Magmatic Hiatus Research Articles

Magmatic records of subduction and closure of the Meso-Tethys Ocean in the northern-central Tibetan Plateau

The evolution of the Tethys Ocean has received much research attention; however, the timing and subduction mechanisms involved in the closure of the Bangong-Nujiang Meso-Tethys Ocean remain poorly constrained. In this study, we present geological, geochronological, geochemical, and zircon isotopic data from Cretaceous magmatic rocks and combine these with previously published data within the central Bangong-Nujiang suture zone in the north-central Tibetan Plateau. The integrated data update the regional tectonic framework and enable a comprehensive geodynamic model to be developed for the subduction and ocean closure events. In detail, temporal and spatial variations in the Jurassic−Cretaceous magmatism reflect the influence of the northward subduction of the Dongqiao-Amdo oceanic basin and the bidirectional subduction of the Bangong-Nujiang Meso-Tethys oceanic crust. The evolution of the Bangong-Nujiang Meso-Tethys Ocean further involved initial intra-ocean subduction, slab rollback, and flat subduction, as evidenced by two phases of north-south migration of magmatism at ca. 190−160 and 160−130 Ma and a magmatic hiatus at 160−140 Ma. The ocean closed during a two-stage process, including the closure of the Dongqiao-Amdo oceanic basin to the north and the Bangong-Nujiang Meso-Tethys Ocean to the south. The Dongqiao-Amdo oceanic basin closed soon after ca. 180 Ma, accompanied by continent-continent collision between the Amdo microcontinent and the South Qiangtang terrane. The Bangong-Nujiang Meso-Tethys Ocean closed at 130−120 Ma, corresponding to a period of waning magmatism. This closure represented complete oceanic closure and caused an arc-continent collisional event involving the Baingoin magmatic arc, the Amdo microcontinent, and the Lhasa terrane. The Lower Cretaceous terrestrial strata and their basal unconformity mark the final closure of the ocean, and the Early Cretaceous ocean islands might have formed before ca. 130 Ma.

Long-term magmatism in the Ribeira Orogen, Western Gondwana: SHRIMP U-Pb and O isotope data for two Ediacaran-Cambrian magmatic events in the Rio de Janeiro area

In the southeastern Brazilian margin, a Neoproterozoic to the Early Paleozoic long-term magmatic interval is recorded within the Oriental Terrane, part of the Ribeira Orogen, characterizing it as a hot orogenic belt. Metaigneous and igneous rock units of ca. 570–540 Ma and 510–480 Ma are coeval with two main collisional orogenies related to the Western Gondwana assembly: the Ribeira Orogeny (590–565 Ma) and the Búzios Orogeny (540–490 Ma). The Oriental Terrane, interpreted as a continental microplate amalgamated during the convergence between the São Francisco and Congo cratons, comprises Cryogenian to Ediacaran metaigneous rocks and Cambro-Ordovician plutons intrusive in metasedimentary sequences. Zircons from an orthogneiss and a porphyritic leucogneiss sample gave SHRIMP U-Pb crystallization ages of 558 ± 2 Ma and 560 ± 2 Ma, respectively, with high δ18O values from 7 ‰ to 10 ‰. Zircons from a tonalite pluton and tonalitic dikes, gave SHRIMP U-Pb crystallization ages of 483 ± 2 Ma and 492 ± 2 Ma, with lower δ18O values, between 6 ‰ to 8 ‰. These two magmatic pulses, integrated with a compiled database from other plutons in this terrane, corroborate with the hypothesis that they represent crustal scale magmatic provinces here interpreted as pre- and post-collisional in relation with the Búzios Orogeny. Their distribution within the tectonic map of the Oriental terrane reflect a paleogeography linked to their tectonic origin. The 570–540 Ma plutons are aligned ENE -WSW, parallel and close to the suture zone with the Cabo Frio Tectonic Domain, representing a crustal melt related to a NW-subduction. A subsequent magmatic hiatus of 30 ma is interpreted as the collision of the Cabo Frio Tectonic Domain. The post collisional period is marked the intrusion of non– linear 510–480 Ma pluton, widespread in the Oriental terrane.

Lithospheric thinning and ignition of a Cordilleran magmatic flare-up: Geochemical and O-Hf isotopic constraints from Cretaceous plutons in southern Korea

