Slab Break-off Event Research Articles

The Manhattan Schist, New York City: Proposed Sedimentary Protolith, Age, Boundaries, and Metamorphic History

There are some persistent basic questions pertaining to the bedrock schist of New York City (NYC). How many mappable schist formations are exposed in NYC, and what was the sedimentary protolith of the Manhattan schists? Our proposed answers are based in part on a blending of published paleontological and radiometric dating results that constrain the timing of Taconic subduction and the best choice of a pelitic protolith for the schists of NYC. We have chemically analyzed some samples of schist and shales at key locations to evaluate the plausibility of our proposals. The compelling published evidence indicates that the Taconic Orogeny began about 475 Ma, when peri-Laurentian plates began the process of east-dipping subduction under the Moretown Terrane, resulting in a magmatic flareup of the Shelburne Falls arc that carried the Moretown Terrane west across NYC. East-dipping subduction accounts for early Ordovician metamorphism until an oceanic slab break-off event at about 466 Ma. Our review of the biostratigraphic data indicates a continuation of subduction and the deposition of pelitic sediments until about 455 Ma, during the transition to deep-water turbiditic sediment deposition. This disqualifies all post-455 Ma turbidites as viable protoliths for the NYC Manhattan schists but does include the Late Cambrian to lowermost Late Ordovician pelites of the Jutland Sequence that are exposed directly west of NYC in New Jersey. Our new chemical analyses of Jutland sediments and each of the three named schists from the NYC plot as a single geochemical population. We, therefore, propose that the schists of NYC could collectively be referred to as the Manhattan schist of the Late Cambrian to lower Late Ordovician.

Geochemical and geochronological evidence of a Neoproterozoic island-arc on the easternmost Sierras Pampeanas. New U-Pb LA-ICP-MS data from amphibolites and metacarbonate depositional sequences

The first basement outcrops west of the Río de La Plata craton are metabasaltic and metasedimentary units of the Sierra Chica belt, part of the Eastern Sierras Pampeanas. These are orthoamphibolites and subordinate garnet-biotite gneisses and metacarbonate sequences that are intruded by Cambrian calc-alkaline and peraluminous rocks of the Pampean Orogeny. In accordance with one of the leading tectonic models for the region, new provenance data from this research suggest that the metabasaltic-metasedimentary association represents the proximal deposits of a Late Neoproterozoic island-arc. This contribution brings new geochemical and geochronological evidence from orthoamphibolites located on different sites along the Sierra Chica belt, which are interbedded with metacarbonate and gneisses outcrops, showing tholeiitic compositions, island-arc related tectonic settings, and Cryogenian to Ediacaran crystallization ages. Moreover, silicate-rich metacarbonates interbedded within the same sequences yielded a maximum depositional age of 569 Ma, reinforcing the assumed age for the sequence. All things considered, these rocks are interpreted as old basic lava flows and gabbro dikes linked to an island-arc activity, interspersed within the carbonatic and siliciclastic shelf sediments, together constituting the Neoproterozoic Córdoba Arc from the Sierra Chica belt. A depositional analogy with the present-day Caribbean island-arcs deposits suggests that these sequences could have been part of Oceanic Arc Depositional Systems (OADS) during the diachronous closure of the Goiás ocean and the final assembly of West Gondwana. Overlapping thermo-tectonic processes associated with the Pampean Orogeny are ubiquitous throughout all the studied outcrops, as evidenced by the petrography, the zircon morphology, and the U-Pb results, indicating a complex scenario towards a Late Cambrian slab break-off event.

Terminal stage of divergent double subduction: Insights from Early Cretaceous magmatic rocks in the Gerze area, central Tibet

Divergent double subduction (DDS) plays an important role in facilitating tectonic processes such as ocean closure, accretion and amalgamation of magmatic arcs, and continental growth. However, DDS geodynamics remain poorly constrained, especially its evolution and the accompanying magmatism at the terminal stage. According to geological and geophysical data, the Meso-Tethys Ocean (also known as the Bangong–Nujiang Tethyan Ocean) in central Tibet has been considered a Mesozoic example of DDS, which provides a rare opportunity to evaluate the geodynamic processes of the terminal stage of a DDS system. Here, we present new geochronological, geochemical, and isotopic data of volcanic rocks from the Gerze area in the western segment of the Bangong–Nujiang suture zone (BNSZ). Combined with previous data, we found that the ca. 105 Ma magmatic rocks in the Gerze area are distributed not only along the two sides of the BNSZ but also as manifold types. On the northern side, the magmatism ca. 105 Ma varied, including A-type rhyolites, bajaitic latites, high-Nb basalts, N-MORB-type basalts, adakites and bimodal volcanic rocks, which originated from sources including a sinking oceanic slab, asthenospheric mantle, metasomatized mantle and overlying continental crust. The lithology on the southern side only includes A-type rhyolite and magnesia-rich andesite and adakitic intrusions. Spatial and compositional variations in ca. 105 Ma magmatism should have been the response to slab breakoff, which may have occurred only in the northern branch of the Meso-Tethys oceanic slab after ocean closure. Our results not only verify the existence of slab breakoff in the classical DDS model but also emphasize that breakoff can cause upwelling of the subslab asthenosphere, which leads to a complex magmatic response. • Manifold magmatism from the Gerze area, central Tibet, is active at ca. 105 Ma. • The ca. 105 Ma Gerze magmatic rocks record a slab breakoff event. • Slab breakoff plays an important role in the divergent double subduction system.

