Crust-mantle Interaction Research Articles

Subduction, collision and plumes in the epoch of the Late Paleozoic magmatism of the Magnitogorsk zone (the Southern Urals)

Subject. A systematization of Late-Paleozoic magmatic formations of the Magnitogorsk zone of the Southern Urals in the process of an accretion of the Magnitogorsk paleoarc to the margin of the East European continent (EEC) with formation in Famenian and Carboniferous active continent margin of South-Uralian accretionary-collisional belt was given in the work.Materials and methods. A generalization of published and manuscript materials characterizing magmatism and ore-mineralization of Magnitogorsk zone for the Devonian-Carboniferous-Permian time carried out, additional investigations of chemical composition of rocks (XRF, ISP-MS) characterizing process of accretion, subduction and plume activity, microelement distribution in them was made, the composition of rock-forming and accessory minerals (EPMA) was studied.Results. It is found that the South-Uralian accretionary-collisional belt was beginning to form in the late phase of the development of the Magnitogorsk island arc in the process its collision with EEC margin with formation in the Frasnian and Carboniferous of active continental margin. The products of Late-island-arc volcanism are represented by the porphyrite formation and in the eastern frame of the arc - by subalkaline monzonite-shoshonite-latite volcanic-intrusive association with intermediate characteristics between the subductional and interplate formations. Synchronously with them, in the backarc setting, picrite and meymechite volcanics − derivatives of a mantle plume are formed. In process of substitution of tectonic-magmatic regime from island-arc to margin-continental intraplate-type mantle series were forming. During this period, hot asthenospheric diapirs (plumes) were rising to the bottom of new-formed (accreted) margin-continental lithosphere. Along with the magmatic associations of intraplate type and rock series of intermediate geochemical type, this geodynamic situation in the Southern Urals is characterized by a presence of great volumes of mantle-crust granitoids of gabbro-tonalite-granodiorite-granite type, that were formed with a manifold manifestation of anatexis in a time interval of 365-290 Ma.Conclusion.On the whole the originality of Magnitogorsk zone geological history in the Devonian and Carboniferous, peculiarities of magmatic complexes formed here due to various geodynamic settings, are making this zone an extraordinary interesting and important object to study of processes of plume-lithosheric and mantle-crust interaction.

Discrepancy between bulk-rock and zircon Hf isotopes accompanying Nd-Hf isotope decoupling

Abstract Zircon is an important accessory mineral for studying the crust-mantle interaction and crustal growth through time because zircon crystals not only allow precise dating but also record initial Hf isotope ratios of the host magma. Our study on a suite of gabbronorite, mafic diorite cumulate, diorite and granite from the Kekeli Batholith in the North Qilian Orogenic Belt, Northern Tibetan Plateau, shows (1) a significant Hf isotope discrepancy between zircon and their bulk rocks; and (2) bulk-rock Nd-Hf isotope decoupling. These observations thus demonstrate that zircons do not always capture the full history of magmatic system. The significant positive correlation between bulk-rock Hf isotope ratios and TiO2 content (R2 = 0.94) indicates that Ti-rich minerals (e.g., ilmenite, amphibole) are likely important Hf hosts. The early-formed Ti-rich minerals possibly record different Hf isotopes from those of zircons crystallized subsequently, thus causing discrepancy between zircon and bulk-rocks and leading to bulk-rock Nd-Hf isotope decoupling. Correlations between bulk-rock TiO2 content, Mg isotopes and Hf isotopes indicate a mixing process, with granite and gabbronorite representing two compositional endmembers. Because Ti minerals have higher crystallization temperatures than zircons, when the mixing melts have contrasting isotopes (or from heterogeneous sources/have a strong crustal contamination), the bulk-rock and zircon Hf isotope discrepancies reflect mineral crystallization sequence during mafic and felsic magma mixing. It is thus imperative to consider early formed minerals such as Ti-rich minerals and the bulk rock composition, not just zircons, when using Hf isotopes to track melt evolution and precisely constrain mantle contribution to granitoid petrogenesis.

Zircon from Orogenic Peridotite: An Ideal Indicator for Mantle-Crust Interaction in Subduction Zones