Northeast Asian continental margins contain the products of magma emplacement driven by prolonged subduction of the (paleo-)Pacific plate. As observed in many Cordilleran arcs, magmatic evolution in this area was punctuated by high-volume pulses amid background periods. The present study investigates the early evolution of the Cretaceous magmatic flare-up using new and published geochronological, geochemical, and O-Hf isotope data from plutonic rocks in the southern Korean Peninsula. After a long (∼50 m.y.) magmatic hiatus and the development of the Honam Shear Zone through flat-slab subduction, the Cretaceous flare-up began with the intrusion of monzonites, granodiorites, and granites in the inboard Gyeonggi Massif and the intervening Okcheon Belt. Compared to Jurassic granitoids formed during the former flare-up, Albian (∼111 Ma) monzonites found in the Eopyeong area of the Okcheon Belt have distinctly higher zircon εHf(t) (−7.5 ± 1.3) and δ18O (7.78‰ ± 0.25‰) values and lower whole-rock La/Yb and Sr/Y ratios. The voluminous coeval granodiorite and granite plutons in the Gyeonggi Massif are further reduced in Sr/Y and to a lesser extent, in La/Yb, and have higher zircon εHf(t) values (−13 to −19) than the Precambrian basement (ca. −30). These chemical and isotopic features indicate that Early Cretaceous lithospheric thinning, most likely resulting from delamination of tectonically and magmatically overthickened lithospheric keel that was metasomatized during prior subduction episodes, and consequent asthenospheric upwelling played vital roles in igniting the magmatic flare-up. The O-Hf isotopic ranges of synmagmatic zircons from the Albian plutons and their Paleoproterozoic and Jurassic inheritance attest to the involvement of lithospheric mantle and crustal basement in magma generation during this decratonization event. Arc magmatism then migrated trenchward and culminated in the Late Cretaceous, yielding widespread granitoid rocks emplaced at shallow crustal levels. The early Late Cretaceous (94–85 Ma) granites now prevalent in Seoraksan-Woraksan-Sokrisan National Parks are highly silicic and display flat chondrite-normalized rare earth element patterns with deep Eu anomalies. Synmagmatic zircons in these granites mimic their host rock’s chemistry. Delamination-related rejuvenation of crustal protoliths is indicated by zircon εHf(t) values of granites (−6 to −20) that are consistently higher than the Precambrian basement value. Concomitant core-to-rim variation in zircon O-Hf isotopic compositions reflects a typical sequence of crustal assimilation and fresh input into the magma chamber.

Stratigraphy of the Agnew-Wiluna Greenstone Belt: review, synopsis and implications for the late Mesoarchean to Neoarchean geological evolution of the Yilgarn Craton

Our ability to understand the geological evolution of Archean cratons is fundamentally dependent on stratigraphic reconstructions across greenstone belts. Here, we integrate published stratigraphic, geochemical and geochronological data to produce a revised stratigraphic model for the Agnew-Wiluna Greenstone Belt of the Yilgarn Craton. The Agnew-Wiluna Greenstone Belt records ca 130 m.y. of magmatism that occurred between ca 2820 and 2690 Ma and can be divided into three cycles. Cycle 1 is marked by ca 2820–2740 Ma mafic–ultramafic volcanic rock successions. Cycle 2 is associated with ca 2740–2720 Ma felsic volcanic rocks, associated fragmental deposits and contemporaneous mafic–ultramafic intrusions. Cycle 3 consists of ca 2720–2690 Ma mafic–ultramafic volcanic rock successions and co-magmatic intrusions. Exhumation and reworking of these continental rift basin successions after ca 2690 Ma resulted in two younger, additional, depositional cycles in the Agnew-Wiluna Greenstone Belt. Cycle 4 is associated with intermediate to felsic volcanoclastic rocks and epiclastic rocks erupted/deposited between ca 2690 and 2665 Ma. Cycle 5 is associated with the deposition of polymict conglomerates and sandstones in localised depocentres between ca 2665 and 2655 Ma. The stratigraphic scheme identified at the scale of the Agnew-Wiluna Greenstone Belt is the local expression of craton-wide tectono-thermal events. Our revised stratigraphic model for the Agnew-Wiluna Greenstone Belt and proposed correlations at the craton scale support a para-autochthonous model whereby crust from a proto-Yilgarn continental mass was extended/thinned, favouring the activation of translithospheric pathways associated with the ca 2720–2690 Ma Kalgoorlie Large Igneous Province. Inversion of this intra-cratonic rift basin after ca 2680 Ma led to the protracted activation of shear zones and multiple shifts in sedimentary depocentres. The widespread occurrence of previously unidentified fragments of ca 2820–2720 Ma substrate within the Kalgoorlie-Kurnalpi Rift has significant implications for craton-scale stratigraphic reconstructions and challenges existing terrane and domain boundaries. KEY POINTS We propose a revised, coherent and testable stratigraphic model for the Agnew-Wiluna Greenstone Belt of the Kalgoorlie Terrane. We compare our revised stratigraphic model for the Agnew-Wiluna Greenstone Belt with that of the Murchison and Southern Cross domains of the Youanmi Terrane in order to build a new space–time–integrated stratigraphic cross-section across the western half of the Yilgarn Craton. We demonstrate for the first time the existence of a magmatic continuum (i.e. no significant magmatic hiatus) between ca 2820 and 2690 Ma, providing the missing link between the geological record of the Youanmi Terrane and that of the Eastern Goldfields Superterrane.

