Delamination Of Lower Crust Research Articles

Crustal anatexis and mantle-derived magmas forming Neoarchean A-type granitoids in Carajás Province, northern Brazil: Petrological evidence and tectonic control

Neoarchean hybrid granitoids from the Vila União suite are located in the central portion of the Canaã dos Carajás domain, Carajás Province (Amazonian craton, Brazil), and suggest that magma mixing events played an important role in their origin. According to complete evidence, including field observations, petrography, geochemical modeling, zircon SHRIMP and LA-MC-ICP-MS U-Pb ages, and Nd-Hf isotope data, we propose a geodynamic model based on initial mafic underplating promoted by lower crustal delamination, which generated mantle-derived mafic melts that were injected into the lower/middle crust (composed of Mesoarchean felsic granulite) and caused partial melting, producing leucogranitic melts. The interaction between these contrasting felsic and mafic magmas occurred by both mixing and mingling processes. The hybrid magma ascent and emplacement occurred in a N–S pure shear-dominated transpressional tectonic regime, with an E–W sinistral sense of tectonic movement, in which the hybrid melt was channeled into pre-existing shear zones trending E–W (Itacaiúnas shear zone). This tectonic regime (transpressional) proceeded coevally with the hybrid magma cooling, producing 2.75–2.73 Ga Neoarchean deformed A-type granitoids with a syntectonic nature in the Vila União suite, in addition to the Pium diopside-norite, which represents the mafic endmember generated by underplating.

Reply to comment by Qi and Wang on “Similar crust beneath disrupted and intact cratons: Arguments against lower-crust delamination as a decratonization trigger”

• Their discussion was based on misinterpretation of our data. • Their comments do not alter our original interpretation. • Both crust beneath disrupted and intact parts of the craton are low-Vp and andesitic.

Discussion on “Similar crust beneath disrupted and intact cratons: Arguments against lower-crust delamination as a decratonization trigger” by Ma et al., 2019

Discussion on “Similar crust beneath disrupted and intact cratons: Arguments against lower-crust delamination as a decratonization trigger” by Ma et al., 2019

Similar crust beneath disrupted and intact cratons: Arguments against lower-crust delamination as a decratonization trigger

The continental lithosphere is not forever; some cratons have lost their original roots during the course of their evolution. Yet, it is not clear whether gravitational instability of dense lower crust is the primary driver of decratonization. This is addressed here with emphasis being placed on the North China Craton (NCC), because it represents one of the best examples of craton-root disruption in the world, and a place where models can be tested. If lower-crustal delamination was the trigger for decratonization, we would expect a clear contrast in crustal structure and composition between disturbed (rootless) and intact cratons. However, the eastern (disturbed) and western (intact) parts of the NCC show virtually identical physical structure and composition (a thin mafic lower crust and a predominantly intermediate composition overall) although the crust in the disturbed part is thinner than in the intact craton. This suggests that delamination of the lower crust was not a viable mechanism of craton-root disruption in the NCC case. Indeed, the crust beneath the NCC largely resembles those of stable Archean cratons worldwide. Therefore the delamination, if it occurred, may have taken place much earlier (Archean) than previously thought, rather than in the Mesozoic. Delamination may have been a common phenomenon in the early evolution of cratons, probably due to relatively higher mantle temperatures in the Archean Eon.

A gravity study along a profile across the Sichuan Basin, the Qinling Mountains and the Ordos Basin (central China): Density, isostasy and dynamics

In order to investigate the structure of the crust beneath the Middle Qinling Mountains (MQL) and neighboring areas in the North China Block and South China Block, a north-south gravity profile from Yuquan in the Sichuan Basin to Yulin in the Ordos Basin was conducted in 2011. The Bouguer gravity anomaly is determined from a high-quality gravity dataset collected between 31°N and 36°N of latitude, and varies between −200 and −110mGal in the study region. Using accredited velocity density relationships, an initial crust-mantle density model is constructed for MQL and adjacent areas, which is later refined interactively to simulate the observed gravity anomaly. The present study reveals the features of the density and Bouguer gravity with respect to the tectonic units sampled by the profile. The lithosphere density model shows typical density values that depict a layered structure and allow differentiate the blocks that extend along the reference profile. The gravity field calculated by forward modeling from the final density distribution model correlates well with the measured gravity field within a standard deviation of 1.26mGal. The density in the crystalline crust increases with depth from 2.65g/cm3 up to the highest value of 2.95g/cm3 near the bottom of the crust. The Conrad interface is identified as a density jump of about 0.05g/cm3. The average density of the crust in MQL is clearly lower than the density in the formations on both sides. Starting from a combined Airy-Pratt isostatic compensation model, a partly compensated crust is found below MQL, suggesting future growth of the crust, unlike the Ordos and Sichuan basins that will remain stable. On the basis of the density and isostatic state of the crust and additional seismological research, such as the P-wave velocity model and Poisson's ratio, it is concluded that the lower crust delamination is a reasonable interpretation for the geophysical characteristics below the Qinling Orogen.