The orogenic peridotites can be subdivided into crust- and mantle-derived types. They record complex geological processes in subduction and collision zones. The crust-derived peridotites are derived from cumulates crystallized from ultramafic-mafic magmas in deep continental crust, an early mantle-crust interaction, prior to subduction. The mantle-derived orogenic peridotites are originated from subcontinental lithospheric mantle (SCLM) wedge and other mantle domains, and are later involved in the subduction channel and orogenic system. The mantle-derived peridotites usually record complex metasomatism, ultra-high pressure (UHP) metamorphism and mantle-crust interaction during the orogenic processes. Zircons are rarely found in orogenic peridotites. These zircons in orogenic peridotites are generally formed during metasomatism, they can be divided into old zircons (mainly the cores of residual magmatic and recrystallized) and newly grown zircons. Three key factors for zircon formation in orogenic peridotites are that: (1) zircon has strong crystallization ability, and Zr is easier to exchange Si in zircon crystal structure than other elements in the mantle; (2) metamorphic destruction of Zr-bearing minerals and precipitation of intergranular melts during the high-grade metamorphism can nucleate zircon under sub-solidus conditions; (3) the melts/fluids released from the subducted crust can metasomatize the mantle wedge to form zircons. In-situ studies on zircons and zircon inclusions enclosed in mantle minerals indicate that zircon can be an ideal indicator for mantle- crust interaction in subduction zones. The inclusions in zircons and Hf-O isotope of zircons are effective to reflect the composition of the melts/fluids, source properties, and the physical and chemical conditions. Dating of the zircons has been widely used in the studies of lithospheric evolution and crust-mantle interaction. During the complex processes of plate convergence, the orogenic peridotites can be subjected to the melt/fluid metasomatism, modifying the mineral and elemental compositions of peridotites. Thus, zircon is very useful to unravel the history of specific lithospheric mantle and the relationship between continental cratonic cores and their margins.

Crust-mantle mixing and crustal reworking of southern Tibet during Indian continental subduction: Evidence from Miocene high-silica potassic rocks in Central Lhasa block

Collisional zones are commonly considered as important regions for crustal reworking, but the reworking mechanism remains debated. The well-known Himalayan-southern Tibetan orogen, built by India-Asia collision and convergence, has the thickest continental crust on Earth and is therefore an ideal region for studying crustal reworking during collisional orogenesis. Here we revisit the Miocene high-silica potassic rocks (trachytes) in the Konglong area of the central Lhasa block, southern Tibet. Integrated studies of geochronology, mineral compositions, bulk-rock major- and trace-element geochemistry, and Sr-Nd-Pb-Hf-O isotopes unequivocally indicate that the Konglong trachytes formed by mixing between enriched mantle-derived ultrapotassic and thickened ancient crust-derived magmas. Combined with post-collisional magma mixing recently identified in the southern Lhasa block, we suggest that magma underplating and subsequent mantle-crust interaction (i.e., the matter and energy transfer from the mantle to the crust) has been a common and important crustal reworking process in southern Tibet during Indian continental subduction. This process may be related to Indian plate flat subduction and subsequent foundering during the post-collisional stage. In combination with the nature of Cenozoic magmatism in the Himalaya block, we suggest that in addition to partial melting of the subducted continental crust, magma underplating and subsequent crust-mantle mixing beneath the obducted continent has also played an important role in crustal reworking of the collisional zone.

Crystal fractionation of contaminated melts and re-melting of newly underplated basaltic crust generated Late Triassic andesitic and dioritic intrusions in the southern Yidun Terrane, SW China

The genesis of andesitic and dioritic rocks in continental arcs is hotly debated, and proposed models include slab melting, juvenile mafic lower-crustal melting, mafic and felsic magma mixing, and mantle-derived melts that underwent a MASH (melting, assimilation, storage, and homogenization) process. The controversy comes from the possibility of their polygenesis and the ambiguity of bulk-rock isotope tracers, not to mention the processes of magma evolution. Here, we report on our investigations in the southern Yidun Terrane (a Triassic continental arc in SW China) into the petrogenesis of two stages of the dioritic Xiuwacu intrusions (phases 1 and 2), the mafic microgranular enclaves (MMEs) hosted by the phase 1 intrusion, as well as the andesitic Hongshan intrusions. We used U-Pb zircon dating, in situ zircon Hf-O isotope analyses, bulk-rock Sr-Nd isotope analyses, and trace element analyses of the zircons from different stages of the intrusions and the MMEs to shed light on their sources and mantle–crust interactions involved in their petrogenesis. Our results suggest that the Xiuwacu MMEs from the phase 1 intrusion crystallized from a melt. Their variable zircon Hf-O isotopic compositions (εHf(t) = +0.4 to +5.3 and δ18O = 4.80‰ to 7.79‰) indicate they were formed by the mixing of mantle-derived basaltic melts and felsic melts. The phase 1 intrusion have an origin similar to that of the MMEs, as indicated by their homogeneous zircon Hf-O isotopic compositions (εHf(t) = +1.3 to +3.9 and δ18O = 6.58‰ to 7.31‰), which fall within the range of the isotopic compositions of the MMEs. The discovery of several inherited zircons and a small recrystallized felsic microgranular enclave hosted by an MME reveals that the MMEs contain relatively more felsic components. Even so, the MMEs still have lower silica contents (62.1–63.4 wt%) than those of the phase 1 intrusion (63.4–70.5 wt%), and this indicates that direct magma mixing of basaltic and felsic melts generated the andesitic MMEs, whereas a crystal fractionation process is required for the dioritic phase 1 intrusion. The porphyritic phase 2 and Hongshan intrusions formed at ~216 Ma, after the phase 1 intrusion, and they had higher oxygen fugacities and crystallization temperatures than the phase 1 intrusion. Given the conclusions of previous researchers that slab breakoff probably occurred at ~216 Ma, we propose that the Xiuwacu phase 2 and Hongshan intrusions formed during the slab breakoff process, the former by partial melting of the newly underplated basaltic lower crust in the MASH zone, and the latter by differentiation of mantle-derived basaltic melts at relatively high pressures.