Catastrophic slab loss in southwestern Pangea preserved in the mantle and igneous record

The Choiyoi Magmatic Province represents a major episode of silicic magmatism in southwestern Pangea in the mid-Permian-Triassic, the origin of which remains intensely debated. Here, we integrate plate-kinematic reconstructions and the lower mantle slab record beneath southwestern Pangea that provide clues on late Paleozoic-Mesozoic subducting slab configurations. Also, we compile geochronological information and analyze geochemical data using tectono-magmatic discrimination diagrams. We demonstrate that this magmatic event resulted from a large-scale slab loss. This is supported by a paleogeographic coincidence between a reconstructed 2,800-3,000-km-wide slab gap and the Choiyoi Magmatic Province and geochemical data indicating a slab break-off fingerprint in the latter. The slab break-off event is compatible with Permian paleogeographic modifications in southwestern Pangea. These findings render the Choiyoi Magmatic Province the oldest example of a geophysically constrained slab loss event and open new avenues to assess the geodynamic setting of silicic large igneous provinces back to the late Paleozoic.

Lithospheric Structure of Eastern Tibetan Plateau from Terrestrial and Satellite Gravity Data Modeling: Implication for Asthenospheric Underplating

Abstract The lithosphere of the eastern Tibetan plateau is underlain by a low-velocity zone at shallow depths which is interpreted as asthenospheric material in the upper-most mantle in various seismic tomography studies. The driving mechanism for the presence of asthenospheric material in the upper-most mantle is not well understood, and the spatial extent of the asthenospheric material is not well delineated. We use 2.5D gravity models to assess what drove the asthenospheric flow upwards in the past and determine the lateral extent of the asthenospheric material in the upper-most mantle. The models also allow us to determine the Indian slab configuration below the Tibetan plateau. The gravity models show that lithospheric thickness increases from ~120 km in the central and eastern parts of the plateau to ~150 km in the west, indicating that the lithosphere in the central and eastern parts of the plateau may have been delaminated. The ~30 km shallower Lithosphere-Asthenosphere Boundary in the central and eastern Tibetan plateau may indicate that asthenospheric flow could have been induced in the past by a combination of lithospheric delamination and a slab break-off event of the Greater Indian slab. The spatial extent of the asthenospheric material in the upper-most mantle beneath the Tibetan plateau is ~15,000 km2 (N−S length=500 km and thickness=30 km) between 85°E and 88°E, which could even extend east of 92°E. The Indian slab is dipping more steeply in the east. The slab dip along the Indian plate increases from ~10° in the west to ~18° in the central (~87°E) and ~25° in the eastern part (~91°E) of the plateau, indicating that the style of lithospheric deformation changes from underthrusting to slab roll-back from west to east.

Tracking the timing of Neotethyan oceanic slab break-off: Geochronology and geochemistry of the quartz diorite porphyries, NE Turkey

The initiation of the break-off of the northern branch of the Neotethyan oceanic lithosphere is an important but poorly understood event in the geology of the Sakarya Zone (SZ) in northeastern Turkey. Although it is well-known that Latest Cretaceous intrusives (~70 Ma) and early Eocene adakitic magmatic rocks are present in the eastern SZ, the outcrops of the early Eocene non-adakitic rocks are very limited, and their tectono-magmatic evolution has not been studied. We describe a small outcrop of non-adakitic quartz diorite porphyry in the Kov area of the Gümüşhane region in northeastern Turkey. The genesis of these porphyries is significant in evaluating the syn- to post-collision-related magmatism. The LA-ICP-MS zircon U-Pb dating revealed that the Kov quartz diorite porphyries (KQDP) formed at ca. 50 Ma, coeval with adakitic rocks, and ~20 Myr later than the slab roll-back-related intrusive rocks. The KQDPs are calc-alkaline in composition and enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti), with significant negative anomalies of Nb, Ta, and Ti but positive anomalies of Th, U, and Pb. Isotopic compositions of the samples show limited range of variation and slight enrichment of 87Sr/86Sr(t) (0.70489 to 0.70555), εNd(t) (−1.4 to −1.2) with TDM of 1.11 to 1.61 Ga. Pb isotopic ratios of the samples point to an enriched mantle source. They were likely crystallized from the melt that originated from an EM2-type spinel-facies subcontinental lithospheric mantle (SCLM), followed by the fractionation with insignificant crustal assimilation. The SCLM was metasomatically enriched, and the metasomatic agent was likely H2O-rich fluids rather than sediments released from subducting oceanic crust during the Late Cretaceous closure of the Neotethyan oceanic lithosphere. In conjunction with the geological background and previous data, we propose that the generation of the KQDPs resulted from a slab break-off event that caused ascending or infiltration of hot asthenosphere, triggering mantle melting. Such sporadic occurrences of the KQDPs, with coeval adakitic rocks in the SZ, are likely associated with the onset of extensional tectonics due to the earlier stage of slab break-off along the region during the early Eocene period.