A synthesis of geochemistry of Mesozoic igneous rocks in NE China and tectonic superposition and transformation of the easternmost Central Asian Orogenic Belt

As the product of the amalgamation of multiple microcontinental massifs, Northeast China is located in the easternmost segment of the Central Asian Orogenic Belt (CAOB), which underwent multi-stage tectonic superposition in Mesozoic. Extensive and intensive igneous activities occurred throughout northeast China and the related igneous rocks can be considered as an effective probe for constraining tectonic evolution. In this study, numerous available ages and geochemical data are collected from published literature. The spatial-temporal distribution, rock assemblage, and geochemical characteristics of Mesozoic igneous rocks are systematically reassessed and summarized. A conclusion is drawn that NE China experienced the superposition and transformation of the Paleo-Asian Ocean (PAO) domain, the Mongol-Okhotsk Ocean (MOO) domain, and the Paleo-Pacific Ocean (PPO) domain in Mesozoic and detailed tectonic evolution is presented as follows. Firstly, NE China was under the joint effect of the MOO and the PAO during Early-Middle Triassic (ca. 252–237 Ma). Arc-type igneous rocks in the Erguna-Xing’an Block were induced by the southward subduction of the MOO Plate and adakites with high Sr/Y and (La/Yb)N ratios in the south segment of NE China were emplaced in the post-collision orogenic environment related to the closure of the PAO. Secondly, during Late Triassic, the influence of the PAO domain was gradually weakened, and igneous activities developed sporadically along the Solonker-Xar Moron-Changchun-Yanji suture zone. The MOO continued to subduct south until the Early Jurassic (ca. 220–190 Ma), and continental igneous rocks with arc-like features were widely distributed in the Erguna-Xing’an Block. During ca. 220–200 Ma, the Jiamusi-Khanka Massif was slipped away from the Songnen-Zhangguangcai Range Massif and this progress may be related to the PPO domain. Thirdly, in the Early Jurassic-Early Cretaceous (ca. 200–120 Ma), NE China came under the joint influence of the MOO and PPO tectonic domains. The subduction and closure of the MOO Plate and corresponding thickening of the lower crust resulted in widespread and intensive igneous activities in the Erguna and the Great Xing’an Range (GXAR). Two-stage subduction of the PPO and between which a period of change of subduction direction, induced the two-stage arc igneous activities of ca. 185–164 Ma and ca. 135–120 Ma, and the magmatic hiatus between ca. 163.5–135 Ma in the Zhangguangcai Range (ZGCR) and its east. Fourthly, the influence of the MOO domain was completed at ca. 120 Ma and numerous volcanic rocks corresponding chemically to A-type granite began to erupt violently, due to the rollback of the Paleo-Pacific oceanic slab. Since then, NE China has been completely under the control of the PPO domain.

Mesoarchean trondhjemitic continental nucleus and pre-plate tectonic crustal-mantle interactions of the Western Shandong Complex, North China Craton

Plate tectonics play an important role in crustal-mantle interactions since late Archean, yet detailed processes of pre-plate tectonic crustal-mantle interactions remain enigmatic. In the Western Shandong Complex of the North China Craton, magmatic rocks recorded successive tectonic transition processes from vertical (plume) to lateral (subduction) during the early Neoarchean. These magmatic records, together with our newly discovered a pre-Neoarchean continental nucleus in the Western Shandong Complex, documented key information on pre-plate tectonic crustal-mantle interactions. In this study, petrology, whole-rock geochemical and Sm–Nd isotopic data, and zircon U–Pb and Lu–Hf isotopes are reported for Neoarchean supracrustal and intrusive rocks for the Western Shandong Complex. Early Neoarchean komatiitic basalts and ultramafic rocks (∼2.77 Ga) were plume-related and generated by crustal contamination of fractional crystallized komatiitic magma. A series of Mesoarchean (∼3.0–2.8 Ga) xenocrystic zircon grains were discovered in the Late Neoarchean charnockites (∼2.56–2.52 Ga), indicating contributions of continental crustal material earlier than regional oldest magmatic records. Based on our new geochronological and geochemical data, a trondhjemitic Mesoarchean (∼3.1–2.8 Ga) continental nucleus was identified and reported for the first time in the Western Shandong Complex. During the early stage of the early Neoarchean, this continental nucleus dismembered during the bottom-up plume processes and the correlated plume-continental nucleus interactions occurred and ubiquitously documented in regional Neoarchean magmatic rocks with different degrees. The plume-continental nucleus interactions consequently resulted in the opening of an oceanic basin and different continental evolutionary processes during the subsequent lateral subduction in the early Neoarchean. The buoyant oceanic plateau was possibly dragged to the trench and blocked subsequent subduction and led to a ∼40 Myr magmatic hiatus between 2.60 Ga and 2.56 Ga.