Nd Isotopic and Model Age Study of the Shandong Province, North China Craton: Implications for Correlation with South Korea

The geological units in Shandong Province, North China are important parts of the North China Craton and offer important insights into their crustal evolutionary history. This work compiled 611 sets of Nd isotopic data of Archean-Mesozoic rocks from Shandong including the Luxi, Jiaobei, and Sulu terranes, which provided important constraints for crustal growth and reactivation. Nd-depleted mantle model ages (TDM) of Archean rocks with positive eNd(t) values showed that ca. 2.9 and 2.8-2.7 Ga were the most important periods of crustal growth in the Jiaobei and Luxi terranes, respectively, while the period of ca. 2.6-2.5 Ga in the Jiaobei terrane likely indicated a coherent event of crustal growth and reworking. During the Proterozoic, multi-stage rifting and collisional orogenic events possibly led to the reworking of Archean crust in the source region. The Nd isotopic data of the Paleoproterozoic and Neoproterozoic rocks from Sulu indicated significant reworking of older crust with juvenile magmatic input. Crustal reactivation occurred during the Mesozoic. The younger TDM ages of the Mesozoic rocks with low negative eNd(t) values indicated that a juvenile crustal/mantle component was added to the ancient basement. The reactivation reflected significant crust-mantle interaction via the mechanism of crustal subduction and mantle-derived magma un-derplating, or possibly asthenospheric upwelling. Our work also attempted to establish the crustal correlation between Shandong and Korea (including the Gyeonggi massif, Ogcheon belt, and Yeongnam massif). The TDM age distribution provided evidence favoring the affinity relationship between the Gyeonggi massif and Ogcheon belt of South Korea and the Jiaobei and Sulu terranes of Shandong, while the Yeongnam massif was more correlated with the South China Block.

The transformation of the lithospheric mantle beneath South China Block (SCB): constraints from petrological and geochemical studies of Daoxian and Ningyuan basalts and their melt inclusions

ABSTRACTThe lithospheric mantle beneath the South China Block (SCB) underwent a dramatic transformation from depleted to enriched during late Mesozoic. With a view to deeply understand this process, here we investigate the Mesozoic basalts and their melt inclusions from the Daoxian and Ningyuan regions within the central SCB. The geochemical features of the melt inclusions in these basalts suggest that these rocks originated from the lithospheric mantle enriched through interaction with K-rich aqueous fluids released from subducted Palaeo-Pacific oceanic sediments, whereas the Ningyuan basalts were mainly derived from the asthenospheric mantle source. Geochemical modelling indicates that the Daoxian basalts were generated from 15%-25% of partial melting of garnet lherzolite, whereas the Ningyuan basalts originated from 10%-20% of partial melting of garnet pyroxenites. Our data, combined with those from other Jurassic basalts suggest a temporal evolution of the SCB mantle sources during the Late Mesozoic. Diverse crust–mantle interactions through mixing of the asthenospheric melts with variable proportions of subducted Palaeo-Pacific oceanic sediments might account for the spatial heterogeneity of mantle sources observed beneath the SCB. The transition from Tethyan tectonic realm to the Palaeo-Pacific tectonic regime might have played a significant role in the transformation of the lithospheric mantle beneath the SCB.

Petrogenesis of Early Silurian granitoids in the Longshoushan area and their implications for the extensional environment of the North Qilian Orogenic Belt, China

Analyses of the zircon U—Pb age, mineralogical chemistry and whole-rock geochemistry of the Jiling pluton along the southern margin of the Alxa Block were conducted. The results of these analyses will provide crucial information for investigations of Paleozoic crust-mantle interactions in the North Qilian Orogenic Belt (NQOB) and explorations of the transition age from a compressional to extensional environment during the collision between the Alxa Block and the Qilian-Qaidam Block. The zircon dating results reveal that the Jiling pluton is composed of Early Silurian granitoids (435–442 Ma) with contemporaneous mafic microgranular enclaves (MMEs). Most granitoids contain amphibole and titanite and have A/CNK values <1, illustrating that they belong to amphibolite-rich calc-alkaline granitoids (ACGs) and K-rich and K-feldspar porphyritic calc-alkaline granitoids (KCGs). Both the granitoid and MME samples display variable initial Sr and Nd isotopic compositions. We suggest that the Jiling pluton was formed by crust-mantle-derived magma mixing. The high HFSE content of the MMEs indicates that the mantle-derived magmas were derived from melting of the mantle wedge metasomatized by adakitic magma, and the crustal-derived magmas were derived from melting of Paleoproterozoic meta-basaltic rocks in the Longshoushan Group. The discovery of the Jiling granitoids indicates that the NQOB was dominated by a compressional environment during Middle Ordovician-Early Silurian in response to the collision between the Qilian-Qaidam Block and the Alxa Block, and the initial extensional environment began before 442 Ma.

