This relationship is the key to reconstruct the Mesozoic tectonics of Japan as a part of East Asia, and its evolution through time.

Early Paleozoic arc magmatism in the Kalamaili orogenic belt, Northern Xinjiang, NW China: Implications for the tectonic evolution of the East Junggar terrane

The Kalamaili orogenic belt in northwestern China preserves a record of the magmatic and tectonic evolution of the Paleo-Asian Ocean and is thus an important region for reconstructing the Paleozoic evolution of the East Junggar terrane and the Central Asian Orogenic Belt. This study presents detailed field investigations, new SHRIMP U-Pb ages, whole-rock geochemistry, and in situ zircon Hf isotopic data for the early Paleozoic igneous rocks exposed in the Kalamaili orogenic belt. These igneous rocks are predominantly felsic plutons and mafic–intermediate volcanic rocks but include minor mafic–intermediate dikes. The igneous rocks have zircon U-Pb ages ranging from 463 Ma to 433 Ma and unconformably overlain by Late Silurian and Early Devonian sedimentary rocks. These early Paleozoic igneous rocks exhibit signatures similar to those of arc-related magmatic rocks such as enrichment in large-ion lithophile elements and depletion of high field strength elements. They have high positive εHf(t) values of +10.7 to +19.0, suggesting a juvenile source and considerable continental growth in the East Junggar terrane during the early Paleozoic. The geochemical signatures of the mafic–intermediate rocks suggest that they originated from the partial melting of mantle wedge material that had been metasomatized by slab-derived fluids; the felsic plutons were derived from the partial melting of juvenile crust. This early Paleozoic arc magmatic event, together with other evidence, indicates that the Kalamaili Ocean formed in the early Paleozoic, was subducted during Middle Ordovician–Early Silurian, and closed before Middle Silurian. Stratigraphic correlations reveal that the East Junggar terrane might not have experienced full extension to form a broad ocean during the late Paleozoic.

Geochronology and geochemistry of early Paleozoic igneous rocks from the Zhangguangcai Range, northeastern China: Constraints on tectonic evolution of the eastern Central Asian Orogenic Belt

Geochronology and geochemistry of early Paleozoic igneous rocks from the Zhangguangcai Range, northeastern China: Constraints on tectonic evolution of the eastern Central Asian Orogenic Belt

Geochronology and geochemistry of early Paleozoic intrusive rocks from the Khanka Massif of the Russian Far East

The Russian Far East and Northeast (NE) China are located in the eastern part of the Central Asian Orogenic Belt (CAOB), which consists of a series of micro‐continental massifs including the Erguna, Xing'an, Songnen–Zhangguangcai Range, Bureya, Jiamusi, and Khanka massifs. The Khanka Massif is located in the easternmost part of the CAOB, mainly cropping out in the territory of Russia, with a small segment in NE China. To the north and west of the Khanka Massif are the Jiamusi and Songnen–Zhangguangcai Range massifs, respectively. The boundary between these massifs is marked by the Dunhua–Mishan Fault. To the south lies the North China Craton, and to the east is the Sikhote–Alin Orogenic Belt separated by the Arsenyev Fault.However, the early Paleozoic evolution and tectonic attributes of the Khanka Massif are debated. These conflicting ideas result from the lack of systematic research on early Paleozoic igneous rocks from the Russian part of the Khanka Massif.It is generally accepted that the CAOB represents the largest known Phanerozoic accretionary orogenic belt. However, questions remain concerning the nature of the deep crust beneath the Khanka Massif, and whether Precambrian crust exists within the massif itself.In this paper, we report new zircon U–Pb ages, Hf isotopic data, and major‐ and trace‐element compositions of the early Paleozoic intrusive rocks from the Khanka Massif of the Russian Far East, with the aim of elucidating the early Paleozoic evolution and the tectonic attributes of the Khanka Massif, as well as the nature of the underlying deep crust.New U–Pb zircon data indicate that early Paleozoic magmatism within the Khanka Massif can be subdivided into at least four stages: ∼502 Ma, ∼492 Ma, 462–445 Ma, and ∼430 Ma.The ∼502 Ma pyroxene diorites show negative Eu anomalies, and the ∼492 Ma syenogranites, intruding the ∼502 Ma diorites, show positive Eu anomalies. These observations indicate that the primary parental magmas of these rocks were derived from different origins.The 462–445 Ma magmatism is made up of syenogranites and tonalites. The ∼445 Ma Na‐rich tonalites contain low REE concentrations, and are enriched in Eu and Sr. These observations, together with the positive εHf(t) values, indicate that they were derived from magmas generated by partial melting of cumulate gabbros.The ∼430 Ma I‐type granodiorites and monzogranites from the northern Khanka Massif, and the A‐type monzogranites from the central Khanka Massif display zircon εHf(t) values ranging from −5.4 to +5.8. This suggests that they formed from magmas generated by partial melting of heterogeneous lower crustal material.Zircon Hf isotopic data reveal the existence of Precambrian crustal material within the Khanka Massif. The geochemistry of the Middle Cambrian intrusive rocks is indicative of formation in an extensional setting, while Late Cambrian–middle Silurian magmatism was generated in an active continental margin setting associated with the subduction of a paleo‐oceanic plate beneath the Khanka Massif. Regional comparisons of the magmatic events indicate that the Khanka Massif has a tectonic affinity to the Songnen–Zhangguangcai Range Massif rather than the Jiamusi Massif.