Tectonic evolution of Southwest Japan at the Cretaceous time inferred from the zircon U–Pb geochronology along the Maana Belt, western Shikoku, Japan

The Mesozoic geological history of proto-Japan holds a crucial role in understanding the tectonic evolution during this time in East Asia, as a whole. In this context, the evolutionary history of the Median Tectonic Line (MTL) is important but still under debate. Most of the studies on the MTL were carried out on well-preserved clear zonally distributed areas such as the central Shikoku Island, Southwest Japan. However, these clear zonal arrangements are absent and several tectonic units including the MTL appear in close proximity at the westernmost Shikoku Island and the Maana Belt occurs here in a restricted area. Such unclear arrangements of lithounits pose a difficulty for a comprehensive understanding of the large-scale tectonic evolution of Southwest Japan. Hence, the study on and along the Maana Belt is crucial to unravel the holistic tectonic evolution of the MTL. The zircon age dating of the tonalites, geochemically of volcanic arc affinity, revealed 470.6 ± 3.2 and 468.3 ± 2.7 Ma as a magmatic age, which confirmed them as members of the Kurosegawa tectonic zone. In contrast, the scarce chemically-altered leucocratic mylonite shows the zircon age of 151.8 ± 1.8 Ma as a possible timing of magmatism. This may be the exotic block because its age is known as a magmatic hiatus in Southwest Japan. The detrital zircon dating of three (meta-)sedimentary rocks along the Maana Belt confirmed that they can be attributed to the Northern Chichibu Belt or the Kurosegawa tectonic zone. Newly characterized rock unit (= the Northern Chichibu Belt with the Kurosegawa tectonic zone) is in contact with the mid-Cretaceous igneous province of the Oshima metamorphic rocks. The suggested juxtaposition in the present study area is a remnant of the original settings including the initial phase of the MTL but is completely disrupted by the exhumation of the Sambagawa Belt in major parts of the Shikoku Island. Identifying this original setting provides not only the initial arrangement of different lithounits but also a clear understanding of the preexisting large-scale tectonic modification systems at the margin of the East Asian continent.

Re-initiation of plutonism at the Gondwana margin after a magmatic hiatus: The bimodal Permian-Triassic Longwood Suite, New Zealand

The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261–243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 ± 2.5 Ma, 256.0 ± 1.8 Ma, 247.8 ± 2.7 Ma and 243.2 ± 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/YbN < 10), unradiogenic 87Sr/86Sr(t) (0.7029 to 0.7032) and radiogenic ε143Nd(t) (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (δ146/144Nd = 233 ppm) compared to global plutonic rocks (δ146/144Nd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km2/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite.

Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan: initiation of Cretaceous high-T metamorphism at eastern margin of Eurasia

ABSTRACT We present Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan. The existence of the partially melted rock in the complex may imply initiation of long-lived high-T metamorphism in proto-southwest Japan at Cretaceous eastern margin of Eurasia. The gneisses are composed of three garnet-bearing layers and one felsic lens, and they show mylonitic foliation. Application of Zr-in-rutile geothermometer and quartz-in-garnet Raman geobarometer for inclusion rutile and quartz in garnet provided prograde condition of 670–692°C and 1.01–1.26 GPa and peak condition of 755–769°C and 1.17–1.38 GPa. These conditions correspond to those of lower crustal depths between volcanic arc and forearc where thermal advection by wedge mantle corner flow occurs. Presence of the felsic lens whose mineral assemblage can undergo partial melting under the peak condition and presence of a crystallized melt inclusion in garnet suggest that distinct partial melting occurred during the metamorphism. The concordant U–Pb ages of metamorphic zircon rims are ranging from 131.5 ± 4.3 to 120.1 ± 3.9 Ma. The mean square weighted deviation (MSWD) of the metamorphic age was as large as 6.4, implying a significant duration of high-T condition for growth of metamorphic zircon. The obtained ages are within the period of the magmatic hiatus that has previously proposed around southwest Japan and Korean Peninsula (ca. 160–120 Ma) that is due to stop of wedge mantle corner flow by flatting of subducting slab. Our results suggest that high-T metamorphism associated with partial melting at lower crustal depths along Early Cretaceous proto-southwest Japan has already been initiated at the late stage of the hiatus (ca. 130 Ma). This may be due to slab rollback and trench retreat at eastern margin of Eurasia.

Late Cretaceous to Paleocene magmatic record of the transition between collision and subduction in the Western and Central Cordillera of northern Colombia