Early Cretaceous Granitic and Monzonitic Rocks of the Southern Part of the Zhuravlevka Terrane (Sikhote-Alin): Geochemical Composition and Melt Sources

The article presents new geological, geochronological, mineralogical, geochemical, and isotopic data on the Early Cretaceous granitic rocks of the southern part of the Zhuravlevka Terrane (Sikhote-Alin). It is shown that four intrusive complexes containing significant amounts of granitic rocks were formed almost simultaneously in this area in the Early Cretaceous (about 100 Ma). These magmatic associations differ in rock set, their mineralogical characteristics, and chemical composition, varying from medium-potassium tonalites and granodiorites depleted in incompatible elements to shoshonitic monzonitic rocks enriched in HFSE and REE. The geochemical and isotopic characteristics of the granitic rocks indicate that the source of their melts was dominated by essentially juvenile metabasite crust with a limited contribution of the upper-crustal metasedimentary rocks. The diversity of geochemical types of the granitic rocks is explained by variable metabasite and metapelite contributions to their source, upper crustal contamination during magma ascent, as well as the variable contribution of the mantle source and different mechanisms of mantle–crust interaction.

Subduction-zone peridotites and their records of crust-mantle interaction

Subduction is the core process of plate tectonics. The mantle wedge in subduction-zone systems represents a key tectonic unit, playing a significant role in material cycling and energy exchange between Earth’s layers. This study summarizes research progresses in terms of subduction-related peridotite massifs, including supra-subduction zone (SSZ) ophiolites and mantle-wedge-type (MWT) orogenic peridotites. We also provide the relevant key scientific questions that need be solved in the future. The mantle sections of SSZ ophiolites and MWT orogenic peridotites represent the mantle fragments from oceanic and continental lithosphere in subduction zones, respectively. They are essential targets to study the crust-mantle interaction in subduction zones. The nature of this interaction is the complex chemical exchanges between the subducting slab and the mantle wedge under the major control of physical processes. The SSZ ophiolites can record melt/fluid-rock interaction, metamorphism, deformation, concentration of metallogenic elements and material exchange between crust and mantle, during the stages from the generation of oceanic lithosphere at spreading centers to the initiation, development, maturation and ending of oceanic subduction at continental margins. The MWT orogenic peridotites reveal the history of strong metamorphism and deformation during subduction, the multiple melt/fluid metasomatism (including silicatic melts, carbonatitic melts and silicate-bearing C-H-O fluids/supercritical fluids), and the complex cycling of crust-mantle materials, during the subduction/collision and exhumation of continental plates. In order to further reveal the crust-mantle interaction using subduction-zone peridotites, it is necessary to utilize high-spatial-resolution and high-precision techniques to constrain the complex chemical metasomatism, metamorphism, deformation at micro scales, and to reveal their connections with spatial-temporal evolution in macro-scale tectonics.

Geochemistry and zircon U-Pb geochronology of Miocene plutons in the Urumieh-Dokhtar magmatic arc, east Tafresh, Central Iran

ABSTRACT The Tafresh plutons that include Ahmadabab diorite, Vasfonjerd monzonite, Mehrezamin diorite and Chahak diorite, located to the east of Tafresh city, north-central Iran, are part of Urumieh-Dokhtar magmatic arc. U-Pb dating of zircon grains provides emplacement ages of 22.3 ± 1 Ma for the Ahmadabad diorite, and tightly clustered ages of 22.2 ± 0.2 Ma, 21.3 ± 0.2 Ma, and 21.7 ± 0.4 Ma for Vasfonjerd monzodiorite, Mehrezamin diorite-monzonite, and Chahak diorite-monzonite plutons, respectively. These rocks are metaluminous to weakly peraluminous, calc-alkaline, and characterized by enrichment in light rare earth elements, Nb-Ta negative anomalies, and high LILE/HFSE ratios. Tafresh plutonic rocks originated from a parental magma source and experienced different degrees of partial melting. Geochemical signatures of Tafresh plutonic rocks, such as a wide range of Y/Nb (2.7–8.4) and low Zr/Nb (19.5–35.) ratios, Nb/Ta (11.46–18.15), argue for mantle–crust interaction during generation of Tafresh magmas. Relatively low Nb/La ratios further indicate that the lithospheric mantle played a significant role in melt generation. HREE signatures (i.e. decrease Dy/Yb with increasing SiO2) preclude substantial involvement of garnet either in the residue, both during partial melting and fractionation of the magma. The plutons are a product of final stages of subduction-related magmatism prior to the collision between the Arabian and Eurasian tectonic plates.