Geochemical evidence for a Ganderian arc/back-arc remnant in the Nashoba Terrane, SE New England, USA

New geochemical data including Sm/Nd isotopic data show evidence for an early Paleozoic arc/back-arc complex in the Nashoba terrane of southeastern New England. The Nashoba terrane lies between rocks of Ganderian affinity to the northwest and Avalonian affinity to the southeast. It consists of early Paleozoic mafic to felsic metavolcanic and metasedimentary rocks that were intruded by intermediate to felsic plutons and metamorphosed to upper amphibolite facies conditions in the mid-Paleozoic. Major and trace element geochemical data indicate that the early Paleozoic igneous rocks contain a mix of arc, MORB, and alkaline signatures, and that the terrane formed as a primitive volcanic arc/back-arc complex built on thinned continental crust. Amphibolites have +4 to +7.5eNd(500) values consistent with formation in a primitive volcanic arc with minimal crustal contamination. Intermediate and felsic gneisses have eNd(500) values between +1.2 and –0.75 indicating a mixture of juvenile arc magmas and an evolved (likely basement) source. Depleted mantle model ages of 1.2 to 1.6 Ga point to a Mesoproterozoic or older age for this source. Metasedimentary rocks yielded –6 to –8.3 eNd(500) values and 1.6 to 1.8 Ga model ages, indicating an isotopically evolved source (or sources) that included Paleoproterozoic or older material. The eNd(500) values and model ages of the intermediate and felsic and metasedimentary rocks indicate that the basement to the Nashoba terrane is Ganderian rather than Avalonian. The Nashoba terrane therefore represents a Ganderian arc/back-arc complex similar to the Cambrian Penobscot arc/back-arc seen in Maritime Canada and Newfoundland, and particularly in the Annidale and New River terranes of southern New Brunswick. This correlation has not previously been recognized in southeastern New England. The Ganderian affinity of the Nashoba terrane also extends Ganderia farther SE in New England than previously established and indicates that the Nashoba terrane did not originate as a separate oceanic arc/back-arc complex or microcontinent.

Ages, geochemistry and tectonic implications of the Cambrian igneous rocks in the northern Great Xing’an Range, NE China