Deformational field relations, compositional and U–Pb geochronological data of magmatic rocks of the northwestern Colombian Andes provide insights to recognize the different pre to post-collisional stages in an arc-arc collisional scenario until the re-initiation of a new subduction zone. A western belt of undeformed to mylonitized volcanic rocks with flat REE patterns and a weak LREE depletion present similar geochemical features of adjacent oceanic plateau basalts that were intruded by arc-related dykes at ca. 82 Ma, before their accretion to the continental margin. A sequence of greenschist and quartz + muscovite + graphite schist, and their associated mylonitized dikes, are part of the eastern continental arc, and present ages between 69 and 71 Ma, show tholeiitic affinity, flat rare-earth patterns, and Nb–Ti anomalies. These characteristics mark a decrease in the mantle wedge and older crustal signatures during magma genesis, as the oceanic-continent collision was advancing. Both suites were metamorphosed in a high geothermal gradient with temperatures between 490 and 400 °C and pressures between 2 and 4 Kbar. After this metamorphic event associated to the end of the collision a series of dykes intrude the different units at ca. 58 Ma, including basalts and andesites with Nb–Ti anomalies, adakite-like, and high niobium compositional signatures, suggesting that they were probably formed associated with slab melting in a young subduction environment that evolved after 62 Ma, following a ≤7 Ma magmatic hiatus. This magmatic and metamorphic record can be related to an arc-arc collision scenario in which mantle involvement in the late stages of collision influence both the magmatic and thermal regimes.

New constraints on tectonism and magmatism from the eastern sector of the Trans-Mexican Volcanic Belt (Chignahuapan Horst, Puebla, México)

The Quaternary Acoculco (<2.7 my) and Los Humeros (<46 ky) calderas, located in the eastern sector of the Trans-Mexican Volcanic Belt (TMVB), are separated by a regional structure known as the Chignahuapan Horst (ChH). The tectonic and magmatic evolution of the ChH comprises four stages: 1) Upper Cretaceous-Paleocene Laramide orogeny, related to the deformation of sedimentary successions; 2) a sedimentary hiatus followed by the initial stage of the TMVB (Middle Miocene); 3) a Miocene NE-SW oriented extension responsible for the NW-SE normal faults that controlled the volcanic processes and the emplacement of calc-alkaline magmas that formed the Aquixtla sequence; 4) a magmatic hiatus prolonged until 1.6 ± 0.15 my; 5) Pliocene-Quaternary normal faults controlled the emplacement of tholeiitic-transitional magmas and the monogenetic volcanism distribution that formed the Apizaco-Chignahuapan sequence.Coevally to the Miocene NE-SW and Pliocene-Quaternary NW-SE tectonic extensional phases, NE-SW and NW-SE transfer faults were developed. NE-SW normal faults and NW-SE strike-slip faults correspond to fault systems optimally oriented with the current stress field, thus, these faults can be considered as potentially active structures controlling magma emplacement and the distribution of volcanic structures at the surface.

Detrital Zircon U-Pb Geochronology and Hf Isotope Geochemistry of the Hayang Group, SE Korea and the Himenoura and Goshoura Groups, SW Japan: Signs of Subduction-Related Magmatism after a Long Resting Period

There was a hiatus in magmatism in Korea and Japan, located on the eastern continental margin of Asia, during a period of about 40 Ma from 160 Ma to 120 Ma. The cause of the resumption of magmatism since then is not yet well understood. In this study, we analyzed the Hf isotope composition of detrital zircons in the Cretaceous sediments of Korea (Hayang Group) and Japan (Goshoura and Himenoura groups) to investigate the tectonic evolution of eastern Asia in the Early Cretaceous period. εHf(t) in Cretaceous zircons from Japanese samples values from +8.2 to +0.1, suggesting that magmatism was sourced from the depleted juvenile materials, which is compatible with ridge subduction and subsequent melting of the young oceanic crust. εHf(t) values from Cretaceous zircons in the Hayang Group are negative, except for the Jindong Formation, which had a sediment supply from Japan, indicating that the old continental crust material of the Korean Peninsula was included in the magma generation. The detrital zircons of this study exhibit a depleted isotopic character at the beginning of subduction-related magmatism in Permian and Early Cretaceous, and then gradually change to a more enriched composition. This trend may be a typical example of the Pacific-type orogenic cycle.

Origins and tectonic implications of Late Cretaceous adakite and primitive high-Mg andesite in the Songdo area, southern Lhasa subterrane, Tibet

Late Cretaceous igneous rocks in the southern Lhasa subterrane, Tibet, include primitive high-Mg andesites and adakites, which provide important constraints on the tectonic evolution of the Neo-Tethys Ocean. Here, we present detailed zircon UPb and Hf isotopic and whole-rock geochemical data for granodioritic and dioritic porphyry samples from the Songdo area in the southern Lhasa subterrane. Zircon UPb dating indicates that the granodiorite crystallized at 88 Ma, whereas the diorite yields ages of 68 and 66 Ma. The granodiorite has adakite-like geochemical characteristics, including high Sr (801–1005 ppm) and low Y (6.8–15.2 ppm) and Yb (0.6–1.3 ppm) concentrations, and high Sr/Y (62–145) and La/Yb (39–93) ratios. We infer that the adakitic granodiorites formed through partial melting of subducted oceanic crust. The dioritic porphyry has intermediate moderate SiO2 (53–58 wt%) and high MgO (5.6–8.2 wt%) contents, and high Mg# (66.4–69.5) values, and is therefore classified as a primitive high-Mg andesite that was derived from interaction between subducted sediment and mantle. The presence of coeval adakite and charnockite, as well as high-temperature granulite-facies metamorphism, indicates that mid-ocean ridge subduction occurred at 100–80 Ma, followed by a 10 Myr hiatus in magmatism and subsequent rollback of the Neo-Tethys slab at 68 Ma. These processes resulted in significant crustal growth within the Lhasa terrane.