Mesozoic felsic dikes in the Jiaobei Terrane, southeastern North China Craton: Constraints from zircon geochronology and geochemistry, and implications for gold metallogeny

The Jiaobei Terrane in the southeastern North China Craton (NCC) witnessed intense magmatic pulses during Mesozoic in response to lithospheric thinning. These include sporadic felsic-porphyritic dikes which preserve important information on the magmatic processes and of its implications on lithospheric thinning. Here we present a detailed investigation of representative felsic-porphyritic dikes based on whole-rock geochemical analysis, zircon LA-ICPMS U-Pb geochronology, trace elements and hafnium isotopic compositions. Our results identify stages of felsic dikes in the Jiaobei Terrane with ages in the range of 131–127 Ma and 123–116 Ma. The early stage felsic dikes (ESFDs) have negative zircon ɛHf(t) (−22.4 to −11.1), high zircon EuN/EuN* (0.41–0.92) and Ce4+/Ce3+ (62–944), low Ti-in-zircon crystallization temperatures (524–738 °C), and are geochemically comparable to the coexisting Guojialing granodiorite. Abundant inherited zircons and scattered zircon Hf, Yb/Gd, Th/U, and Ce/Sm values suggest that the magmas from which the dikes formed involved crustal contamination. They are interpreted to be the hypabyssal phase of the Guojialing suite sourced from the Paleoproterozoic lower crust of the eastern NCC with input of mantle components, associated with extensive crust-mantle interaction during the peak lithospheric thinning. The late stage felsic dikes (LSFDs) are characterized by variable zircon ɛHf(t), EuN/EuN* and Ce4+/Ce3+ values (−20.1 to −12.2, 0.38–0.88 and 64–956 for the granodiorite- and quartz-porphyritic dikes; −27.6 to −16.2, 0.17–0.75 and 22–503 for the monzogranite-porphyritic dikes), low Ti-in-zircon crystallization temperatures (562 to 774 °C), and display geochemical similarity with the Aishan granitoids. Systematic variations of zircon Hf, Yb/Gd, Th/U and Ce/Sm suggest that fractional crystallization of hornblende played a significant role in the formation of the late dikes. The LSFDs originated from the Neoarchean-Paleoproterozoic lower crust of the NCC with heterogeneous involvement of lesser amounts of mantle materials than the early dikes, consistent with weak crust-mantle interaction during the waning stage of lithospheric thinning. In the LSFDs, the granodiorite and monzogranite porphyries are broadly coeval (123–118 Ma) with large scale gold mineralization in the area as also favored by the high zircon Ce4+/Ce3+ ratios. Our study also provides new insights into the formation of gold deposits.

Petrogenesis of Early Cretaceous high-Mg# granodiorites in the northeastern Lhasa terrane, SE Tibet: Evidence for mantle-deep crustal interaction

High-Mg# (molar Mg/[Mg + Fe]) intermediate to felsic rocks, such as high-Mg andesitic rocks (HMA) and Mg andesites (MA), are usually produced by melting of mantle-derived peridotites and/or interaction with mantle-derived peridotites. Both are crucial in tracking mantle-crust interaction processes in the deep crust. We present LA-ICP-MS in-situ zircon U-Pb isotopic data, whole-rock geochemistry and whole-rock Sr-Nd-Pb-Hf isotopic data for the Basu granodiorites which are exposed between the Jiali fault and the Bangong-Nujiang suture, SE Tibet. The LA-ICP-MS zircon U-Pb data show that the granodiorites were formed at ca. 122.1 ± 1.0 Ma, coeval with other granitoids in this region. The granodiorites belong to the high-K calc-alkaline and high-Mg# (49.7–53.7) series, similar to typical MA. The decreasing A/CNK from 0.97 to 0.87 and increasing FM (molar Fe + Mg) from 0.08 to 0.16 indicate a clinopyroxene entrainment and I-type trend. The rocks also display enriched Rb, Th, U and Pb, but depleted P, Ti and obvious negative Eu anomalies. Evolved whole-rock isotopic compositions of Basu granodiorites include high initial 87Sr/86Sr (0.7097–0.7114), negative εNd(t) (−12.0 to −7.2) and εHf(t) values (−9.6 to −7.9), and lower to upper crustal affinity of Pb isotopes which imply that they were derived from an ancient and evolved lower crust source. Therefore, we suggest that these granodiorites were products of interaction between partial melting of ancient basaltic lower crust and mantle-derived peridotites. Together with a geochronology dataset from the literature, we suggest that there was intensive granitic magmatism occurred in the Early Cretaceous range from 133 to 113 Ma in the northeastern Lhasa terrane, similar to those in the central and northern Lhasa terrane. Furthermore, we have compared with the dataset of in-situ zircon Lu-Hf isotopes, whole-rock Sr-Nd from the literatures and the relationship with zircon U-Pb ages in the northeastern Lhasa terrane, all of which indicate the importance of evolved and ancient continental crust source in the Early Cretaceous magmatism in the northeastern Lhasa terrane, but also that the mantle-derived materials play a necessary role in the generation. In addition, the amount of enriched and evolved isotopic component gradually decreased from ca.140 to 100 Ma.

