Mafic Crust Research Articles

Crustal silica content of East China: A seismological perspective and its significance

East China has experienced multiple periods of tectonic movements, which have contributed to the composition and rheological properties of its present crust. Estimating the composition of the crust is crucial for understanding the tectonic processes. Based on the teleseismic receiver functions with data from the National Seismic Network of China, we applied the H-κ-c method to obtain the crustal bulk VP/VS ratio and to constrain the SiO2 content in the crust. We estimated the SiO2 content to range from 50.89 wt% to 73.51 wt%, with an average value of 65.87 wt%, indicating a predominant felsic composition of the East China's crust. Our study suggests that the North-South Gravity Lineament (NSGL) is an approximate delimitator of the felsic and mafic crust in East China, hinting at a widespread deficiency of mafic lower crust in the east of the NSGL. The mafic crust is extensively distributed in the Taihang orogenic region (TSR) and WuLingshan gravity gradient belts (WLG), particularly in the Datong volcanic area, which manifests the mantle materials intraplating. The scatteredly distributed mafic crust at the east of the NSGL is mainly concentrated in the southeast coast of China and in the intersection region of the Tanlu Fault (TLF) and Sulu region. In Sulu region, the TLF may primarily provide a channel for the thermal intrusion from the underlying mantle lithosphere, which has increased the mafic content. The Pacific/Philippine Sea plate subduction has triggered a significant amount of crust-mantle material exchange below southeast China that resulted in a high degree of mafic crustal composition.

Widespread ca. 800 Ma granitoids in the southern Dabie Orogen: Petrogenesis and implications for Neoproterozoic accretion-type orogeny in the northern Yangtze Block

Large-scale Neoproterozoic granitoids occur widely in the Dabie Orogen of the northern Yangtze Block and provide significant clues to understanding the geological evolution of the Block within the Rodinia supercontinent. We present evidence for a prominent episode of granitoid intrusion at ca. 800 Ma in the southern Dabie Orogen (SDO), verified by new zircon LA-ICP-MS ages from four separate granitoid intrusions ranging from 807 Ma to 795 Ma in age. The granitoids are dominated by monzogranite and granodiorite with bulk-rock A/CNK and zircon δ18O values of 0.90–1.10 and 5.03–6.45‰, respectively, corresponding to I-type affinity. This is also supported by negative correlations of SiO2 vs. P2O5 and Sr vs. P2O5, and positive correlations of SiO2 vs. A/CNK, Rb vs. Y and Rb vs. Th. Low zircon εHf(t) values (−15.03 to −3.84) and relatively old two-stage Hf model ages (mainly between 1900 and 2200 Ma) for these granitoids are consistent with primary derivation of magma from partial melting of Paleoproterozoic metabasites. Relatively low Ga/Al ratios, Zr contents, and zircon saturation temperatures and consistently negative zircon εHf(t) and bulk-rock εNd(t) values are distinct from those of 780–720 Ma A-type granites in the Dabie Orogen that formed in a rift setting. We propose that the ca. 800 Ma granitoids in the SDO were products of variable degrees of partial melting of ancient mafic crust, promoted by heating of the upwelled asthenosphere due to oceanic slab breakoff in an accretion-type orogeny. Our observations suggest that the formation of the unified Yangtze Block was achieved by diachronous assembly of multiple micro-blocks during the Neoproterozoic, consistent with a peripheral location of the Yangtze Block in the Rodinia supercontinent.

Underplating-induced trans-crustal melting and maturation of Neoarchean continental crust in the North China Craton

A global change in granitoid compositions from early predominantly sodic tonalite-trondhjemite-granodiorites (TTGs) to later TTGs and more potassic granites occurred during the late Archean, coupled with a major period of crustal maturation. However, the detailed relationship between granitoid chemical evolution and the maturing crustal process remains enigmatic. Successive granitoid magmatism including late Neoarchean TTGs and high-K granites occurred in the Western Shandong Province granite-greenstone belt (WSP) of the North China Craton and thus preserves crucial clues of the crustal maturation process. In this study, petrology, whole-rock geochemistry, and zircon U-Pb and Lu-Hf isotopes are reported for the late Neoarchean TTG gneisses, monzogranites, and minor metabasaltic to andesitic rocks from the WSP. The ca. 2560−2540 Ma TTG gneisses show low MgO, K2O/Na2O, but high (La/Yb)N, Sr/Y, and absence of Eu anomalies, indicating their derivation from partial melting of the thickened lower mafic crust. The ca. 2530−2500 Ma monzogranites are characterized by systematically high SiO2 and K2O/Na2O, but low MgO and Sr/Y, and moderately negative Eu anomalies, revealing they were formed by intracrustal reworking of local TTGs and sedimentary rocks in the middle to upper crust. Geochemical variations of these crustal-derived granitoids suggest that they were formed by melting at gradually higher crustal levels with the melt zone moved gradationally from the eclogite stability field into the plagioclase stability field. The ca. 2530−2500 Ma calc-alkaline metabasaltic to andesitic rocks sourced from metasomatized mantle outline roles of mantle-derived magma underplating in contributions of heating and trans-crustal melting magmatism. The long-term melting processes facilitated the upward movement of volatiles and heat-producing elements from deep to shallow crustal levels, and introduced K-enriched monzogranites into the upper crust, leaving a refractory, strengthening, and tectonically stable lower crust. Secular compositional evolution of crustal-derived granitoids reveals that continuous crustal reworking drove lithosphere differentiation and paved the way for the maturation of the Archean continental crust.