Petrogenetic and tectonic controls on magma fertility and the formation of post-subduction porphyry and epithermal mineralization along the late Cenozoic Anatolian Metallogenic Trend, Turkey

The late Cenozoic Anatolian Metallogenic Trend in Turkey is a manifestation of westerly Aegean subduction and easterly Arabian continental collision along the Western Tethyan Orogenic Belt. The 1500 × 200 km late Cenozoic Anatolian magmatism hosts many gold-rich porphyry and epithermal deposits and prospects (~ 27 Moz Au) that formed during the tearing of the Aegean slab and Arabian slab break-off events that initiated at 15 Ma in western Anatolia and 25 Ma in eastern Anatolia. Although the temporal and spatial correlations between slab segmentation, late Cenozoic magmatism, and associated gold-rich porphyry and epithermal mineralization has been intuitively proposed, the genetic relationships between asthenospheric upwelling, subcontinental lithospheric mantle instability, and magma fertility have not been well established. Herein, new geochemical and radiogenic Sr, Nd, and Pb isotope data from Miocene to Pliocene magmatic rocks that are genetically related to porphyry, epithermal, and skarn systems from western, central, and eastern Anatolia are presented and interpreted with previously published geochemical data from late Cenozoic intrusion-related mineral systems throughout Anatolia. The magma fertility of the late Cenozoic post-subduction igneous suites along the late Cenozoic Anatolian Metallogenic Trend is assessed and is best indicated by the SiO2 (61.8 ± 5.5 wt%) content, Sr/Y (> 20) and Ba/Ta ratios (> 428), and (eNd)i values (< 0). Fertile magmatism dominantly resulted from the melting of the metasomatized Anatolian SCLM and was accompanied by amphibole ± clinopyroxene and plagioclase fractionation in the absence of garnet residue as indicated by low (Dy/Yb)N and Eun/Eu* ratios. Magma fertility indicators evolved through time along the late Cenozoic Anatolian Metallogenic Trend. In western Anatolia, fertile magmatism peaked in the middle to late Miocene (15–9 Ma) and was preceded by less-fertile early Miocene magmatism (21–18 Ma) and followed by barren Pliocene to Quaternary, alkaline lavas. The geochemical signatures of mineralized igneous units in central and eastern Anatolia indicate that fertile magmas formed early as regional magmatism evolved towards less hydrous, less oxidized, shallower, and mantle-dominant signature. The reduction of magmatic fertility since the late Miocene throughout Anatolia was caused by the upwelling of hot, dry asthenosphere through the Aegean slab tear and Arabian slab break-off, SCLM instability, and subsequent removal in absence of an over-thickened crust. The spatial migration of slab ruptures and slab gap opening modified mantle fluxes, reduced the SCLM thickness and volume of fusible material, and caused the migration of melting source. Thus, the segmentation of the Southern Neotethyan oceanic slab triggered the metallogenic events along the Anatolian Trend but subsequently contributed to the cessation of production of fertile magma.

Timing of two separate granulite-facies metamorphic events in the Helanshan complex, North China Craton: Constraints from monazite and zircon U–Pb dating of pelitic granulites

This study presents the results of monazite and zircon U–Pb dating of garnet–biotite gneiss and recently discovered spinel-bearing ultrahigh-temperature (UHT) pelitic granulites from the Paleoproterozoic Helanshan complex of the North China Craton, and considers implications for the formation of the complex and the tectonic history of this region. SHRIMP monazite U–Pb dating of the garnet–biotite gneiss yielded a weighted mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma, which is the same within error as a weighted mean laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) 207Pb/206Pb age of 1959 ± 18 Ma obtained from metamorphic zircons within the same unit. The age of 1959 ± 18 Ma is similar to the published metamorphic ages of ca. 1.96–1.95 Ga from high-temperature (Tmax < 900 °C) pelitic granulites in the Helanshan, which represent the timing of continental collision between the Yinshan and Ordos terranes as suggested by previous published works. Metamorphic monazites from the spinel-bearing UHT pelitic granulite unit in this area yielded weighted mean 207Pb/206Pb ages of 1930.8 ± 2.6 and 1933.6 ± 3.1 Ma, which are consistent with the ages of metamorphic zircons. These ages record the timing of UHT metamorphism within the Helanshan complex and are similar to 1.93–1.92 Ga ages previously reported from typical UHT pelitic granulites within the Jining and Daqingshan complexes of the Khondalite Belt, where ca. 1.92 Ga metamorphic zircons are thought to record the cooling of the UHT rocks to the solidus. The new data presented here indicate that the ca. 1.93 Ga UHT metamorphic event occurred across a wider area than previously thought and extended into the Helanshan complex. Combining these new data with the results of previous research suggests that the entire Khondalite Belt was influenced by a ca. 1.93 Ga UHT metamorphic event that was preceded by metamorphism associated with continental collision between the Yinshan and Ordos terranes at ca. 1.95 Ga. This suggests that the Khondalite Belt underwent a tectonic transition from compression to extension between 1.95 and 1.93 Ga, a process likely controlled by a late-stage shallow slab-breakoff event.

Gabbroic–dioritic dykes from the Sanandaj–Sirjan Zone: windows on Jurassic and Eocene geodynamic processes in the Zagros Orogen, western Iran