Metallogenic Dynamics Background of Ga’erqiong Cu-Au Deposit in Tibet, China

The Ga’erqiong Cu-Au deposit, which sits on the north side of the Coqên-Xainzamagmatite belt, is a large-scale skarn-type deposit, whose ore body has formed in the skarn zone in the contact part of quartz diorite and marble of Duoai formation or the cracks of quartz diorite. Its mineralization is closely related to quartz diorite. And granite porphyry-related molybdenum ore still exists in its deep part. Currently, there are disputes about the metallogenic dynamics background of this deposit. From previous studies, this paper carried out zircon LA-LCPMS U-Pb dating and petrogeochemistry study for quartz diorite of Ga’erqiong Cu-Au deposit. The testing result indicates: quartz diorite and granite porphyry were formed respectively in 88±2Ma and 83±1Ma, belonging to the magmatic activity of the early stage of Upper Cretaceous; quartz diorite and granite porphyry have geochemical characteristics similar to those of island arc rock of subduction zone and geochemical indexes similar to “adakite.” Combining with the regional tectonic evolution, we think that quartz diorite and granite porphyry were all formed in the extension environment after the collision of Lhasa block and Qiangtang block. Quartz diorite is the result of the migmatization of basic melt and acid melt evoked by asthenosphere material raise caused by lower crustal delamination; the formation of granite porphyry may be crust-mantle material’s partial melting results due to delaminated lower crustal. Therefore, Ga’erqiongskarn-type Cu-Au deposit belongs to the metallogenic response to the collisional orogeny in the closing process of Meso-Tethys.

Late Mesozoic magmatism and metallogeny in NE China: The Sandaowanzi–Beidagou example

ABSTRACTThe Sandaowanzi (>22t Au) and Beidagou (>5t Au) tellurium–gold deposits are located in the northeastern Central Asian Orogenic Belt (Heilongjiang Province, NE China). The ore-hosting volcanic rocks unconformably overly monzogranite and were intruded by adakitic granodiorite. In this study, we report new-age, geochemical, and Sr–Nd–Pb isotopic data to elucidate the genetic link between the igneous rocks and the Te–Au mineralization. New-age data indicate that local magmatism occurred in the Early Jurassic (ca. 177.2 Ma) and Early Cretaceous (ca. 118.7 − 122.0 Ma). Geochemically, the igneous rocks are enriched in LREEs, Pb, K, and U, and depleted in Nb, P, and Ti, showing calc-alkaline affinity. The Early Jurassic monzogranite rocks are featured by 87Sr/86Sr = 0.7111−0.7118; εNd(t) = −4.6 to −4.7; 206Pb/204Pb = 18.098−18.102, 207Pb/204Pb = 15.558−15.580, and 208Pb/204Pb = 37.781−37.928, whereas the Early Cretaceous adakitic granodiorite contains: 87Sr/86Sr = 0.7071−0.7073; εNd(t) = − 3.4 to −3.2; 206Pb/204Pb = 17.991−18.080, 207Pb/204Pb = 15.483−15.508, and 208Pb/204Pb = 37.938−37.985. Initial isotopic ratios for the Early Cretaceous volcanic rocks: 87Sr/86Sr = 0.7061−0.7087; εNd(t) = − 3.6 to −2.9; 206Pb/204Pb = 18.136−18.199, 207Pb/204Pb = 15.512−15.628, and 208Pb/204Pb = 38.064−38.155. The pyrite, chalcopyrite, and telluride grains yielded δ34S of −6.52 ‰ to 2.13 ‰ (mean = − 0.82 ‰) and δ13CPDB of the calcite samples are in the range of −6.64 ‰ to −5.24 ‰, implying the ore materials were derived from mantle. The geochemical and isotopic results indicate that primary melts of Late Mesozoic magmatic rocks have features by partial melting of the continental crust. The adakitic rocks may have been the products of the thickened lower crustal delamination and the subsequent asthenospheric upwelling during the intra-continental extension in NE China. Regionally, intrusive activity and molybdenum mineralization during the Jurassic was affected by subduction setting, whereas gold mineralization was controlled by the Early Cretaceous tectonothermal events associated with a superposition extension.