Cassiterite and zircon U-Pb ages and compositions from ore-bearing and barren granites in Thailand: Constraints on the formation of tin deposits in Southeast Asia

The Southeast Asian Tin Province comprises western, central, and eastern belts and hosts significant granite-related Sn deposits. The genetic links between granites and Sn mineralization are still unclear. Most Sn deposits are in Thailand’s western and central belts, but their origin remains poorly elucidated due to the absence of direct dating of mineralization. Herein in-situ U-Pb age data of wolframite and cassiterite grains from nine representative Sn deposits in Thailand are obtained, which fall into two stages. Triassic deposits (224–210 Ma) are found in the Central belt, with Cretaceous deposits (78–67 Ma) in the Central belt being younger than those in the Western belt (84–74 Ma). However, ore-bearing granites, spanning two periods (227–205 Ma and 85–69 Ma), occur in central and western belts. Some Triassic ore-bearing granites exhibit significantly older ages than ore-forming ages. Newly identified ore-forming granites contain zircon grains with relatively low εHf(t) values (−29.5 to + 4.1; average = − 11.8), indicative of an origin from supracrustal sediments from the Sibumasu block. In contrast, barren granites have ages from 303–224 Ma and higher zircon εHf(t) values (−9.9 to + 13.2; average = +1.2), which suggests that they were derived from the juvenile mafic crust. Even after experiencing hydrothermal fluids exsolution, some low-fractionated ore-forming granites (D.I. < 90) still maintain remarkably high tin contents. Both ore-forming and barren granites crystallized under reducing conditions. Our study highlights the importance of Sn-rich sources of parental magmas in forming Sn deposits. The metasediment-rich basement of the Sibumasu block distributing along the continental margin is likely the Sn-rich source. These sources played a crucial role in forming two stages of tin deposits in distinct tectonic settings, that are syn-collisional crustal thickening in Paleo-Tethys and post-collisional extension-related settings in Neo-Tethys.

Unraveling abiotic organic synthesis pathways in the mafic crust of mid-ocean ridges

The aqueous alteration of the oceanic lithosphere provides significant energy that impacts the synthesis and diversity of organic compounds, which are crucial for the deep carbon cycle and may have provided the first building blocks for life. Although abiotic organic synthesis has been documented in mantle-derived rocks, the formation mechanisms and complexity of organic compounds in crustal rocks remain largely unknown. Here, we show the specific association of aliphatic carbonaceous matter with Fe oxyhydroxides in mafic crustal rocks of the Southwest Indian Ridge (SWIR). We determine potential Fe-based pathways for abiotic organic synthesis from CO2 and H2 using multimodal and molecular nano-geochemical tools. Quantum mechanical modeling is further employed to constrain the catalytical activity of Fe oxyhydroxides, revealing that the catalytic cycle of hydrogen may play a key role in carbon-carbon bond formation. This approach offers the possibility of interpreting physicochemical organic formation and condensation mechanisms at an atomic scale. The findings expand our knowledge of the existence of abiotic organic carbon in the oceanic crustal rocks and emphasize the mafic oceanic crust of the SWIR as a potential site for low-temperature abiotic organic synthesis.

From onset of Cadomian subduction to forearc-arc-continent collision: Insights from the Neoproterozoic Calzadilla Metaophiolite (SW Iberia)

ABSTRACT The Cadomian basement of SW Iberian Massif appears in the so-called Ossa-Morena Complex, which contains several allochthonous terranes that are key for deciphering the evolution of the Gondwana margin during the Neoproterozoic. In this context, the Calzadilla Ophiolite represents the remnants of an oceanic Ediacaran lithosphere formed by a boninitic mafic crust that overlies a serpentinized ultramafic section, the latter being the biggest outcrop of Cadomian serpentinites in Europe. In this work, the petrological and geochemical nature of the mantle section is analysed, as well as the tectonothermal evolution of both, the mafic crust and the ultramafic section. The algebraic analysis of whole rock geochemistry of the serpentinites indicates that the protolith corresponds to harzburgite. The geochemical fingerprint of the serpentinites, as well as the primary spinel crystals composition, shows that peridotites underwent high degrees of partial melting (up to 16%) in a forearc setting. Thermodynamic modelling and average PT methods were applied to serpentinites and mafic dykes from the ultramafic section as well as to amphibolites from the mafic section, both of which experienced progradation up to 4–5 kbar and 525–575°C. According to the data, the protoliths of the Calzadilla Ophiolite were formed in a peri-Gondwanan arc system developed during the subduction of oceanic crust below Gondwana. The roll-back of the subducting slab would induce the opening of the forearc basin at c. 602 Ma, protolith age of amphibolites from crustal section, with an extensive partial melting of the mantle wedge and the formation of boninitic-affinity magmas that constitute the mafic section of the Calzadilla Ophiolite. Metasomatism and serpentinization of the mantle wedge would occur via melt- and fluid–rock interactions. A compression-dominated stage at c. 540 led to the emplacement of the Calzadilla Ophiolite onto the Cadomian magmatic arc, with the development of the amphibolite-facies metamorphic imprint.