Craton Destruction 2: Evolution of Cratonic Lithosphere After a Rapid Keel Delamination Event

AbstractCratonic lithosphere beneath the eastern North China Craton has undergone extensive destruction since early Jurassic times (approximately 190 Ma). This is recorded in its episodic tectonic and magmatic history. In this time, its lithosphere changed thickness from approximately 200 km to &lt;60 km. This change was associated with a peak time (approximately 120 Ma) of lithospheric thinning and magmatism that was linked with high surface heat flow recorded in rift basins. We believe that these records are best explained by a two‐stage evolutionary process. First, approximately 100 km of cratonic “keel” underlying a weak midlithospheric discontinuity layer (approximately 80–100 km) was rapidly removed in &lt;10–20 Ma. This keel delamination stage was followed by a protracted (approximately 50–100 Ma) period of convective erosion and/or lithospheric extension that thinned the remaining lithosphere and continuously reworked the former cratonic lithospheric mantle. This study focuses on numerical exploration of the well‐recorded second stage of the eastern North China Craton's lithospheric evolution. We find that (1) lithospheric mantle capped by thick crust can be locally replaced by deeper mantle material in 100 Ma due to small‐scale convective erosion; (2) asthenospheric upwelling and related extension can replace lithospheric mantle over horizontal length scales of ~50–150 km, and account for observed “mushroom‐shaped” low‐velocity structures; (3) modeling shows conditions that could lead to the multiple eastern North China Craton magmatic pulses between 190 and 115 Ma that are associated with temporal and spatial changes in magma source petrology and a magmatic hiatus; and (4) a “wet” midlithospheric discontinuity layer provides a potential source material for on‐craton magmatism.

Elucidating tectonic events and processes from variably tectonized conglomerate clast detrital geochronology: Examples from the Permian Hongliuhe Formation in the southern Central Asian orogenic Belt, NW China

AbstractThis article expands upon detrital zircon geochronology with a sampling and analysis strategy dating variably tectonized granitoid conglomerate clasts. Its purpose is to elucidate details of the provenance's tectonomagmatic history from deformation‐relative age distributions. The method involves bulk samples of clasts, sorted based on the degree of internal ductile deformation. Isolating granitoid clasts, we divide them into three subsets: undeformed, slightly deformed, and deformed. Laser ablation, inductively coupled plasma mass spectrometry U‐Pb geochronology is performed on zircon separates of each subset. Our case study, involving the Permian Hongliuhe Formation in the southern Central Asian Orogenic Belt, analyzes each of the three clast subsets, as well as sandstone detrital samples, at three stratigraphic levels and yields a profile of the unroofed provenance. Clast age distributions exhibit different, wider‐age spectra than sandstone samples, an effect of proximity to the respective provenance. They indicate a minimal lag time, implying rapid exhumation rates, whereas sandstone data alone would indicate a 90 Myr lag time. Early Paleozoic arc‐building episodes appear as Ordovician peaks in sandstone data and Silurian‐Devonian peaks in clast data, indicating a younging of magmatism toward proximal provenance. A magmatic hiatus starts in the Devonian, correlating with the latest age of deformed clasts, interpreted as timing of collisional tectonics. The detailed age spectra provide regional tectonic context and interpretation of processes, as well as more robust provenance interpretation than could be determined from sandstone samples alone. The variably tectonized clast detrital geochronology method removes human sampling bias and the practical limits of studying regional granitoid distributions.

Geochemical evidence for provenance of Ordovician cherts in southeastern Australia

Early to Middle Ordovician cherts of the Girilambone and Adaminaby groups are widespread in the Lachlan Orogen in central New South Wales. Their ages are well constrained biostratigraphically by conodonts ranging from the late Tremadocian to earliest Sandbian. Broadly contemporaneous cherts are exposed in the Narooma terrane (latest Cambrian to Darriwilian), and in allochthonous blocks (of late Middle and Late Ordovician age) of the New England Orogen at Port Macquarie. In the Kiandra–Tumut region of southern NSW Darriwilian to earliest Gisbornian cherts are interbedded with volcaniclastics of the Macquarie Volcanic Province. To determine their provenance and compare depositional settings, 60 chert samples representative of these regions were analysed for selected major, trace and rare earth elements (REE). Al2O3/TiO2 ratios enable recognition of two dominant sources from which the sediment fraction of the cherts was derived at different times in the evolution of the Tasmanides, one indicative of old continental crust and the other sourced from a juvenile continent or plateau. All cherts analysed, regardless of geological province, carry continental margin signatures demonstrated by high Al2O3/Fe2O3 ratios, LREE enrichment, small negative Ce anomalies, prominent negative Eu anomalies, low total REEs and near-chondritic Y/Ho ratios. The late Cambrian and Early Ordovician Narooma terrane cherts display a clastic component derived chiefly from a juvenile continent or plateau, whereas cherts of the Hermidale and Albury-Bega terranes contain detritus of mixed origin. During the Early to Middle Ordovician magmatic hiatus in the Macquarie Volcanic Province, all cherts regardless of tectonic affiliation incorporate terrigenous detritus solely of Gondwana origin. Upon resumption of magmatism in the Macquarie Volcanic Province, a mixed terrigenous source is recorded in all analysed cherts, before a dominant Gondwana source is re-established in the late Darriwilian for cherts of the Hermidale, Albury-Bega and Kiandra regions.