Origin of adakite-like plutons in southern Korea

We present Sr–Nd–Pb isotopic compositions for two adakite-like intrusions in southern Korea (Jindong and Bongnae), including major and trace element concentrations, and the sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb age. Our aim is to constrain the origin and tectonomagmatic processes that gave rise to the plutons. A SHRIMP U–Pb zircon age of 88.7±0.7Ma was obtained from a Jindong granodiorite sample. The Jindong plutons belong to the medium-K calc-alkaline series, and have an overall enrichment in large ion lithophile elements (LILE), such as K, Rb, Ba, and Pb, and a relative depletion in high field strength elements, such as Nb, Ta, and Ti, compared with the neighboring elements in the primitive mantle-normalized multi-trace element variation diagram. It was found that (87Sr/86Sr)i=0.70475–0.70596, (143Nd/144Nd)i=0.512547–0.512604, [(εNd)i=+0.4 to 2.2], and (206Pb/204Pb)i=18.19–18.37, which fall within the field of the arc-type Cretaceous to Tertiary Bulguksa intrusive rocks in the Sr–Nd and Pb–Pb isotopic correlation diagrams. The elevated Sr/Y (17–40), but low La/Yb (3.7–8.8) ratios of the Jindong plutons, together with their coherent geochemical trends and U-shaped rare earth element (REE) patterns, indicate that they were not formed by slab melting, but by amphibole-dominated fractional crystallization of the Bulguksa-like arc magma. A SHRIMP U–Pb zircon age of 226.5±2.5Ma was obtained from a Bongnae tonalite sample. The Bongnae plutons belong to the shoshonitic series. Their trace element patterns resemble the Jindong samples, but are characterized by significantly negative U anomalies in mafic rocks. They also have elevated MgO, Ni, Co, Cr, Rb, Ba, Sr, and low Al2O3 and Na2O contents at a given SiO2 concentration compared with the Jindong plutons, and are typified by highly radiogenic Sr and unradiogenic Nd and Pb isotopic compositions: [(87Sr/86Sr)i=0.71096–0.71290, (143Nd/144Nd)i=0.511641–0.511681, (εNd)i=−13.0 to −13.8, and (206Pb/204Pb)i=17.53–17.45]. The Bongnae plutons have adakite-like elevated Sr/Y (36–56) and La/Yb (12.9–42.7) ratios, but are highly potassic compared with slab melts. The petrogenesis of the C-type Bongnae adakites is consistent with formation by the partial melting of two components (a lower crust and metasomatized lithospheric mantle) in a post-collisional tectonic setting. Asthenospheric upwelling induced by slab breakoff after continental collision between the north and south China blocks may be a heat source for melting of the continental lithosphere. The Paleo-Tethys subduction before continental collision in this area or lower crustal delamination and subsequent low fractions of melting may have induced metasomatic overprinting in LILE in the overlying lithospheric mantle.

Crustal growth during island arc accretion and transcurrent deformation, Natal Metamorphic Province, South Africa: New isotopic constraints

The Natal Metamorphic Province consists of, from north to south, the Tugela, Mzumbe, and Margate Terranes. These were accreted to the southeastern margin of the Kaapvaal Craton in the late Mesoproterozoic, and followed by intrusion of a large suite of A-type granitoid bodies. New U–Pb data from zircon, titanite, and monazite further constrains the temporal framework of these geological events.The Tugela and Mzumbe Terranes record protracted magmatism in an island arc complex from ∼1200Ma to 1160Ma, followed by the accretion of these terranes onto the southern margin of the Kaapvaal Craton at ∼1150Ma. Arc magmatism in the Margate Terrane continued until ∼1120Ma and was followed by extension and bimodal volcanism immediately prior to accretion to the Kaapvaal/Mzumbe continental margin at ∼1090Ma. This accretion was accompanied by high-pressure and high-temperature metamorphism, juxtaposition of the Mzumbe and Margate Terranes along the Melville Thrust, and the formation of a number of syntectonic intrusive units derived from melting of the pre-existing arc crust. After accretion, extensional collapse is evidenced by the intrusion of mafic/ultramafic and alkaline intermediate magmatic suites at ∼1085Ma, resulting from mafic underplating and/or lower crustal delamination. Nd and Hf isotopic data imply the magmatic rocks of the Natal Metamorphic Province were derived from relatively juvenile continental crust, initially generated by island arc magmatism and subsequently reworked during the accretion event(s). The combined Kaapvaal-NMP region (the southern margin of the enlarged Kalahari Craton) then experienced extensive sinistral transcurrent deformation centred along a series of discrete steep shear zones that are found from the Kaapvaal cratonic margin to the southernmost portion of the Natal Metamorphic Province. This deformation is accompanied by low-pressure, (ultra)high-temperature metamorphism, isobaric cooling, and intrusion of the voluminous A-type Oribi Gorge Suite porphyritic granites and charnockites throughout the Mzumbe and Margate Terranes.