The significance of Paleoarchean granitoids from the Saglek Block, Labrador, Canada

The Saglek Block in the Nain Province of northeast Canada is part of the North Atlantic Craton. It comprises Archean gneisses that record magmatic and metamorphic ages between ca. 3.9 and 2.5 Ga. In this study, a grey banded gneiss from Maidmonts Island records an age of 3.72 Ga, which is equated with Uivak I gneisses reported from across the Saglek Block, and contains 3.8 Ga xenocrysts. These rocks were deformed and metamorphosed prior to the intrusion of augen gneiss protoliths on both Maidmonts and Mentzel islands that record U-Pb zircon ages of 3.33 Ga. These rocks are composed of ferroan calc-alkaline granite and granodiorite and were likely generated by partial melting of pre-existing quartzo-feldspathic crust, as attested by the presence of ca. 3.8 Ga xenocrysts. Such augen gneiss was previously classified as ca. 3.6 Ga ‘Uivak II gneiss’, a term we argue should now be abandoned. The final magmatic events recorded on Maidmonts and Mentzel islands took place in the Neoproterozoic with the emplacement of granitic stocks at ca. 2.72 Ga, and sills and dykes at ca. 2.57 Ga. This sequence of magmatic events from the Eoarchean to Neoarchean is very similar to that recorded in the Itsaq Gneiss Complex of southwest Greenland, where, based on the Hf signature, it has been suggested that a change in tectonic environment resulted from the initiation of subduction at ca. 3.2 Ga. Although Lu-Hf and Sm-Nd isotopic signatures from the ca. 3.33 Ga gneisses have been interpreted in the literature as due to partial melting of Hadean mafic crust, alternatively, they can be generated by partial melting of Eoarchean continental crust in a continental arc setting. We argue this is more consistent with the non-TTG geochemistry of the augen gneisses.

What controls structural variations along the Zagros Collision Zone? Insights from geophysical observations and thermo-mechanical modelling

The Zagros Collision Zone is a complex tectonic region formed as a consequence of the collision between Arabia and Eurasia after the subduction of the Neo-Tethys ocean. The NW-SE striking Zagros orogen consists of the following parallel tectonic units (from SW to NE): Zagros Fold and Thrust Belt (ZFTB), Sanandaj–Sirjan Metamorphic Zone (SSZ), and Urumieh–Dokhtar Magmatic Arc (UDMA). In this study, we perform a combined analysis of recent geophysical data, revealing pronounced differences in the crustal and lithospheric structure along the Zagros Mountains. The northwestern sector shows a fairly uniform crustal thickening across the broad symmetric orogen from the ZFTB to the UDMA. In contrast, in the central Zagros, the transition from a relatively narrow zone of high elevations and high-frequency relief in the ZFTB to a smoother surface topography of the SSZ and UDMA occurs with an abrupt increase in Moho depth below the SSZ. The last observation has recently been interpreted as a result of “relamination” process, where the felsic upper crust of the Arabian plate underthrust the mafic crust of the Iranian plate. We present geodynamic numerical models of crustal relamination during continental collision and compute static gravity field of the resulting structures. We show that oblique closure of the Neo-Tethys affects lateral variations in the style and extent of crustal relamination, which control the observed along-strike changes in crustal configuration and orogen altitude. In particular, a narrow and higher orogen (as in the central Zagros) develops in the experiments with a young and wide oceanic plate, whereas an old and narrow subducting plate tends to form a broad and lower topography (as in the northwestern Zagros). This is geometrically consistent with the progressive closure of the Neo-Tethys from NW to SE during the oblique continental collision between Arabia and Eurasia.

Insights into the late Archean evolution of the Carajás Province: Geochemical and geochronological constraints from the Santana do Araguaia complex, Amazonian craton, Brazil