The Sanandaj–Sirjan Zone (SaSiZ) is a magmatic terrane within the Zagros Orogen, western Iran, marking the Tethyan suture zone between the Afro-Arabian Plate and the Central Iran Micro-Continent. Mafic–intermediate dyke swarms with Middle Jurassic (Group 1: hornblende gabbro and diorite) and Late Eocene (Group 2: hornblende–pyroxene gabbro) ages are recognized in the Malayer–Boroujerd Plutonic Complex of the northern SaSiZ. Group 1 dykes have elemental and isotopic signatures consistent with melting of a mantle source modified during Neo-Tethyan subduction. Some Group 1 magmas evolved to intermediate compositions through assimilation and fractional crystallization. Group 2 dykes have within-plate trace element geochemical signatures, modelled as deriving from low-degree melting of asthenospheric mantle without a subduction influence. Published models postulate either a Cretaceous–Eocene Neo-Tethyan flat-slab scenario or a Latest Cretaceous–Paleogene Neo-Tethyan break-off event beneath the SaSiZ. Such models do not reconcile with the Late Eocene presence of within-plate magmatism in westernmost Iran, very close to the Zagros Suture. We argue that a period of flat-slab subduction concluded with sub-parallel subduction of a Neo-Tethyan ridge to the trench. The resulting slab break-off event in the Late Eocene is responsible for generation of the distinct Mesopotamia and Zagros slabs in mantle tomography models. Break-off was followed by small-volume within-plate type magmatism before short-lived re-establishment of Tethyan subduction prior to the final Arabia–Eurasia collision. Supplementary material: Field photographs, photomicrographs, additional geochemical plots, descriptions of analytical methods and tables of geochemical modelling parameters are available at https://doi.org/10.6084/m9.figshare.c.4126196

The volcanic succession of Baoligaomiao, central Inner Mongolia: Evidence for Carboniferous continental arc in the central Asian orogenic belt

The Baoligaomiao Formation, within the Hegenshan ophiolite-arc-accretion complex is an important segment to understand the tectonic evolution of the Central Asian Orogenic Belt (CAOB), world's largest Phanerozoic orogenic belt. In this study, we present an integrated study of zircon U-Pb isotopic ages, whole rock major-trace elements, and Sr-Nd-Pb isotopic data from the volcanic succession in the Baoligaomiao Formation. The volcanic succession can be divided into the lower sequence with zircon U-Pb ages in the range of 326.3Ma–307.4Ma and the upper sequence of 305.3Ma. The succession belongs to two suites: calc-alkaline volcanics and high-Si rhyolites. The calc-alkaline volcanic rocks include basaltic andesite through andesite and dacite to rhyolite and their pyroclastic equivalents. These rocks exhibit a well-defined compositional trend from basaltic to rhyolitic magma, reflecting continuous fractional crystallization. These rocks show obvious enrichment in LILEs and LREEs and relative depletion of HFSEs, typical of subduction-related magma. The calc-alkaline rocks have low initial 87Sr/86Sr (0.7023–0.7052), positive ɛNd(t) values (2.75–4.80), and their initial Pb isotopic compositions are 17.875–18.485 of 206Pb/204Pb, 15.481–15.520 of 207Pb/204Pb and 37.467–37.764 of 208Pb/204Pb, respectively. Geochemical and isotopic results suggest that the volcanic succession represents Carboniferous subduction-related, mature, continental arc volcanism. The outcrops of high-Si rhyolites are restricted to the northern edge of the continental arc, marking a transition zone between volcanic arc and back-arc basin, where they are interbedded with the calc-alkaline rocks in the lower sequence, and the upper sequence is composed only of high-Si rhyolites. The high-Si rhyolites have high SiO2 (71.12–81.76wt%) and varied total alkali contents (K2O+Na2O=5.46–10.58wt%), low TiO2 (0.06–0.27wt%), MgO (0.09–0.89wt%) and CaO (0.08–0.72wt%). Based on the presence of mafic alkali phenocrysts, such as arfvedsonite and siderophyllite, high Zr/Nb ratios (>10) and peralkalinity index (PI) near unity, the high-Si rhyolites can be classified as peralkaline comendites. The high-Si rhyolites are characterized by unusually low Sr and Ba, and high abundance of Zr, Th, Nb, HREEs and Y. They show geochemical characteristics similar to those of typical A2-type granites including their high K2O+Na2O, Nb, Zr and Y, and high ratios of FeOT/MgO, Ga/Al and Y/Nb. Our study suggests that the high-Si rhyolites were derived from discrete trachytic parent magma with fractional crystallization within shallow magma reservoirs. Their Nd-Pb isotopic characteristics are similar to those of the calc-alkaline arc rocks and are compatible with partial melting of pre-existing juvenile continental arc crust. We observe that the widespread eruptions of A2-rhyolitic magmas (305.3Ma–303.4Ma) following a short period of magmatic quiescence was temporally and spatially associated with voluminous intrusion of the bimodal magmas (304.3Ma–299.3Ma) in the pre-existing arc volcanic-plutonic belt (329Ma–307Ma). We envisage northward subduction and slab breakoff process resulting in an obvious change of the regional stress field to extensional setting within the Carboniferous continental arc running E-W for thousands of kilometers. Therefore, we propose the existence of an east-west-trending Carboniferous continental arc in the Hegenshan ophiolite-arc-accretion complex, with the slab breakoff event suggesting that the age of the upper sequence (305.3±5.5Ma) likely indicates the maximum age for the cessation of the northward subduction of the Hegenshan oceanic lithosphere.