Crustal thickening and uplift of the Tibetan Plateau inferred from receiver function analysis

The India–Eurasia collision, crustal thickening and northward convergence leading to uplift of the Tibetan Plateau have been topics of multidisciplinary studies. Here we employ the H–k stacking technique and depth domain receiver function to constrain the crustal thickness and P- and S-wave velocity ratio as well as the upper mantle discontinuity. Our results identify the signature of slab break-off and lower crustal delamination in southern Tibet, which might have induced the mantle upwelling and crustal melting. The convergence of the Indian plate inducing northward crustal flow might be a major factor that contributes to the uplift and crustal thickening of Tibet. We also evaluate the E–W trending cumulative compression as another dominant factor that contributes to the increase in crustal thickness in the northern Tibet.

A short-lived but significant Mongol–Okhotsk collisional orogeny in latest Jurassic–earliest Cretaceous

The Central Asian Orogenic Belt (CAOB) is one of the largest orogenic collages on Earth, which records a long history of accretionary orogenesis from the late Proterozoic to the Mesozoic. Although paleomagnetic studies have unanimously suggested that the major Mongol–Okhotsk Ocean in the eastern CAOB closed mainly during the Late Jurassic–Early Cretaceous, few conclusive and self-consistent geological studies in Central Asia and East Asia have confirmed the occurrence of this significant collisional orogeny. Based mainly on basin evolution in the CAOB, Tarim Block, North China Block, and Qaidam–Qialian Block during the Late Jurassic–Early Cretaceous, the present study proposes that a short-lived but significant Mongol–Okhotsk collisional event occurred in the latest Jurassic–earliest Cretaceous. The rapid closure of the Mongol–Okhotsk Ocean was possibly related to the clockwise rotation of Siberia, which was driven by the continental collision between the Siberia Craton and the Kolyma–Omolon microcontinent in northeast Russia in the Late Jurassic. Owing to the clockwise rotation of Siberia and the subsequent closure of the Mongol–Okhotsk Ocean, a number of areas to both the east and the west of the Mongol–Okhotsk suture underwent uplift and erosion, including Lake Baikal, Transbaikalia, Sayan ranges, Altai Shan, Junggar Basin, Turfan Basin, Tarim Basin, Tian Shan, Mohe Basin, Great Xing'an ranges, Hailar–Tamsag Basin, Erlian Basin, Songliao Basin, East Gobi Basin, Yingen Basin, Bei Shan, Yin Shan–Yan Shan belt, Ordos Basin, Chaoshui–Yabulai Basin, and northern Qaidam Basin, forming a giant fold and thrust belt in northern China and Mongolia. The collision probably caused a prominent gap between Late Jurassic and Early Cretaceous magmatic episodes in the eastern CAOB and North China Block. After lasting for about 10millionyears, the main portion of the Mongol–Okhotsk Ocean closed, forming a northeast-trending suture. During the Early Cretaceous, a remnant ocean still remained in the easternmost part of the Mongol–Okhotsk Ocean, and the regional compressional setting in East Asia related to the closure of the Mongol–Okhotsk Ocean changed to escape tectonics, driven by the collision of the Lhasa Block at the southern margin of Asia. Strike–slip faulting triggered gravitational collapse of upper crust and delamination of lower crust and mantle lithosphere that was thickened during the Mongol–Okhotsk collision, resulting in the development of a continental-scale extensional setting in East Asia, associated with rift basins, metamorphic core complexes, and igneous rocks.

Crustal structure of Hubei Province of China from teleseismic receiver functions: Evidence for lower crust delamination

Western Hubei Province is at the southern end of the 3000-km-long north-south-oriented Xing'anling–Taihangshan–Wulingshan topographic step in China, which separates high-rising plateaus and mountain ranges in the west from low-elevation plains in the east. We calculated teleseismic P receiver functions of 32 permanent broadband seismic stations in Hubei Province and estimated crustal thicknesses under them using the H-κ method. We also obtained detailed crustal structural images along three profiles using the CCP stacking method. The results show an east-west crustal thickness increase in the study area from 30–35km to 45–50km in less than 20km of horizontal distance, most likely in a step-wise fashion. The thin crust beneath the Nanxiang and Jianghan basins in eastern Hubei extends into the interior of the Wuling Uplift and the Huangling Massif in western Hubei. The lack of mirror symmetry between the Moho and surface topography suggests that part of the mountain ranges in western Hubei is either compensated by non-Airy-type isostasy models or is not in isostatic equilibrium but supported by the strength of the lithosphere. The brittle or localized ductile deformation in the lower crust/uppermost mantle as indicated by the abrupt Moho steps seems to be decoupled with brittle deformation in the upper crust. The CCP images also reveal an apparent double Moho beneath the Wudang Mts. which is interpreted to be due to a partially eclogitized lower crust after the original cratonic mantle lithosphere was replaced by warm and hydrated mantle materials in eastern China in the Late Mesozoic. The Moho steps were formed when a segment of eclogitized lower crust became gravitationally unstable and foundered into the mantle.