To the south of the Carajás Province, Amazonian Craton, 2.85–1.88 Ga old rock units underlie the Santana do Araguaia Domain (SAD), bounded by the Iriri-Xingu and Rio Maria domains and the Tocantins Province. The SAD differs from the Rio Maria Domain, as indicated by the scarcity of TTGs and the dominance of both high-K to weakly sodic granites, sodic granodiorites, and monzodiorite/quartz monzodiorite with high-Ba + Sr contents. Therefore, the SAD is a particular case to assess tectonic settings in the Archean, continental crust generation and the timing/nature of the Meso-Neoarchean transition from TTG-type to calc-alkaline high-K and sodic granitoid magmatism to the south of the Rio Maria Domain. The magmatic evolution of the SAD started at 2.85 and extended up to ∼1.88 Ga with a significant contribution from the pre-existing crust, differing from that of the Rio Maria Domain (3.0–2.86 Ga). The Santana do Araguaia Complex (SAC) comprises deformed to non-deformed rocks divided into (i) tonalite (TTG with high Al2O3 and medium La/Yb ratio), (ii) sodic granodiorite group (K2O/Na2O < 1) with Ba- and Sr-rich varieties, which can be divided into type 1 displaying high Sr/Y and La/Yb ratios, and type 2 with low Sr/Y and low to intermediate La/Yb, both resembling the sanukitoids low-Ti, (iii) hybrid granite group (high-K calc-alkaline), composed of two sub-types of high-Ba + Sr monzogranites, one of high Sr/Y that shares many features with the CA2-type of Archean calc-alkaline granites and HSG, and a second type, with low Sr/Y ratio and akin chemistry with the CA1 and normal granite (NG). Group (iv) is of biotite or low Ba–Sr granites classified as syenogranites CA1/NG strongly fractionated calc-alkaline rocks. Group (v) consists of rocks with high Ba–Sr but with moderate Mg# (mean = 44.8), low contents of transition elements, and moderate K2O/Na2O ratios (mean = 0.72); the group is divided into monzodiorite with high Sr/Y, La/Yb ratios, and low Y, and quartz monzodiorite with low Sr/Y, moderate La/Yb ratio and Y values, both with sanukitoids high-Ti characteristics. The rocks that constitute the studied complex show arc-like geochemical signatures, suggesting emplacement during the end of phase B (collisional) and the start of phase c (thermal relaxation/colappse) of stage 2 in the two-stage tectonic-magmatic model (subduction/collision-post-collision). In the Carajás Province the first stage of the model is marked by the genesis of TTG (3.0–2.94 Ga) –juvenile magmatism of the Rio Maria Domain, and the second stage, collisional phase, between 2.89 and 2.85 Ga with sanukitoid rocks, hybrid and biotite granites as well as significant occurrences of high Ba + Sr rocks and post-collisional granitic suites (∼2.74 Ga). However, the elemental whole-rock geochemistry and geochronology datasets show that the Santana do Araguaia Complex (SAC) is marked by high-K and Ba + Sr rocks such as abundant sanukitoid suites, hybrid and biotite granites, but with very scarce TTGs. The ages between 2.85 and 2.81 Ga obtained from SAC suggest the final phase “a” and beginning of phase “b” of the second evolutionary stage, collisional phase, and slab-breakoff/post-collisional, probably the terminal tectonic-thermal episode of Archean accretion and differentiation in the Amazonian Craton. The SAC rocks' contrasting Sr/Y and La/Y ratios cannot be explained exclusively by variation in the melting pressure (melts of mafic crust at >50 km) in a thickened continental crust. Multiple crustal petrogenetic processes or heterogeneous sources can produce the heterogeneous geochemical signatures that characterize the rocks of the complex. The admitted sources of these Archean granitoid magmas include basaltic andesite, sanukitoids, and metagreywackes, subordinate TTGs, at low to moderate pressures melting as they suggest low values of (Dy/Yb)n below 1.9 and relatively high-Sr contents. The mesostructure data indicate that the studied rocks were affected by high-temperature deformation (>600 °C) with partial melting.

Generation of Archean TTGs via sluggish subduction

Abstract The trondhjemite-tonalite-granodiorite (TTG) suite of rocks prominent in Earth’s Archean continents is thought to form by melting of hydrated basalt, but the specific tectonic settings of formation are unclear. Models for TTG genesis range from melting of downgoing mafic crust during subduction into a hotter mantle to melting at the base of a thick crustal plateau; while neither uniquely defines a global tectonic regime, the former is consistent with mobile lid tectonics and the latter a stagnant lid. One major problem for a subduction model is slabs sinking too quickly and steeply in a hotter mantle to melt downgoing crust. I show, however, that grain size reduction in the lithosphere leads to relatively strong plate boundaries on the early Earth, which slow slab sinking. During this “sluggish subduction,” sinking plates can heat up enough to melt when the mantle temperature is ≳1600 °C. Crustal melting via sluggish subduction can thus explain TTG formation during the Archean due to elevated mantle temperatures and the paucity of TTG production since due to mantle cooling.