Early Palaeozoic high-Mg basalt-andesite suite in the Duobaoshan Porphyry Cu deposit, NE China: Constraints on petrogenesis, mineralization, and tectonic setting

High-Mg basalt-andesite suites are extremely rare in the modern Earth but genetically important for indicating essential crust-mantle interactions, ascertaining critical geodynamic settings, and understanding the formation of porphyry copper deposits. Secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating and geochemistry document an early Palaeozoic high-Mg basalt-andesite suite from the Duobaoshan porphyry Cu deposit, eastern Central Asian Orogenic Belt (CAOB). Dating results reveal that the Duobaoshan high-Mg basalt and andesite erupted ca. 506 Ma and ca. 485 Ma, respectively. These high-Mg samples are typical subduction-related volcanic rocks with SiO2 contents of 47.73 to 55.16 wt%, high MgO (6.08 to 10.82 wt%), and high Mg# (58 to 67) and feature enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs). The samples have juvenile whole-rock initial 87Sr/86Sr ratios of 0.70272 to 0.70451, ɛNd(t) of 4.8 to 8.7 (average 7.23), and zircon ɛHf(t) of 7.3 to 15.9. Additionally, they have high Sr (average 619.36 ppm) and low Y (average 11.92 ppm) and Yb (average 1.21 ppm) contents that show affinity with adakitic high-Mg basalts and andesites worldwide. These high-Mg, depleted mantle-like isotopic and adakitic geochemical features imply a depleted mantle source variably assimilated by slab-derived melts under a sustained subduction tectonic setting. We also propose that the Duobaoshan high-Mg basalt-andesite suite, as the parental source magma, fertilized the overlying Duobaoshan porphyry Cu deposit by providing water, copper, and sulphur and high oxygen fugacity. A comprehensive comparison of the post-ore volcanic rocks shows that they might have originated from the slab-derived fluid metasomatized depleted mantle wedge, which had different properties from the mantle that produced the pre- and syn-ore volcanic intrusive rocks. The post-ore volcanic rocks underwent little crustal evolution en route to the surface during a reworked subduction event, which indicates a relatively immature island arc environment.

Tracing magma mixing and crystal–melt segregation in the genesis of syenite with mafic enclaves: Evidence from in situ zircon Hf–O and apatite Sr–Nd isotopes

Alkaline rocks contain essential information on post-collisional/intraplate extensional magmatic processes within the continental lithosphere. The Erhulai syenitic pluton in the Liaodong Peninsula, North China Craton, consists of quartz syenite, mafic enclaves, and porphyritic K-feldspar granite. Field observations and zircon U–Pb dating reveal that the quartz syenite and the hosted mafic enclaves were coeval (with identical emplacement ages of 129–126 Ma), and slightly older than the porphyritic K-feldspar granite (ca. 123 Ma). The mafic enclaves have relatively low SiO2 and high MgO contents (up to 3.0%), and lack Ba or Eu anomalies. Whole-rock analyses of the mafic enclaves and accessory minerals in the enclaves show relatively low initial 87Sr/86Sr ratios (0.7056 to 0.7078) and variable εNd(t) (−11.2 to −2.0) and εHf(t) (−15.8 to −5.5) values. Large proportion of zircons from the mafic enclaves possess mantle-like δ18O values. These geochemical features suggest an enriched mantle source, with the addition of crustal material. The quartz syenites have relatively high SiO2 and low CaO and MgO contents, as well as significantly positive K, Ba, and Eu anomalies. All the quartz syenites show variable (87Sr/86Sr)i ratios (0.7073 to 0.7081) and εNd(t) values (−10.4 to −8.6) for the whole-rocks and apatites, εHf(t) values of −12.4 to −3.6 for the zircons, and zircon δ18O values of +4.4‰ to +7.0‰ (mainly greater than +5.3‰). All these features can be isotopically produced by the partial melting of ancient crustal material, with the involvement of mantle-derived magma. Crystal fractionation process, such as K-feldspar crystallization and accumulation, played an important role in the formation of the quartz syenite. The porphyritic K-feldspar granites possess relatively evolved geochemical compositions with high silica contents (SiO2 > 69 wt%) and remarkable negative Ba and Eu anomalies, which are chemically complementary to the quartz syenite. A crystal mush model was employed to establish the genetic link between the porphyritic K-feldspar granite and the quartz syenite, with the granite representing the interstitial melt extracted from the crystal mush and the quartz syenite representing the residual accumulation of crystals. The variable isotopic compositions of the quartz syenite and the porphyritic granite were mainly the result of extensive interaction of isotopes between mantle- and crust-derived magmas during magma mixing.In situ analyses of the Sr, Nd, Hf, and O isotopes of accessory minerals, coupled with field observations and whole-rock geochemical data for the Erhulai syenitic pluton, indicate a complex multi-stage magmatic process involving magma mixing, crystal–melt segregation, and wall-rock assimilation. Our study also shows that in situ isotopic analyses of accessory minerals provide unique information on, and are powerful geochemical tracers of, mantle–crust interactions and magmatic processes.