Geochronology and geochemistry of the Parashi granitoid, NE Colombia: Tectonic implication of short-lived Early Eocene plutonism along the SE Caribbean margin

The Parashi granitoid of northeasternmost Colombia intrudes the Upper Cretaceous to Lower Paleocene accretionary complex formed by the collision of the Caribbean arc and the continental margin of South America. This granitoid presently separated of the continental margin includes a major quartzdiorite body with andesite to dacite dikes and mafic enclaves. Zircon U–Pb LA-MC-ICP-MS and K–Ar geochronology on the quartzdiorite and the dikes suggest that crystallization extended from ca. 47 to 51 Ma. Major and trace elements are characterized by a medium-K, immature continental arc signature and high Al2O3, Na2O and Ba–Sr contents. Initial 87Sr/86Sr isotopic values range between 0.7050 and 0.7054, with 143Nd/144Nd = 0.51235–0.51253, εNd and εHf values from −0.81 to −4.40 and −4.4 and −5.2. Major and trace element ratios and isotopic modeling suggest that sedimentary and/or quartzofeldspathic crustal sources were mixed with a mafic melt input. The petrotectonic and geological constraints derived from this granitoid suggest that Parashi plutonism records an immature, oblique subduction-zone setting in which the presence of a high-temperature mantle realm and strong plate coupling associated to upper crust subduction caused the partial fusion of a previously tectonically underplated mafic crust and associated metasediments exposed in the continental margin. The limited temporal expression of this magmatism and the transition to a regional magmatic hiatus are related to a subsequent change to strongly and slow oblique tectonics in the Caribbean–South America plate interactions and the underflow of a relatively thick slab of Caribbean oceanic crust.

Geochemistry of Tertiary Igneous Rocks of Northern Luzon, Philippines: Evidence for a Back-Arc Setting for Alkalic Porphyry Copper-Gold Deposits and a Case for Slab Roll-Back?

Oligocene to early Miocene volcanic rocks are preserved in the Central Cordillera range and Cagayan Valley of northern Luzon, Philippines. Basaltic and andesitic rocks of the Pugo Formation in the Baguio district of the Central Cordillera were intruded by the ~27 to 20 Ma calc-alkaline Central Cordillera Diorite Complex. In the southern Cagayan Valley the subalkaline to alkaline late Oligocene Mamparang Formation overlies the Cretaceous Caraballo Formation and has been intruded by the Didipio Igneous Complex, the Cordon Syenite Complex, and the Palali batholith. The Didipio complex comprises an early suite of diorites, which were intruded by the strongly mineralized stocks of the Dinkidi Cu-Au porphyry deposit. Whole-rock geochemical data for intrusive and extrusive rocks of the Baguio district range from low K calc-alkaline to shoshonitic basalts to dacites with rare earth element (REE) and high field strength element (HFSE) characteristics of suprasubduction zone magmas and are all interpreted to have been sourced from the same parent melt. Samples from Didipio display higher alkali contents but similar trace element characteristics. New age dates for the Didipio area range from 25.7 to 24.8 Ma. The potassic magmas of the Cagayan Valley are interpreted to have formed in a back-arc coeval to the mainarc sequence that is preserved in the Baguio Miocene rocks. This contradicts earlier models, which invoke an early Miocene arc reversal in the northern Luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes. The lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back-arc magmas in the east in northern Luzon is best interpreted as the result of eastward subduction since the late Oligocene. The presence of ~20 Ma adakitic magmas in the Baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back-arc rifting.