An Overview on the Composition and Age of Upper Crust of Proto‐Tethyan Lajishan Intra‐oceanic Arc, NE Tibet Plateau

AbstractIdentification and anatomy of oceanic arcs within ancient orogenic belt are significant for better understanding the tectonic framework and closure process of paleo‐ocean basin. This article summarizes the geological, geochemical, and geochronological characteristics of upper crust of Proto‐Tethyan Lajishan intra‐oceanic arc and provides new data to constrain the subduction evolution of the South Qilian Ocean. The intra‐oceanic arc volcanic rocks, including intermediate–mafic lava, breccia, tuff, and minor felsic rocks, are distributed along southern part of the Lajishan ophiolite belt. Geochemical and isotopic compositions indicate that the intermediate–mafic lava were originated from depleted mantle contaminated by sediment melts or hydrous fluids, whereas the felsic rocks were likely generated by partial melting of juvenile mafic crust in intra‐oceanic arc setting. Zircons from felsic rocks yield consistent and concordant ages ranging from 506 to 523 Ma, suggesting these volcanic rocks represent the relicts of upper crust of the Cambrian intra‐oceanic arc. Combined with the Cambrian forearc ophiolite and accretionary complex, we suggest that the Cambrian intra‐oceanic arc in the Lajishan ophiolite belt is belonging to the intra‐oceanic arc system which was generated by south‐directed subduction in the South Qilian Ocean at a relatively short interval between approximately 530 and 480 Ma.

Petrogenesis of early-middle jurassic igneous rocks in the Central Jilin Province, NE China: implications for the early subduction of the Paleo-Pacific ocean and crustal growth

ABSTRACT Crustal growth is closely associated with the oceanic subduction process, although the precise mechanism involved remains poorly constrained. Northeast (NE) China is an important component of the largest accretive orogenic belt worldwide – Central Asian orogenic belt. Its accretionary process was dominated by the Paleo-Asian and Paleo-Pacific Oceans; thus, NE China can provide a promising insight to explore this complex mechanism. We report new geochronological, geochemical and Hf isotopic compositions of the igneous rocks found in the Central Jilin Province, NE China. The geochronological results reveal that these igneous rocks were formed during 174.7 − 167.7 Ma (i.e. Early−Middle Jurassic). The quartz diorite and monzogranites from the northeastern margin of the North China Craton have high Sr (314 − 757 ppm) content, (La/Yb)N (7.11 − 29.9) and Sr/Y (41.50 − 157) ratios, low Y (2.03 − 12.97 ppm) and Yb (0.30 − 1.34 ppm) contents, exhibiting adakitic features. Coupling with their significant negative εHf(t) values (−32.51 to − 5.45), we propose that these igneous rocks were originated from the partial melting of an ancient thickened lower crust. In contrary, the syenogranite, granite porphyry, and porphyritic monzogranite from the Zhangguangcai and Jiamusi blocks are characterized by high K calc-alkaline, low MgO (0.03 − 0.88 wt.%), Zr + Nb + Ce + Y (141 − 462 ppm) contents and Sr/Y (0.93 − 17.40) ratios, indicating an affinity of I−type granite. In view of their positive εHf(t) values (5.41 to 9.47), we suggest that they were originated from the partial melting of a juvenile lower mafic crust. Despite they have different magma sources, all the Jurassic igneous rocks exhibit typical geochemical signatures associated with subduction, such as enriched in large ion lithophile elements and depleted in high field strength elements. Considering the spatial-temporal distribution of coeval igneous rocks within the region, we propose that the Early Jurassic is the early subduction of the Paleo-Pacific Ocean. The Jurassic igneous rocks can be divided into two categories, one with high Sr/Y ratios and negative Hf isotope, and the others with low Sr/Y ratios and positive Hf isotope, which may indicate that crustal architecture in subduction zone has a significant effect on continental crustal growth and reworking in NE China.

Subsurface distribution of granites and greenstones in the Paleoarchean East Pilbara Terrane from 3-D gravity inversion

The Paleoarchean East Pilbara Terrane contains some of the oldest rocks on Earth, and has a distinctive surface architecture of granite domes surrounded by arcuate greenstone belts. Explanations for creation of the domes include fold interference, core complex formation and the subsidence of a dense, mafic crust between relatively buoyant granitic complexes. In the latter “sagduction” process, greenstones are often viewed as descending to the base of the crust. Here we present a 3-D inversion of Bouguer gravity data for density contrast constrained by shear wave velocities and a seismic reflection profile. We show that the East Pilbara Terrane is predominantly characterized by a two-layer crust with high-density greenstone rocks largely confined to the upper crust, which is inconsistent with vertical tectonic models requiring large-scale greenstone preservation in the lower crust. The eastern margin of the terrane is marked by a west-to-east transition from low-density ovoid bodies to linear northwest-trending sources associated with the Proterozoic Paterson Orogen. We identify the previously unknown 10–15 km wide Tumbinna Pool Dome, between the Mount Edgar and Yilgalong domes. Higher density sources related to the surface distribution of greenstone rocks largely occur above 10–15 km depth, though they may locally reach >15 km depth on the north side of the Mount Edgar Dome. Some granitic domes have higher density cores related to distinct granite suites, while the Split Rock Supersuite appears to exhibit systematically lower density at depths of 5–10 km. Low-density sources due to small intrusions such as the North Pole Dome only extend to depths up to 10 km, while the large Mount Edgar Dome can be identified at depths as great as 15–20 km. Some low-density bodies in the lower crust, one of which underlies the Yilgalong Dome, correlate with regions of reduced seismic reflectivity, which we interpret to be possible relict migmatisation zones or melt pathways. A weakly constrained ESE-trending high-density region immediately below the Moho correlates with isolated sub-Moho reflectors, and may be a relict zone of rifting and delamination.