Late Triassic high Mg diorites of the Wulong pluton in the South Qinling Belt, China: Petrogenesis and implications for crust-mantle interaction

High Mg diorites provide important insights into crust-mantle interaction during tectonic evolution of orogenic belt. An integrated study including zircon U-Pb dating, whole-rock geochemistry and isotopic compositions of Sr and Nd was carried out on high Mg diorites of the Wulong pluton located in the narrowest part of the South Qinling Belt. Crystallized ages of 218-210 Ma are obtained in combination with the published data. These rocks are characterized with relatively high MgO (2.76–7.17%), Cr (68.7-310 ppm) and Ni (21.3–76.3 ppm) contents. Most diorites exhibit high Sr/Y and La/Yb ratios with negligible Eu anomaly. Isotopic compositions of whole-rock show enriched attributes (87Sr/86Sr(i) = 0.7050–0.7071, εNd(t) = −7.56 to −3.45). Geochemical models indicate that these rocks did not result from melting of subducted slab, magma mixing, fractional crystallization or melts derived from delaminated lower crust reacted with mantle. Instead, high Mg diorites were generated through basaltic melts derived from asthenosphere mantle assimilating crustal components with crystallized phase of garnet-bearing amphibolite (AFC). Together with regional geological setting of the South Qinling Belt, we propose that the parental magma of these high Mg diorites was probably originated from upwelled asthenosphere mantle at the base of thinned crust after delamination of thickened crust during Late Triassic. The continental crust of the South Qinling Belt was thickened as a result of collision with the Yangtze Block during Middle Triassic. Then eclogitized lower crust collapsed and foundered into asthenosphere mantle due to partial melting of thickened crust during Late Triassic. In response, the upwelling and underplating of asthenosphere mantle took place at the base of continental crust. Moreover, upwelling of hot mantle may provide the heat source for migmatites and initiate the doming of Foping area.

Early Cretaceous potassic volcanic rocks from the northern margin of the Sulu orogenic belt, Shandong Province, eastern China: Constraints on petrogenesis and crust–mantle interaction

A major and trace element, Sr–Nd–Hf isotope, and zircon U–Pb geochronology study was conducted on Early Cretaceous volcanic rocks from the northern margin of the Sulu orogenic belt in Shandong Province, eastern China. These results constrain the petrogenesis of the volcanic rocks and provide important insights into the significance of crust–mantle interaction. The rocks comprise high‐K basaltic andesite, shoshonites, latites, high‐K dacites, and high‐K rhyolites. U–Pb zircon dating of four representative samples of latite, two high‐K dacites, and high‐K rhyolite indicates that these rocks formed from 126 to 122 Ma. The volcanic rocks are characterized by high K2O (2.04–6.70 wt.%) and total alkalis (Na2O + K2O; 5.3–10.9 wt.%) that plot within the field for shoshonitic rocks. All the rocks are enriched in light rare earth and large‐ion lithophile elements (e.g., Rb, Th, U, and K) and are depleted in high‐field‐strength elements (e.g., Nb, Ta, and Ti). The rocks have high initial 87Sr/86Sr ratios (0.706760–0.708898), low εNd(t) values (−19.5 to −12.5), and negative εHf(t) values (−31.3 to −19.7), similar to the Early Cretaceous Jiaodong mafic dikes and intermediate–mafic volcanic rocks. These geochemical and isotopic characteristics indicate that the volcanic rocks formed through simultaneous fractional crystallization and crustal assimilation of a basaltic magma derived from metasomatized mantle. The metasomatized mantle source might have been produced by the recycling of subducted Yangtze Craton crust into the ancient lithospheric mantle of the North China Craton.

A-type volcanic–intrusive complex in the Huanggangshan Basin: Implications for early cretaceous crust–mantle interaction in the Gan-Hang Belt and adjacent areas, South China