The geochemical evolution of the granitoid rocks in the South Qinling Belt: Insights from the Dongjiangkou and Zhashui intrusions, central China

Widespread Late Triassic granitoid rocks in the South Qinling Belt (SQB) represent excellent subjects to study the geochemical and geodynamic evolution of magmatic rocks during the collision between the North China Craton and South China Block. In this study, we report new geological, geochemical, zircon U–Pb geochronology and zircon Hf isotopic data for the Dongjiangkou and Zhashui intrusions, two large igneous complexes typical of the SQB. We also summarize published geochemical data of similar granitoid rocks. Our results show that the Dongjiangkou intrusion is composed of ca. 223–214Ma quartz diorites, tonalites and granodiorites with abundant coeval mafic magmatic enclaves (MME), and the Zhashui intrusion consists of ca. 203–197Ma monzogranites and K-feldspar granites with rare MME. The Dongjiangkou granitoid rocks are characterized by high K2O, MgO, Rb, and Mg# values, but low TiO2, Sc, and A/CNK (molar Al2O3/(CaO+Na2O+K2O)) values, with εHf(t) values of −18.1 to −0.4 and TDM2(Hf) values of 1141–2117Ma. We suggest that the Dongjiangkou granitoid rocks are formed by magma mixing between 70%–90% melts derived from partial melting of the Neoproterozoic basement rocks of the SQB and 10%–30% mantle-derived mafic melts with minor involvement of Archean to Paleoproterozoic old basement materials of the SQB. The Zhashui granitoid rocks exhibit high K2O, Rb, A/CNK, AMF(Al2O3/(MgO+FeOT)), Al2O3/TiO2 and Rb/Sr values, but low MgO, TiO2, Sc, CaO/Na2O values, with εHf(t) values of −3.5 to +2.3 and TDM2(Hf) values of 979–1300Ma, suggesting they are mainly produced by partial melting of Neoproterozoic metamorphic greywackes mixed with minor amounts of mantle-derived melts. The MME are characterized by low SiO2, Sr/Y ratio, but high MgO, K2O, and Rb contents, with εHf(t) values of −0.8 to +4.3 and TDM(Hf) values of 732–905Ma, suggesting they are produced by partial melting of metasomatized continental lithospheric mantle. A flare-up event is recognized during 225–205Ma and the lower SiO2 and Sr/Y of the granitoid rocks during this period indicate prominent involvement of mantle-derived melts mixed with crustal melts. The granitoid rocks with higher SiO2 and Sr/Y during 225–205Ma, however, suggest they are formed under a greater pressure than those of previous period (235–225Ma) granitoid rocks. Few granitoid intrusions are formed after 205Ma, and they display high SiO2 with low Sr/Y reflecting that they may be produced by low pressure partial melting of crustal materials or may represent highly fractionated magmas in the upper crust. These geochemical variations show a regular change with their formation ages, reflecting the possible variation in petrogenesis and crustal thickness. We propose that the granitoid rocks may have been formed in a transitional setting from subduction to collision during ca. 235–225Ma, and a slab break-off event followed during ca. 225–205Ma. Slab detachment was completed during ca. 205–190Ma. We propose that no intensive delamination/convective removal of lower crust occurred in the SQB.

Petrogenesis of Permian A-type granitoids in the Cihai iron ore district, Eastern Tianshan, NW China: Constraints on the timing of iron mineralization and implications for a non-plume tectonic setting

The geochronology and geochemistry of granitoids in the Eastern Tianshan, NW China provide important constraints on the timing of iron mineralization, as well as in understanding evolution history of the southern Central Asian Orogenic Belt (CAOB). Here we present results from a detailed study on granitoid rocks from the Cihai iron ore district in the Beishan region, southern part of the Eastern Tianshan. The granitoid rocks are composed of granodiorite, quartz monzonite, granite, and monzonite. Zircon U–Pb analyses yielded the ages of 294.1±2.2Ma, 286.5±0.7Ma, 284.3±3.3Ma, and 265.6±3.0Ma, respectively, suggesting they were formed in Early–Middle Permian. Among these granitoid rocks, the ages of quartz monzonite and granite are close to the timing of iron mineralization (~282Ma), indicating they may provide a source of iron in the Cihai ore district.Geochemically, the granodiorite, granite, and quartz monzonite samples are characterized by high FeOt/(FeOt+MgO) and Ga/Al ratios (0.84–0.94 and 2.28–3.27, respectively), as well as high zircon saturation temperatures (781–908°C), similar to those of typical A-type granitoids. Isotopically, they display consistently depleted Hf isotopic compositions (εHf(t)=+1.18 to +15.37). Geological, geochemical, and isotopic data suggest that the Cihai A-type granitoids were derived from melting of juvenile lower crust.Some Early Permian A-type granitoids were recently identified in the Tarim and Eastern Tianshan with the ages between 294 and 269Ma. The A-type granitoids in the Eastern Tianshan formed earlier between 294–284Ma and exhibit characteristics of A2 type granitoids, whereas the A-type granitoids in the Tarim formed later between 277–269Ma and show A1 granitoids affinity. We suggest that the Permian Tarim mantle plume does not account for the formation of the A-type granitoids in the Eastern Tianshan area, and the Eastern Tianshan was in a non-plume tectonic setting during Early Permian time. A slab break-off event has been suggested as the possible tectonic mechanism that triggered the formation of Early Permian A-type granitoids in the Eastern Tianshan.