K–Ar age and geochemistry of the SW Japan Paleogene cauldron cluster: Implications for Eocene–Oligocene thermo-tectonic reactivation

Systematic K–Ar dating and geochemical analyses of Paleogene cauldrons in the Sanin Belt of SW Japan have been made to explore the relationship between the timing of their formation and the Paleogene subduction history of SW Japan documented in the Shimanto accretionary complex. We also examine the magma sources and tectonics beneath the backarc region of SW Japan at the eastern plate boundary of Eurasia. Fifty-eight new K–Ar ages and 19 previously reported radiometric age data show that the cauldrons formed during Middle Eocene to Early Oligocene time (43–30 Ma), following a period of magmatic hiatus from 52 to 43 Ma. The hiatus coincides with absence of an accretionary prism in the Shimanto Belt. Resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench, as a common tectono-thermal event within a subduction system. The cauldrons are composed of medium-K calc-alkaline basalts to rhyolites and their plutonic equivalents. These rocks are characterized by lower concentrations of large ion lithophile elements (LILE) including K 2O, Ba, Rb, Th, U and Li, lower (La/Yb) n ratios, lower initial Sr isotopic ratios (0.7037–0.7052) and higher ε Nd( T) values (−0.5 to +3.5) relative to Late Cretaceous to Early Paleogene equivalents. There are clear trends from enriched to depleted signatures with decreasing age, from the Late Cretaceous to the Paleogene. The same isotopic shift is also confirmed in lower crust-derived xenoliths, and is interpreted as mobilization of pre-existing enriched lithospheric mantle by upwelling depleted asthenosphere. Relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the SW Japan arc-trench system during the Eocene to Oligocene. Newly identified Late Eocene low silica adakites and high-Mg andesites in the Sanin Belt and Early Eocene A-type granites in the SW Korea Peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate. The backarc region was an extensional tectonic setting, and some Paleogene rift basins and Sanin Belt cauldrons occur in linear arrays. The Eocene–Oligocene Sanin-SE Korea continental arc lies on the NE extension of the East China Sea Basin, the initial stage of which probably formed by continental arc rifting. This rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab, in response to decreased convergence rate between the Pacific and Eurasian plates.

Age and tectonic implications of Paleoproterozoic mafic dyke swarms for the origin of 2.2 Ga enriched lithosphere beneath the Ungava Peninsula, Canada

Mafic dyke swarms represent short-lived magmatic events that carry important temporal and chemical constraints for the evolution of the lithospheric mantle. Five baddeleyite U–Pb isotopic analyses of Paleoproterozoic mafic dyke swarms intruding the Ungava Peninsula yielded ages of 2508 ± 6, 2220 ± 1, 2212 ± 3, 2199 ± 5 and 2149 ± 3 Ma. These ages fall within the 2.5–2.0 Ga period previously defined by five other swarms, which together comprise a magmatic record of more than 500 Ma. The similar age, composition and surface area of two 2.51 Ga swarms, along with the lack of coeval supracrustal rocks, suggest they correspond to failed rifts. An important magmatic hiatus of 280 Ma follows, but ends with the intrusion of numerous dykes between 2.23 and 2.20 Ga in the thinnest lithosphere of the northern Ungava Peninsula. These dykes exhibit chemical signatures that vary from enriched older 2229–2212 Ma dykes ( ɛNd ( t) < +0.4, La/Yb > 4) to relatively depleted 2209–2199 Ma voluminous younger dykes ( ɛNd ( t) > +1.4, La/Yb < 4). The depleted character, great volume, and occurrence of these younger dykes in a thin lithosphere may represent decompression melting of the asthenosphere, following delamination of the Archean lithospheric keel ca. 2.21–2.20 Ga. Later dykes emplaced ca. 2.17 Ga within a rejuvenated Paleoproterozoic lithosphere also have depleted chemical compositions, and map the extent of rifting coeval with the emplacement of early basalts in the Labrador Trough. Dykes emplaced in the 2.15–2.00 Ga period further to the South are proximal to coeval continental basalts, with which they share enriched isotopic compositions ( ɛNd ( t) = −1.5 to −7) and a positive correlation between La/Yb and Zr/Nb (La/Yb = 4–10; Zr/Nb = 7–20) that suggest assimilation of an Archean crustal component. A progressive decrease in the ɛNd ( t) values of these magmas with decreasing age further implies increasing contamination, until the Archean lithosphere failed ca. 2.0 Ga and permitted the eruption of isotopically depleted basalts. The ca. 2.2 Ga depleted northern dykes and the 2.1–1.9 Ga Povungnituk basalts of the Cape Smith foldbelt share a negative correlation between Zr/Nb and La/Yb that is expected for the incorporation of an enriched lithospheric component. This component is represented by the alkaline rocks comprised in the Circum-Ungava supracrustal belts (carbonatites, lamprophyric lavas, alkaline basalts) and by the Lac Aigneau lamprophyric dykes (1941 ± 3 Ma) intruding the Ungava Peninsula. These alkaline magmas collectively have 2.2–2.1 Ga depleted-mantle Nd model ages ( T DM) similar to a 2.15 Ga Sm–Nd isochron defined by the basaltic rocks of the supracrustal belts. Although these Nd T DM ages and isochron are 100–300 Ma older than magmatic ages, they are identical with the ca. 2.2 Ga mafic dykes of the Ungava Peninsula. This time correlation, along with the similar chemical signature of the northern dykes and younger Povungnituk basalts, suggest that the composition of younger lavas and alkaline magmas was influenced by an enriched component associated with the pervasive metasomatism of the lithosphere, coeval with the emplacement of the many ca. 2.2 Ga dykes.