The origin and petrogenesis of Early Cretaceous A-type volcanic–intrusive complexes in the Gan-Hang Belt and adjacent areas of South China remain controversial. An A-type volcanic–intrusive complex in the Huanggangshan Basin of North Wuyi Mountain comprises syenite porphyry, granite porphyry, granite, and tuffaceous volcanic and pyroclastic rocks. These rocks were subjected to geochronological, geochemical, and isotopic analyses in order to investigate their age and petrogenesis, and crust–mantle interactions in South China. Field observations reveal that this A-type volcanic–intrusive complex contains abundant mafic microgranular enclaves. Zircon UPb dating of the various rock types yielded ages between 136 and 128 Ma. Geochemically, the rocks have A-type affinities, including high large-ion lithophile and high-field-strength element contents, Ga/Al ratios, and zircon saturation temperatures. (87Sr/86Sr)i values vary from 0.702704 to 0.711298, and εNd(t) = −10.63 to −7.28 and εHf(t) = −13.26 to −7.70. Three stages of magmatism are proposed to explain the petrogenesis of the A-type volcanic–intrusive complex in the Huanggangshan Basin: (1) syenitic magmas formed by mingling of crustally derived silicic and mantle-derived mafic magmas; (2) fractional crystallization of anorthitic plagioclase + biotite + ilmenite from the syenitic magmas generated A-type igneous rocks with SiO2 < 71 wt%; and (3) fractional crystallization of plagioclase + K-feldspar + biotite + apatite + allanite + ilmenite then generated A-type igneous rocks with SiO2 > 71 wt%. Combined with previous studies, we suggest that the Early Cretaceous A-type igneous rocks in the Gan-Hang Belt and adjacent areas might have been generated by crust–mantle-derived magma mixing, and the crust–mantle interaction was more intense from southwest to northeast. This increased crust–mantle interaction may have been a response to the Early Cretaceous back-arc extensional environment, possibly as a result of the rollback of the Paleo-Pacific Plate.

Evolution of a Mesoarchean suprasubduction zone mantle wedge in the Dharwar Craton, southern India: Evidence from petrology, geochemistry, zircon U–Pb geochronology, and Lu–Hf isotopes

Petrological, geochemical, and zircon U–Pb geochronological features of Archean ultramafic–mafic complexes formed in subduction‐related settings provide significant insights into mantle source and geodynamic processes associated with subduction–accretion‐collision events in the early Earth. Here, we investigate a suite of serpentinized dunite, dunite, pyroxenite, and clinopyroxenite from an ultramafic complex along the collisional suture between the Western Dharwar Craton (WDC) and the Central Dharwar Craton (CDC) in southern India. We present petrology, mineral chemistry, zircon U–Pb geochronology, rare earth element (REE), Lu–Hf isotopes, and whole‐rock geochemistry including major, trace element, and platinum‐group element (PGE) data with a view to investigate the magmatic and metasomatic processes in the subduction zone. Mineral chemistry data from chromite associated with the serpentinised ultramafic rocks show distinct characteristics of arc‐related melt. Zircon U–Pb data from the ultramafic suite define different age populations, with the oldest ages at 2.9 Ga, and the dominant age population showing a range of 2.8–2.6 Ga. The early Paleoproterozoic (ca. 2.4 Ga) metamorphic age is considered to mark the timing of collision of the two WDC and CDC. Zircon REE patterns suggest the involvement continental crust components in the magma source. Zircon Lu–Hf analysis yields both positive and negative εHf(t) values from −3.9 to 1.5 with Hf‐depleted model ages (TDM) of 3,041–3,366 Ma for serpentinised dunite and −0.2–2.0 and 2,833–2,995 Ma for pyroxenite, suggesting that the magma was sourced from depleted mantle and was contaminated with the ancient continental crust. Geochemical data show low MgO/SiO2 values and elevated Al2O3/TiO2 ratios, implying subduction‐related setting. The serpentinized dunites and dunites show mild LREE enrichment over HREE, with relatively higher abundance of LILE (Ba, Sr) and depletion in HFSE (Nb, Zr), suggesting fluid–rock interaction, melt impregnation, and refertilization processes. The PGE data suggest olivine, chromite, and sulphide fractionations associated with subduction processes. Our study on the Mesoarchean to Neoarchean ultramafic complex provides important insights to reconstruct the history of the crust–mantle interaction in an Archean suprasubduction zone mantle wedge.

Mode of slip and crust–mantle interaction at oceanic transform faults

Oceanic transform faults, connecting offset mid-ocean spreading centres, rupture quasi-periodically in earthquakes up to about magnitude M 7.0 that are often preceded by foreshocks. In addition to seismic slip, a large portion of slip takes place as aseismic creep, which likely influences initiation of large earthquakes. Although oceanic transform faults are one of the major types of plate boundaries, the exact mode of slip and interaction between the seismic and aseismic motion remains unclear. Here we present a detailed model of the mode of slip at oceanic transform faults based on data acquired from a recent temporary deployment of ocean-bottom seismometers at the Blanco Transform Fault and existing regional and teleseismic observations. In the model, the crustal part of the fault is brittle and fully seismically coupled, while the fault in the mantle, shallower than the depth of the 600 °C isotherm, creeps partially and episodically. The creep activates small asperities in the mantle that produce seismic swarms. Both mantle and the crustal zones release most of the plate-motion strain during large-magnitude earthquakes. Large earthquakes appear to be preceded by a brief episode of shallow mantle creep, accompanied by seismic swarms, which explains the observation of foreshocks and shows that mantle creep likely influences initiation of large seismic events. Earthquakes in the crust and mantle at transform faults are distinct yet coupled, with seismic swarms in the mantle apparently preceding large earthquakes, according to ocean-bottom seismic monitoring of the Blanco Transform Fault.