Geology, zircon geochronology, and petrogenesis of Sabalan volcano (northwestern Iran)

Sabalan Volcano (NW Iran) is an isolated voluminous (4821m elevation; >800km2) composite volcano that is located within the Arabia-Eurasia collision zone. Its edifice was assembled by recurrent eruptions of trachyandesite and dacite magma falling into a relatively restricted compositional range (56–67% SiO2) with high-K calc-alkaline and adakitic trace element (Sr/Y) signatures. Previous K-Ar dating suggested protracted eruptive activity between 5.6 and 1.4Ma, and a two stage evolution which resulted in the construction of the Paleo- and Neo-Sabalan edifices, respectively. The presence of a topographic moat surrounding Neo-Sabalan and volcanic breccias with locally intense hydrothermal alteration are indicative of intermittent caldera collapse of the central part of Paleo-Sabalan. Volcanic debris-flow and debris-avalanche deposits indicate earlier episodes of volcanic edifice collapse during the Paleo-Sabalan stage. In the Neo-Sabalan stage, three dacitic domes extruded to form the summits of Sabalan (Soltan, Heram, and Kasra). Ignimbrites and minor pumice fall-out deposits are exposed in strongly dissected drainages that in part have breached the caldera depression. Lavas and pyroclastic rocks are varyingly porphyritic with Paleo-Sabalan rocks being trachyandesites carrying abundant phenocrysts (plagioclase+amphibole+pyroxene+biotite). The Neo-Sabalan rocks are slightly more evolved and include dacitic compositions with phenocrysts of plagioclase+amphibole±alkali-feldspar±quartz. All Sabalan rock types share a common accessory assemblage (oxides+apatite+zircon). High spatial resolution and sensitivity U-Pb geochronology using Secondary Ionization Mass Spectrometry yielded two clusters of zircon ages which range from 4.5 to 1.3Ma and 545 to 149ka, respectively (all ages are averages of multiple determinations per sample). U-Th zircon geochronology for selected Neo-Sabalan rocks agrees with the U-Pb ages, with the youngest zircon rims dating to ca. 110 ka. Because zircon crystallization predates eruption, this age represents the upper limit for the youngest eruptions of Sabalan. Valley-filling ignimbrites yielded variable U-Pb zircon ages which argue against these pyroclastic rocks being generated in a single caldera forming event. These results indicate that eruptions occurred more recently than previously indicated by K-Ar dating. Paleo-Sabalan and Neo-Sabalan volcanic rocks have similar geochemical characteristics, including enrichment of LILE and LREE relative to HFSE and HREE, respectively, and prominent negative Ti, Nb, and Ta anomalies. The trachyandesitic to dacitic rocks of Sabalan also share negative Eu anomalies. This, together with horizontal or slightly increasing Y vs. Rb trends, indicates fractionation of plagioclase-amphibole or plagioclase-clinopyroxene assemblages with negligible crustal assimilation (based on low and invariant Rb/Th). High degrees of mantle partial melting are inferred from high (La/Yb)N (from 28 to 48). Overall, the subduction-affinity of Sabalan volcanic rocks agrees with models of melt generation following a Quaternary slab break-off event coeval with continental collision.

The late stages of the Pampean Orogeny, Córdoba (Argentina): Evidence of postcollisional Early Cambrian slab break-off magmatism

Widespread rhyolitic and mafic volcanism in the northern and southern sectors respectively of Eastern Sierras Pampeanas, central western Argentina, are associated with an important phase of extension and uplift linked to slab break-off on latest stages of the Pampean Orogeny. The main orogenic deformation took place between 540 and 535 Ma based on new ages available for this region. New U–Pb ages in zircons from the Oncán Rhyolite and new and old recalculated zircons from Los Burros Rhyodacite in the northern sector of Sierras de Córdoba, together with new U–Pb ages of the southern sector, indicate that volcanic and subvolcanic rocks of both sectors are partially coeval and unconformably overlying and/or intruding the basement rocks during a period of exhumation and subsequent cooling at 530-520 Ma. These data are in agreement with previous estimates for the final uplift of the Sierras de Córdoba based on the K–Ar cooling ages. The southern sector of Eastern Sierras Pampeanas may represent deeper structural levels within the crust and is characterized by the occurrence of small mafic bodies with OIB-like signature. New and reinterpreted U–Pb SHRIMP and TIMS ages on zircons and monazites in the metamorphic associated rocks date this episode, which is related to a rapid slab break-off event at 519-515 Ma. This episode is also associated with the emplacement of peraluminous granitoids, and with the extension, cooling and final uplift of the rocks affected by the Pampean Orogeny all along the Eastern Sierras Pampeanas.

Origins of Early Mesozoic granitoids and their enclaves from West Kunlun, NW China: implications for evolving magmatism related to closure of the Paleo-Tethys ocean

Early Mesozoic granitoids and microgranular enclaves (MEs) are widespread in the West Kunkun, northwestern Tibetan plateau, and record the tectonic evolution of Eurasia–Tethys in this area. This study reports geochemistry, zircon U–Pb and Hf isotopic data for a suite of granitoids and their MEs from the Middle Triassic Bulunkou pluton (BP) and the Late Triassic Akazishan pluton (AP) from the West Kunlun. LA-ICP-MS U–Pb zircon dating of a sample from the BP host monzogranite and an enclave, as well as a AP monzogranite, yielded ages of 236 ± 2, 230 ± 7 and 208 ± 1 Ma, respectively. The BP monzogranite and its enclaves from the northwestern part of the West Kunlun are mainly weakly peraluminous granites characterised by relatively high Rb, Th, Rb/Sr (2.64–9.03) and HREE contents, and low Mg#, Sr/Y and negative Eu anomalies. Zircons from the BP monzogranite have eHf(t) values from −5.7 to −1.6. Zircons from enclaves of the BP show more variable eHf(t) values from −4.1 to 3.8. We consider that the BP granites are likely to have been formed by partial melting of metasedimentary rocks at shallow crustal depth, and their enclaves, composed of quartz + biotite + plagioclase + garnet + K-feldspar, are relics from the melting of a source at middle crustal depths. The AP host and its enclaves from the southeastern part of the West Kunlun have low Rb, Rb/Sr (0.15–1.90) and weakly negative Eu anomalies but high HREE contents indicating limited fractionation of plagioclase without residual garnet in their source. The inferred protolith is an intermediate igneous rock in the middle or lower crust. MEs hosted in the AP have high Mg# (39.7–45.0) and Nb and weakly negative Eu anomalies, as well as high Sr, P and Ti, corresponding to a medium-K basaltic rock, which may have originated from mixing of partial melting of metasomatised mantle wedge that has been modified by upwelling asthenospheric mantle and crustal melting in the deep source. Post-collisional southeasternward younging of Mesozoic granitoids in the West Kunlun records a transition from shallow level, mid-crustal melting to deeper level, lower-crustal melting. The post-collisional granitoids show significant mantle input increasing from northwest to southeast, which was closely associated with mafic magma-driven partial melting of mantle and crustal sources as a consequence of slab break-off. Syn- to post-collisional Early Mesozoic granites emplaced along the Mazar–Kangxiwar suture zone suggest that these tectonomagmatic processes followed the closure of the Paleo-Tethys ocean during the Tarim and Karakorum–Qiangtang continental collision.

Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane, Canadian and Alaskan Cordillera

Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane is recorded by two phases of regional deformation, metamorphism, and magmatism within basement complexes of the Alexander (Craig and Admiralty subterranes), Wrangellia, and Peninsular terranes in the Canadian and Alaskan Cordillera. New secondary ion mass spectrometry (SIMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) zircon U-Pb ages, whole-rock major- and trace-element and Nd-Sr isotope geochemical compositions, and geological field observations of late Paleozoic igneous rocks were used to identify the precise timing and significance of this tectonism in the Saint Elias Mountains region of southwestern Yukon and eastern Alaska. Middle to Late Pennsylvanian (301–307 Ma) igneous rocks, herein assigned to the Barnard Glacier suite, were preferentially emplaced along the Wrangellia-Craig subterrane boundary and mainly comprise syenitic plutons that intrude Paleozoic country rocks with evidence of Pennsylvanian or older (D1) deformation. We propose that Barnard Glacier suite magmatism was produced by a slab breakoff event after the consumption of a narrow backarc ocean basin and early Pennsylvanian collision between the Wrangellia-Peninsular arc and Craig subterrane passive margin. Early Permian (284–291 Ma) dioritic to granodioritic rocks, herein assigned to the Donjek Glacier suite, comprise the vestiges of an extensive magmatic system within the Craig subterrane of southwestern Yukon and southeastern Alaska. The available data suggest that the Donjek Glacier suite represents part of a short-lived, Early Permian arc that initiated along the outboard margin of the Craig subterrane–Wrangellia–Peninsular block after Pennsylvanian collision and slab breakoff. At two field localities in southwestern Yukon, Paleozoic country rocks with D1 fabrics are also intruded by sills and dikes of the Donjek Glacier suite that show evidence of ca. 285 Ma regional deformation and metamorphism (D2). Field evidence for Early Permian tectonism in the Saint Elias Mountains implies direct connections with coeval deformation and metamorphism in the Admiralty subterrane, a microcontinent in the Admiralty Island region of southeastern Alaska that developed separately from the Craig subterrane prior to the Early Permian. D2 tectonism was likely related to the entry of the Admiralty subterrane margin into the Early Permian subduction zone, which resulted in collision and final amalgamation of the Alexander-Wrangellia-Peninsular composite terrane. Our tectonic scenarios require the currently accepted configuration of the Alexander terrane (composite of the Craig and Admiralty subterranes) to have only existed after the Early Permian collision between the Admiralty subterrane and the previously assembled Craig subterrane–Wrangellia–Peninsular terrane. Biogeographic and other geological data suggest that the two-part assembly of the Alexander-Wrangellia-Peninsular composite terrane took place along a convergent margin to the north of the Cordilleran pericratonic arc terranes (Yukon-Tanana, Quesnellia, and others), in between the paleo–Pacific Ocean and paleo–Arctic Ocean realms, to the northwest of the supercontinent Pangea. The assembly of the Alexander-Wrangellia-Peninsular composite terrane might have been associated with the Early to Middle Permian subduction polarity flip recognized in the Cordilleran pericratonic realm, which led to the closure of a backarc ocean basin and Late Permian arc-continent collision along the western margin of North America.

Geochronology and geochemistry of the Eastern Erenhot ophiolitic complex: Implications for the tectonic evolution of the Inner Mongolia–Daxinganling Orogenic Belt