Fault-scale crustal structure across the Dunhua-Mishan fault (Tanlu northern segment) constrained from teleseismic P-wave receiver functions

The Dunhua-Mishan fault, located in the northern segment of the Tanlu fault zone, experienced multiple tectonic processes associated with the effects of the Pacific Plate subduction and the Indo-Asia collision. The high-resolution fault-scale structure is critical for understanding the fault evolution and potential fault damage. However, the well-defined deep structure of the Dunhua-Mishan fault is still unclear due to the lack of the dense seismic array. In this study, we construct a high-resolution P-wave receiver function imaging based on linear dense seismic array across the fault. Our results reveal the strong Moho depth variation across the Dunhua-Mishan fault zone. The slightly higher Vp/Vs ratio values within the fault zone indicate the presence of a small amount of mafic crust composition. Interestingly, the significant double positive Ps converted phases are observed within the fault zone, which may represent double Moho discontinuities. The double Moho structure may be related to multiple significant tectonic activities in the Tanlu northern segment. These newly observed structures provide new seismic constraints on the formation and evolution of the Tanlu fault zone and probably reflect that the lithospheric structure of the Dunhua-Mishan fault has been modified by a series of tectonic processes.

Plagioclase as archive of the incremental assembly of the Quxu batholith, South Tibet: Implication for the nature of magma reservoir

The formation of magma reservoir and resultant construction of large batholith are increasingly recognized as long-term incremental processes. Here we employed back-scattered electron and cathodoluminescence images, in-situ major and trace elements, and Sr isotopes of plagioclase for five lithological groups from the Quxu complex batholith (south Tibet), which shed new light on the nature and incremental construction processes of batholith. Our results show that plagioclases display diverse textures and show systematic differences in geochemistry and Sr isotopes for different lithologies, indicating that their host rocks originated from multiple magma sources and/or underwent distinct magmatic processes. Plagioclases in Group I granodiorites and Group III mafic dykes and mafic magmatic enclaves exhibit comparable textural features (e.g., resorption and sieved textures) and share similar Sr (< 1500 ppm) and Ba (< 200 ppm) contents, and 87Sr/86Sr ratios (ISr) (0.7035–0.7050) at given lower An (20–50) values, suggesting that they were evolved from similar mafic magma sources. The significant variations in An (20–90), Sr and Ba contents of plagioclase in Group III indicate both fractional crystallization and magma mixing. Plagioclases in Group II monzogranites have a similar An ranges to Group I but higher Sr (900–2200 ppm) contents and ISr (mostly 0.7045 to 0.7066) values, suggesting that they formed from partial melting of juvenile mafic crust without significant mantle contribution. Plagioclases in Group IV granodiorites and Group V monzogranites exhibit higher Sr (500–3000 ppm) contents than other groups at given An values. Their ISr values fall within the range of Group II, indicating a similar magma source. Abundant plagioclases displaying normal and oscillatory zonings in them implies that fractional crystallization occurred, while the presence of typical glomerocrysts in Group V indicates mush disaggregation. The simulation results of the trace elements indicate that the different lithologies of the Quxu batholith represent different states of magma reservoirs, corresponding to crystal mushes with varying degrees of extracted melts. The incremental growth processes of lager batholith operate as an open system, in which magma convection, mixing and recharge, crystal accumulation, recycling and reaction with melts, and mush disaggregation commonly occur.

Petrogenesis of Neoarchean granitic gneisses from the Daqishan area of the Dabie orogen and implications for the early crustal evolution of the Northern Yangtze Craton

ABSTRACT The Yangtze Craton is one of the crucial cratons constitued the eastern Asia continent. It is characterized by Archaean to Paleoproterozoic basement rocks, making it an excellent area to study the Precambrian geologic and tectonic regime. Recently, abundant Archaean rocks have been reported from the Dabie Orogen, which providing important information to understanding the early evolution of the Northern Yangtze Craton. In this contribution, we present petrological, whole-rock geochemical, and zircon U – Pb – Hf isotopic analyses on newly discovered Neoarchean granite in the Dabie Orogen. We identified 2744 ± 10 Ma and 2715 ± 11 Ma (~2.74 Ga) Na-rich granitic gneisses, 2691 ± 6 Ma and 2696 ± 8 Ma (~2.69 Ga) A2-type granitic gneisses, and 2664 ± 8 Ma, 2664 ± 6 Ma, 2672 ± 7 Ma, 2642 ± 10 Ma (~2.66 Ga) A1-type granitic/syenite gneisses from the Daqishan area of the Dabie Orogen. Geochemical and zircon Hf isotope data indicate that the ~ 2.74 Ga Na-rich granitic gneisses were partial melting products of thickened mafic crust with limited depleted mantle input, while the 2.69 Ga and 2.66 Ga A-type granitic gneisses represent felsic magmatism in a post-collisional to an extensional tectonic setting. Based on our research, we categorized the Northern Yangtze Craton into the Kongling domain and Dabie domain by comparing with the distribution of the zircon U-Pb magmatic ages and metamorphic events. We suggest that the initial unified basement of the northern proto-Yangtze Craton formed during the Nuna supercontinent cycle at ~ 2.0 Ga.