The Xinlin–Xiguitu suture zone, located in the Great Xing’an Range, NE China, in the eastern segment of the Central Asian Orogenic Belt (CAOB), represents the boundary between the Erguna and Xing’an micro-continental blocks. The exact location of the Xinlin-Xiguitu suture zone has been debated, especially, the location of the northern extension of the suture zone. In this study, based on a detailed field, geochemical, geochronological and Sr–Nd–Hf isotope study, we focus our work on the Cambrian igneous rocks in the Erguna-Xing’an block. The Xinglong granitoids, mainly include ∼520Ma diorite, ∼470Ma monzogranite and ∼480Ma pyroxene diorite. The granitoids show medium to high-K calc-alkaline series characteristics with post-collision granite affinity. The circa 500Ma granitoids have low εHf (t) values (−16.6 to +2.2) and ancient two-stage model (TDM2) ages between 1317Ma and 2528Ma. These results indicate the primary magmas of the Xinglong granitoids were probably derived from the partial melting of a dominantly Paleo-Mesoproterozoic “old” crustal source with possible different degrees of addition of juvenile materials, and formed in a post-collision tectonic setting after the amalgamation of the Erguna and Xing’an blocks. Compared with the Xinglong granitoids, the Duobaoshan igneous rocks are consisted of the approximately coeval rhyolitic tuffs (491±5Ma) and ultramafic intrusions (497±5Ma) within the Duobaoshan Formation. They are generally enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti), consistent with the geochemistry of igneous rocks from island arcs or active continental margins. The ultramafic rocks have high positive εHf (t) values (+1.3 to +15) and εNd (t) (+1.86 to +2.28), and relatively young two-stage model (TDM2) ages and low initial 87Sr/86Sr ratios (0.70628–0.70853), indicating the partial melting of a depleted mantle source from a subducted slab in the ocean basin between the Erguna-Xing’an and Songliao blocks. The rhyolitic tuffs contain a group of Phanerozoic zircons with εHf (t) values (−4.6 to +15.0), suggesting that the rhyolitic tuffs were derived from juvenile lower crustal material with some ancient crustal material.Coupled with our previous geochemical and isotopic studies on Early Paleozoic igneous rocks, we proposed that the collision of the Erguna and Xing’an blocks at least took place ca. 500Ma ago, and that there exist in a westward subduction of an oceanic plate between the Eruguna-Xing’an and Songliao blocks, took place during the Early Ordovician. Up to now, there are more evidences and constraints that the northern extension location of the Xinlin-Xiguitu suture zone is located in the Jifeng–Xinglong areas.

Geochronology and geochemistry of early Paleozoic igneous rocks of the Lesser Xing'an Range, NE China: Implications for the tectonic evolution of the eastern Central Asian Orogenic Belt

This paper presents new zircon U–Pb, Hf isotope, and whole-rock major and trace element data for early Paleozoic igneous rocks of the Lesser Xing'an Range, NE China, in order to constrain the early Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt (CAOB). Zircon U–Pb dating indicates that early Paleozoic magmatic events within the northern Songnen–Zhangguangcai Range Massif (SZM) can be subdivided into four stages: Middle Cambrian (~505Ma), Late Cambrian (~490Ma), Early–Middle Ordovician (~470Ma), and Late Ordovician (460–450Ma). The Middle Cambrian monzogranites are K-rich, weakly to strongly peraluminous, and characterized by pronounced heavy rare earth element (HREE) depletions, high Sr/Y ratios, low Y concentrations, low primary zircon εHf(t) values (−6.79 to −1.09), and ancient two-stage model (TDM2) ages (1901–1534Ma). These results indicate derivation from partial melting of thickened ancient crustal materials that formed during the amalgamation of the northern SZM and the northern Jiamusi Massif (JM). The Late Cambrian monzonite, quartz monzonite, and monzogranite units are chemically similar to A-type granites, and contain zircons with εHf(t) values of −2.59 to +1.78 and TDM2 ages of 1625–1348Ma. We infer that these rocks formed from primary magmas generated by partial melting of Mesoproterozoic accreted lower crustal materials in a post-collisional extensional environment. The Early–Middle Ordovician quartz monzodiorite, quartz monzonite, monzogranite, and rhyolite units are calc-alkaline, relatively enriched in light REEs (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, Th, and U), depleted in HREEs and high field strength elements (HFSEs; e.g., Nb, Ta, and Ti), and contain zircons with εHf(t) values of −7.33 to +4.98, indicative of formation in an active continental margin setting. The Late Ordovician alkali-feldspar granite and rhyolite units have A-type granite affinities that suggest they formed in an extensional environment. A comparison of early Paleozoic magmatic events and Hf isotopic model ages between the northern SZM and the JM indicates that these two massifs have similar histories of Mesoproterozoic and early Paleozoic crustal accretion and reworking, although the SZM contains much older crustal materials than the JM.