The Eastern Erenhot ophiolitic complex (EOC) is one of the numerous fragments of oceanic lithosphere in southeastern Central Asian Orogenic Belt. It is composed dominantly of serpentinized ultramafic rocks with subordinate gabbros, mafic lavas and minor plagiogranite dikes. Zircons from two gabbros and one plagiogranite yielded weighted mean 206Pb/238U ages of 354.2±4.5Ma, 353.3±3.7Ma and 344.8±5.5Ma. These ages suggest that the oceanic crust of the EOC formed in a maximum time period of 10Ma, and that the plagiogranite may have formed later than the gabbroic section. An undeformed and unmetamorphosed dioritic porphyry dike intruded in the Carboniferous strata near the EOC has an intrusive age of 313.6±2.9Ma and provides a possible younger minimum time limit for the formation of the early Carboniferous ophiolitic complex. All the mafic rocks have similar chondrite normalized REE patterns characterized by moderate depletion in LREE with (La/Yb)N (0.20–0.75) similar to normal middle oceanic ridge basalt (N-MORB). The PM-normalized trace element patterns of the gabbros and massive basalts are also reasonably consistent, essentially similar to those of N-MORB except for some enrichment in LILE (e.g. Rb, Ba) and slightly negative Ti anomalies. The plagiogranite samples are characterized by lower K2O (0.45–0.73wt%) comparable with oceanic plagiogranite. They have LREE-enriched, chondrite-normalized REE patterns with varying Eu anomalies and the trace elements (e.g. Rb, Y, Nb) show similarity to volcanic arc granite. These geochemical features of the EOC show a similar volcanic arc affinity, suggesting that they form in a back-arc-type environment. Their origin is attributed to asthenospheric upwelling and further lithospheric extension during early Carboniferous, formed as a consequence of slab breakoff on collision of the northern early to mid-Paleozoic orogenic terrane and the Hunshandake Block.

Neoarchean metagabbro and charnockite in the Yinshan block, western North China Craton: Petrogenesis and tectonic implications

Properly calibrating the magmatic record of mantle and crustal origin in Archean granite-greenstone terranes is crucial for understanding the petrogenetic and geodynamic processes that generated early continental landmasses. This geochronological and geochemical study documents a Neoarchean bimodal metaplutonic suite of metagabbro and charnockite from the Yinshan block, North China Craton. The meta-gabbroic rocks show an SiO2 range from 45.9 to 51.7% and high MgO content from 6.7 to 16.4%, with enrichment in Ba and light rare earth elements (LaN/YbN=3.11–5.99) and depletion in high field strength elements. Together with their enriched whole-rock Nd (ɛNd(t)=−0.23 to 1.49) and zircon Hf (ɛHf(t)=0.1–6.9) isotopic signatures relative to the Archean depleted mantle in the craton, these rocks are supposed to originate from the second-stage high-temperature partial melting of refractory depleted mantle that experienced prior basaltic magma extraction and subsequent metasomatism by subduction-related fluids. The associated charnockites range in SiO2 from 61.3 to 69.6% and exhibit a magnesian, calc-alkalic and metaluminous character, with variable Sr/Y and LaN/YbN ratios. These elemental features, plus their evolved isotopic compositions (ɛNd(t)=0.93–1.85, zircon ɛHf(t)=−0.3 to 2.5), are consistent with partial melting of newly underplated mafic lower crustal protolith. In combination with widespread occurrence of metasomatized lithospheric mantle-derived magmas (e.g., high-Mg basalts, sanukitoid suites) and juvenile crust-extracted potassic granites in the NCC during Late Neoarchean time, such a mafic and felsic magma association not only attests to the establishment of a craton-scale subduction-related metasomatized sub-continental lithospheric mantle, but also encapsulates a scenario of coupled lithospheric mantle-crust formation at ∼2.7Ga and juvenile crustal reworking at ∼2.5Ga within a modern-style convergent continental margin possibly featuring episodic slab break-off events.

Phanerozoic sanukitoids from Caledonian Scotland: Implications for Archean subduction

Sanukitoids represent a volumetrically minor but important series of Neoarchean granitoids enriched in large-ion lithophile elements (e.g., Ba, Sr, and light rare earth elements), and with relatively high compatible elements (e.g., Mg, Ni, and Cr). Petrogenetic models have favored an origin in the suprasubduction mantle wedge, so their sudden appearance ca. 2.95–2.68 Ga has made them central to the ongoing debate about the beginnings of modern plate tectonics. Caledonian high Ba-Sr granites from the Northern Highlands of Scotland are petrological and compositional equivalents. The Caledonian examples have not undergone subsequent reworking, and their petrogenetic setting can be used to inform genetic models for sanukitoids. Prolonged subduction preceded emplacement of the high Ba-Sr granites in a short interval (∼10 Ma) at the end of the Caledonian orogeny. Sediment subduction was responsible for elemental and isotopic enrichments and slab breakoff triggered melting in the subcontinental lithospheric mantle. This implies that during the Neoarchean the evolving tectonothermal regime moved the locus of melting from the basaltic slab to the sediment-infused mantle wedge, creating the first subcontinental lithospheric mantle. Sanukitoids may then have resulted from widespread, closely subsequent slab-breakoff events, producing evolved magmas by familiar fractionation mechanisms. Careful study of secular variations in high Ba-Sr magmas could constrain the evolution of the subcontinental lithosphere.

Short-term episodicity of Archaean plate tectonics

By combining geochemical data and geodynamical models, evidence is provided to address the existence and style of Archaean plate tectonics, a topic of vigorous debate for decades. Using careful analyses of lithostratigraphic Archaean assemblages and numerical model results, we illustrate that a short-term episodic style of subduction was a viable style of tectonics in the early Earth. Modeling results show how, due to the low strength of slabs in a hotter Earth, frequent slab break-off events prevented a modern-style long-lived subduction system, and resulted in frequent cessation and re-initiation of the subduction process on a typical time scale of a few million years. Results fit with geochemical observations that suggest frequent alternation of arc-style and non-arc-style volcanism on a similarly short time scale. Such tectonics could provide the link between early pre-plate tectonic style of tectonics (or stagnant-lid convection) and modern-style plate tectonics, in which short-term episodes of proto-subduction evolved over time into a longer-term, more successful style of plate tectonics as mantle temperature decayed.