From 3.4 Ga TTG generation to 2.9 Ga crustal anatexis: The Archean crustal evolution of Porteirinha Complex (SE, Brazil)

The Porteirinha Block (PB) provides an important record of the origin and evolution of South American crust. The PB records the generation of Paleoarchean TTG, Mesoarchean anatexis, and Paleoproterozoic crustal reworking. It is exposed as a basement inlier in the Araçuaí Orogen as a result of a tectonic involvement of the continental margins of São Francisco Congo Paleocontinent (PSFC) during the collisional processes of Gondwana amalgamation at the Neoproterozoic. The PB basement unit is the Porteirinha Complex (PC), which consists of TTG orthogneisses, migmatites, amphibolites, metaultramafics rocks and Neoarchean granitoids. In this paper, we discuss the evolution of the TTG orthogneisses and migmatites from PC based on field, geochronological and whole-rock geochemistry data. Zircon U–Pb ages (LA-ICP-MS) show a complex and polycyclic evolution of these rocks. Concordant ages were obtained for crystallization of TTG magma at ca. 3.4 Ga and the geochemical signature supports the idea of TTG generation by partial melting of hydrated mafic crust under garnet-amphibolite facies conditions. Zircon U–Pb ages also suggest crustal reworking events at 3.1 Ga. Metatexite and diatexite domains related to partial melting of TTG orthogneisses are individualized. The migmatites record processes of crustal anatexis and melt generation that occurred between 2.8 and 2.9 Ga (LA-ICP-MS, U–Pb in zircon) and are linked to crustal reworking and heat generation events recognized regionally. The new data presented in this work allow us to establish correlations between Archean and Paleoproterozoic units of the PB and those that occur regionally, particularly those related to Gavião block, where a series of stages and events of early crustal evolution are shared.

Petrogenesis of early Paleozoic granitoids in the North Qinling Orogen, Central China: Implications for crustal evolution in an accretionary orogen

Accretionary orogens are sites of extensive continental crustal growth and modification. The mechanism by which mafic crust is transformed into silicic melts (i.e., maturation of continental crust) is important for understanding the formation of the continental crust. The North Qinling Orogen (NQO) is a composite orogenic belt and contains an early Paleozoic accretion-dominated orogenic system, which is ideal for investigating continental crustal maturation. We obtained zircon and monazite U–Pb age and O isotope data, zircon Lu–Hf isotope data, and whole-rock major- and trace-element and Sr–Nd isotope data for early Paleozoic granitoids of the NQO. The granitoids are divided into three groups. Group 1 includes the Taiping tonalite (445 ± 3 Ma), the Manziying syenogranite (445 ± 2 Ma), and the Huoshenmiao granodiorite (436 ± 2 Ma). The Taiping and Huoshenmiao plutons have relatively high SiO2 contents (68.64–71.67 wt.%) and Na2O/K2O ratios (1.15–4.19), with enrichments in Rb, Ba, Th, and U and depletions in Nb, Ta, P, and Ti, and they are geochemically similar to sodic arc magmas. The Manziying syenogranite is a peraluminous potassic granite with high K2O contents (4.59–5.27 wt.%). Grantioids from Group 1 have similarly depleted Sr–Nd–Hf–O isotopic features (εHf[t] = +5.9 to +8.8; δ18O = 3.98‰–5.41‰), indicative of derivation via partial melting of oceanic arc crust, which suggests that partial melting of oceanic arc crust in a subduction system contributes to the generation of continental crust and causes its maturation. Group 2 consists of the Wuduoshan monzogranite (418 ± 2 Ma) and the Sikeshu granodiorite (423 ± 3 Ma). These plutons have relatively high SiO2 (65.59–72.06 wt.%), K2O (3.26–4.79 wt.%), and Al2O3 (14.65–16.12 wt.%) contents and Sr/Y (33–87) and (La/Yb)N (23–48) ratios. The Wuduoshan monzogranite has positive zircon εHf(t) (+0.4 to +3.1) and uniform δ18O (6.38‰–8.07‰) values, but the Sikeshu granodiorite has more variable isotopic compositions (εHf[t] = −1.9 to +5.0; δ18O = 6.37‰–10.60‰). The Wuduoshan monzogranite and Sikeshu granodiorite have similar whole-rock Sr–Nd isotopic compositions to basement rocks of the NQO. These features indicate that the two plutons formed by partial melting of basement rocks (i.e., subducted into the lower crust) of the North Qinling unit, along with juvenile crustal material. Group 3 is represented by the Xiaguan monzogranite, which formed at 434–430 Ma, and can be subdivided into heavy rare earth element (REE)-depleted and -enriched units. The former has high Sr/Y (56–98) and (La/Yb)N (34–73) ratios and low MgO (0.13–0.24 wt.%), Cr (0.37–1.69 ppm), and Ni (0.32–1.09 ppm) contents, similar to adakites derived from metabasaltic sources. The heavy REE-enriched nature of the Xiaguan monzogranite may reflect modification of its source by melt or fluid. Our results show that partial melting of enriched oceanic arc crust contributed to crustal maturation in an accretionary orogen. The addition of evolved crustal material also facilitated this process; therefore, the basement rocks and crustal thickness should be considered when assessing crustal dynamics in an accretionary orogen.