Geochronology, geochemistry, and Sr–Nd–Hf isotopes of the early Paleozoic igneous rocks in the Duobaoshan area, NE China, and their geological significance

The Duobaoshan area of northwestern Heilongjiang Province is the most important copper resource concentration region in NE China. To date, the Duobaoshan superlarge Cu–Mo deposit and the Tongshan large Cu–Mo deposit have been discovered in the Duobaoshan area. Both the deposits are hosted by granodiorites and volcanic rocks. Zircon LA-ICP-MS U–Pb dating indicates that these granodiorites emplaced approximately 479Ma ago and that those volcanic rocks erupted between 447 and 450Ma. The early Ordovicain granodiorites belong to the high-K to medium-K calc-alkaline series and are characterized by high Al2O3 and Sr contents, low Yb and Y contents, and relatively low Mg# values and Na2O/K2O ratios, with positive Eu or slight negative Eu anomalies (averaging 1.18). All of these geochemical characters are similar to those of the adakites generated by partial melting of a thickened lower crust in the world. Moreover, the granodiorites have low initial 87Sr/86Sr ratios (varying from 0.703474 to 0.704436), very high zircon εHf(t) and whole-rock εNd(t) values (varying from 13.0 to 16.8 and 5.27 to 5.46, respectively), and young zircon Hf and whole-rock Nd single-stage and two-stage model ages. Taking these geochemical characteristics and Sr–Nd–Hf isotope compositions together, we suggest that the early Ordovician granodiorites in the Duobaoshan area occurred in a post-collision environment and were formed by partial melting of a juvenile thickened lower crust dominated by depleted mantle-derived material. These late Ordovician volcanic rocks, which are composed of basalt, basaltic andesite, and andesite, belong to the tholeiitic or calc-alkaline series. They are generally enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Zr, Hf, P, and Ti), consistent with the geochemistry of igneous rocks from island arcs or active continental margins. Compared with the early Ordovician granodiorites, these volcanic rocks have similar Sr, Nd, and Hf isotopic features, such as low initial 87Sr/86Sr ratios (varying from 0.704404 to 0.705820), very high εHf(t) and εNd(t) values (varying from 11.5 to 17.6 and 5.24 to 5.77, respectively), and young Hf and Nd single-stage and two-stage model ages. Both geochemical characteristics and Sr–Nd–Hf isotope compositions of the volcanic rocks suggest that the late Ordovician volcanic rocks occurred in an island-arc setting and were formed by partial melting of a depleted mantle wedge modified by predominant slab fluids. Our study in this work indicates that the collision of the Xing’an and Argun massifs at least took place ∼500Ma ago and that a westward subduction of an oceanic plate, which is located between the Songnen–Zhangguangcai Range and Xing’an massifs, took place during the late Ordovician.

Geochronology and petrogenesis of the early Paleozoic I-type granite in the Taishan area, South China: Middle-lower crustal melting during orogenic collapse

The early Paleozoic Wuyi–Yunkai orogeny represents the first extensive tectonothermal event in South China since the Neoproterozoic break-up of the Rodinia supercontinent. The early Paleozoic igneous rocks in the South China Block (SCB) are important for understanding the processes responsible for the orogeny. The Taishan Batholith in the inner Cathaysia Block consists dominantly of granodiorite and hornblende-bearing granite. Zircon U–Pb dating of four samples gives a consistent crystallization age of ca. 436Ma, synchronous with widespread late-orogenic S-type granitic intrusions in the Wuyi–Yunkai orogen. The Taishan granitoid rocks are metaluminous or peraluminous (A/CNK=0.94–1.15) and have variable SiO2 (61.5–74.9wt.%), MgO (0.4–2.4wt.%), Fe2O3 (1.8–7.4wt.%) and CaO (1.1–5.5wt.%), with gradually decreasing P2O5 from mafic to felsic components. Thus the Taishan Batholith displays the petrological and geochemical characteristics of I-type granites. Additionally, all samples have strongly negative εNd(t) values (−11 to −9), suggesting an overall ancient source. The early Paleozoic I-type granitoid rocks in the Taishan area were derived from partial melting of ancient middle to lower crust at a high thermal gradient (>35°C/km), and underwent extensive fractional crystallization of hornblende, plagioclase, and biotite. The Wuyi–Yunkai orogenic collapse may have been responsible for middle to lower crustal melting of the Cathaysia Block. The early Paleozoic I- and S-type granitoid magmatism lack significant input of a juvenile component, indicating that the processes accompanying orogenic collapse such as asthenosphere upwelling and basaltic underplating, contributed heat to trigger widespread early Paleozoic felsic magmatism in the SCB. An intraplate orogenic event, instead of continental or arc collision, may be the preferred model for the Wuyi–Yunkai orogeny.

Early Paleozoic amalgamation of the Songnen–Zhangguangcai Range and Jiamusi massifs in the eastern segment of the Central Asian Orogenic Belt: Geochronological and geochemical evidence from granitoids and rhyolites

To constrain the early Paleozoic tectonic evolution of the Songnen–Zhangguangcai Range and Jiamusi massifs, we undertook zircon U–Pb dating and geochemical analyses (major and trace elements, and Hf isotopic compositions) of early Paleozoic igneous rocks in the Lesser Xing’an–Zhangguangcai Range of eastern Heilongjiang province, NE China. The zircons from one diorite, two rhyolites, and four granitoids are euhedral–subhedral and display fine-scale oscillatory growth zoning, indicating a magmatic origin. LA–ICP–MS zircon U–Pb age data indicate the occurrence of magmatic events in the late Ordovician (443–451Ma) and middle Silurian (424–430Ma). The late Ordovician igneous rocks are composed mainly of diorite, tonalite, and rhyolite, whereas the middle Silurian igneous rocks consist mainly of monzogranites. The early Paleozoic igneous rocks have SiO2=59.65–76.11wt.%, MgO=0.13–1.23wt.%, and Na2O+K2O=4.68–7.77wt.%, and are chemically a calc-alkaline series. They are characterized by enrichment in light rare earth elements and large ion lithophile elements, and depletion in heavy rare earth elements and high field strength elements such as Nb, Ta, and Ti, consistent with the chemistry of igneous rocks from an active continental margin setting. In situ Hf analyses of zircons show that εHf(t) values of zircons that formed in the early Ordovician (captured and inherited zircons in late Ordovician igneous rocks) range from −0.1 to +7.1; those of zircons that formed in the late Ordovician (including captured and inherited zircons in middle Silurian monzogranite) vary from −2.6 to +5.6; and those of zircons that formed in the middle Silurian vary from −3.0 to +3.2. The progressive decrease in zircon εHf(t) values from the early Ordovician to the middle Silurian suggests an increasing involvement of ancient crust in the magma sources, corresponding to the transition from subduction to collision. Take together, these data suggest that the formation of late Ordovician diorite, tonalite, and rhyolite in the Zhangguangcai Range, adjacent to the Mudanjiang Fault, may have been related to the westward subduction of an oceanic plate between the Songnen–Zhangguangcai Range and Jiamusi massifs. In contrast, coeval rhyolites from the Lesser Xing’an Range, located far from the Mudanjiang Fault, may have formed in the supra-subduction zone. The occurrence of a middle Silurian monzogranite association, combined with geochronological data for detrital zircons from Early Devonian sedimentary rocks in eastern Heilongjiang province, suggests that the amalgamation between the Songnen–Zhangguangcai Range and Jiamusi massifs occurred in the middle Silurian.