A Long-Lived Accretionary Process during the Amalgamation of the North China Craton: Insights from Neoarchean–Paleoproterozoic Polyphase Magmatism in the Lüliang Complex

Abstract There has been a long debate regarding the timing of the final amalgamation of the North China Craton, which is considered to have occurred either during the Neoarchean or Paleoproterozoic era. One major point of contention is whether there existed a long-lived subduction lasting through the Neoarchean to Paleoproterozoic. The Lüliang Complex contains multiphases of magmatism and thus represents the most viable region to address this controversy. In this study, we carried geochronological and geochemical analysis on the representative granitoids. Secondary ion mass spectrometry U–Pb dating revealed four distinct granitoid groups emplaced at 2531 ± 4, 2189–2173, 2027 ± 25, and 1852 ± 41 Ma, respectively. Notably, the 2531 Ma granitic gneiss was identified for the first time in this region. Based on the geochemical characteristics, the granitoids can be divided into two types. The 2531 and 2027 Ma groups display I-type features, while the 2189–2173 and 1852 Ma groups exhibit A-type geochemical affinities. Both I-type groups exhibit enrichment in Rb, depletion in Nb, Ta, and Ti, moderate fractionated REE patterns, substantial negative Eu anomalies, low Sr/Y ratios, and positive εHf(t) (+3.51 to +5.53 and +5.59 to +7.32, respectively), indicating that they were generated from partial melting of the juvenile mafic crust. In contrast, the 2189–2173 Ma granitoids belong to A2-type and were most likely generated by the partial melting of felsic rocks in the back-arc region, while the 1852 Ma granitoids belong to A1-type and were most possibly the result of partial melting of mafic-intermediate rocks during the post-collisional stage. Based on the records of A-type granitic magmatism and the ~1950 Ma peak metamorphism throughout the Trans-North China Orogen, we propose that a long-lived subduction process (2531–1950 Ma) can mostly explain the existing geological phenomena. It is likely that the subduction between the Eastern and Western Blocks should have commenced at ~2531 Ma, followed by a long-lived subduction. The two blocks ultimately collided with each other to form the North China Craton at ~1950 Ma, which triggered post-collisional exhumation and partial melting at ~1852 Ma.

The Singhbhum Craton (India) records a billion year of continental crust formation and modification

The petrogenesis of continental crust from its ultimate mantle source can be reconstructed from the element abundances and radiogenic isotope compositions of ideally pristine igneous rocks. The initial isotope compositions of igneous rocks provide geochemical constraints on the age, composition and evolution of their source(s). Determining initial isotope ratios for rock samples can be challenging, especially in rocks with a long and protracted thermal history. The Rb-Sr system is highly sensitive to parent-daughter element fractionation during magma differentiation. This makes the Rb-Sr isotope systematics ideal to trace the precursor composition of Archean felsic crust and constrain the time of element fractionation during the formation and subsequent modification of continental crust. Initial isotope compositions can be obtained directly from minerals that strongly prefer the daughter element and effectively exclude the parent element of the radio-isotope system of interest. Apatite, having a near zero Rb/Sr ratio, is ideal for preserving its initial 87Sr/86Sr and zircon records initial 176Hf/177Hf compositions. Combined modelling of Sr and Hf isotope data from granitoids of the Archean Singhbhum Craton, indicates that the older Paleoarchean granitoids, emplaced between 3.53 Ga and 3.44 Ga, were derived from a mafic precursor (∼52–54 wt% SiO2) sourced from a depleted mantle at ∼3.71 Ga. Initial 87Sr/86Sr isotope signatures of matrix apatite and apatite inclusions in zircon from the younger Paleoarchean granitoids (3.4–3.2 Ga) of the Singhbhum Craton indicate these younger granitoids were produced by mixing of magma generated from an older mafic source and partial melts derived from the older granitoids. The combined Sr-Hf isotope modelling links the timing of mantle extraction of the precursor material for Paleoarchean Singhbhum granitoids with a known mafic crust extraction event at ∼3.71 Ga. In combination, the new Sr isotope data from apatite combined with whole rock and zircon Hf isotope data from the literature reveal a ∼1 Ga protracted crustal growth and differentiation history of the nucleus of the Singhbhum Craton. By combining radio-isotope systems like 87Rb-87Sr and 176Lu-176Hf, the petrogenesis of Archean felsic crust from the extraction of mafic material from the mantle to reworking in an orogenic cycle to emplacement can be reconstructed. This approach can be applied to other greenstone-gneiss terranes to quantify the spatio-temporal and compositional evolution of voluminous felsic crust and the formation of cratons in the Archean.