Supercontinent reconstruction from recognition of leading continental edges

Repeated amalgamation and subsequent breakup of continental lithosphere have profoundly affected Earth9s evolution since the Archean. Following breakup, distinctive rift and passive margin sequences along the trailing edges of dispersing continents have been used to identify such margins in the geologic past. Using western North America as an analogue, we show that the leading edges of dispersing continents have isotopic characteristics that can likewise be used to identify these margins. For example, the Sm-Nd isotopic signatures of Late Neoproterozoic and early Paleozoic igneous rocks along the northern margin of Gondwana indicate derivation from 0.7 to 1.1 Ga old mantle lithosphere. This lithosphere originated in the Mirovoi Ocean surrounding Rodinia. It subsequently accreted to northern Gondwana in response to Rodinia breakup, and provided a source for subsequent magmatism. Accretion and subsequent recycling of oceanic mantle lithosphere should be common along the leading edges of dispersing continents following supercontinent breakup, providing an additional aid in paleocontinental reconstructions.

Cadomian and Early Paleozoic Magmatic Events: Insights from Zircon Morphology and Geochemical Signatures of I- and S-Type Granitoids (Saxo-Thuringia/Bohemian Massif/Central Europe)

This paper presents studies of zircon morphologies and geochemical signatures of Neoproterozoic and Early Paleozoic metagranitoids and metarhyolites of the Saxo-Thuringian Zone (Central Europe). The Saxo-Thuringian Zone was derived from the northern periphery of Gondwana and became part of the Variscan belt of Central Europe during Paleozoic orogenic processes. Five tectonomagmatic events are preserved in the Saxo-Thuringian Neoproterozoic and Early Paleozoic igneous rocks. These different geotectonic scenarios are each characterized by distinctive geochemical signatures and zircon morphologies. Samples of felsic lithologies with age ranges of ~580-560 Ma, ~500 Ma, and ~380-360 Ma show whole-rock compositions of dry I-type with a tendency to A-type granites and zircon crystals with strong dominance of {100} prisms and {101} pyramids. The ~580-560 Ma and ~500 Ma periods of predominantly juvenile magmatism both were followed by tectonothermal events producing S-type granitoids at ~550-530 Ma and 495-480 Ma. Associated zircon populations are dominated by {110} prisms and {211} pyramids. Inasmuch as the errors of individual geochronological analyses may overlap two different geotectonic episodes, the relationship between geochemical features and zircon morphologies of the Saxo-Thuringian Neoproterozoic and Early Paleozoic igneous rocks can aid in assigning the dated sample to the proper stage of formation.

U–Pb geochronology of detrital zircons from a continental margin assemblage in the northern Coast Mountains, southeastern Alaska

Metamorphic rocks within and west of the northern Coast Mountains in southeastern Alaska consist of an Upper Proterozoic(?) to upper Paleozoic continental margin assemblage that we interpret to belong to the Yukon-Tanana terrane. U–Pb geochronologic analyses of single detrital zircon grains from four samples of quartzite suggest that the zircons were shed from source regions containing rocks of ~495 Ma, ~750 Ma, 1.05–1.40 Ga, 1.75–2.00 Ga, ~2.3 Ga, 2.5–2.7 Ga, and ~3.0 Ga. Multigrain fractions from two samples yield upper intercepts between 2.0 and 2.3 Ga, but the scarcity of single grains of similar age suggests that these fractions comprise a mixture of < 2.0 and > 2.3 Ga grains. Zircons in these rocks generally overlap in age with (i) detrital zircons in metasedimentary rocks of the Yukon–Tanana terrane in eastern Alaska and Yukon, (ii) detrital zircons in strata of the Cordilleran miogeocline, and (iii) plutonic and gneissic rocks that intrude or are overlain by miogeoclinal strata. In addition, the pre-1.7 Ga grains overlap in age with dated crystalline rocks of the western Canadian Shield. These similarities raise the possibility that metaclastic rocks in the northern Coast Mountains accumulated in proximity to western North America. The younger zircon populations were likely shed from mid-Proterozoic to early Paleozoic igneous rocks that now occur locally (but may have been widespread) along the Cordilleran margin. Recognition of a continental margin assemblage of possible North American affinity in the Coast Mountains raises the possibility that some arc-type and oceanic terranes inboard of the Coast Mountains may be large klippen that have been thrust over the North American margin.