Ordovician marginal basin evolution near the palaeo-Pacific east Gondwana margin, Australia

Although the Ordovician Macquarie Arc, part of the eastern Lachlan Orogen of southeastern Australia, has long been considered to be an intra-oceanic arc within an accretionary orogen, key characteristics contrast with more typical examples of accreted arcs. Significantly, multiple stacked phases of mafic to intermediate volcanic rocks, with suprasubduction-zone chemistry, are flanked to the east and west by extensive, coeval continental margin turbidite, chert and black shale sequences. By analysing stratigraphic and contact relationships within and between the volcanic rocks, ophiolitic components ( sensu lato ) and turbidite sequences, constrained by precise biostratigraphy, we document a repeated cycle of uplift, upper-plate extension and collapse common to all sequences. This cycle is interpreted as resulting from localized extension (rifting or hyper-extension and lower-crustal delamination) within a continent-margin sequence developed upon an already established marginal or back-arc basin of probable middle to late Cambrian age. Our interpretation provides a counter-example to prevailing arc accretion models by inferring extensional tectonics at the palaeo-Pacific east Gondwana margin during the Ordovician with development of alkalic and calc-alkalic Cu–Au porphyry deposits away from an active arc system.

Relationships between arc maturity and Cu–Mo–Au porphyry and related epithermal mineralization at the Cenozoic Arasbaran magmatic belt

The actual study deals with the temporal and geochemical evolution of the arc magmatism and the Cu–Mo–Au mineralization in the Neo-Tethyan belt of the Arasbaran region in northwestern Iran. The magmatic activity in this section of the Neo-Tethyan belt periodically occurred from Upper Cretaceous to Quaternary. The Cretaceous magmatic rocks are calc-alkaline to tholeiitic in nature without any mineralization in the Arasbaran region. Cenozoic magmatism begins with alkaline (shoshonitic) volcanism at an early stage and is followed by alkaline to calc-alkaline plutonism. No significant mineralization is associated with the early stage alkaline magmatism and the most porphyry and epithermal mineralizations are related to the late stage adakite-like, calc-alkaline magmatism. The subsequent, post-collisional alkaline magmatism follows in the Plio-Quaternary. Magmatism and the related mineralization decrease in age from the north to the south, and the mineralization changes in time and space from a Cu–Mo to a Cu–Au association.Mineralization stages at the Arasbaran region consist of (a) Upper Oligocene to Lower Miocene (~20Ma) Cu–Mo porphyry and intrusion mineralization, and (b) Upper Miocene (~10Ma), Cu–Au porphyry and epithermal Cu–Au mineralization. Although the exact timing of the porphyry mineralization relative to the final collision is unclear, the presence of most porphyry and epithermal mineralizations in the Upper Oligocene to Upper Miocene, post-collisional, adakite-like intrusions, is related to crustal thickening and delamination of lower crust suggesting that they formed at a late stage of the Arasbaran magmatic arc evolution following a major volcanic stage in the Eocene. Mineralogical and geochemical characteristics of the mineralized porphyry bodies in Arasbaran region indicate that significant amounts of hornblende was fractionated prior to plagioclase crystallization as a consequence of high magmatic water contents. In contrast, the earlier, barren plagioclase-phyric magmas were relatively dry. Fertile plutonic suites in Arasbaran region may be recognized by their relatively late timing with respect to the voluminous Eocene volcanism. Widespread presence of hornblende phenocrysts, and major and trace element compositions with relatively high Sr/Y and La/Yb ratios, high Sr (>400ppm) and Nd (≥20ppm) indicate a long-lived mature arc setting.

Preserved and modified mid-Archean crustal blocks in Dharwar craton: Seismological evidence

We report significant lateral variability in shear wave velocity and Moho depth in the Archean crust, beneath the Dharwar craton, using earthquake waveform data recorded over 50 broadband seismographs. The craton is a continuously exposed Archean continental fragment divided into the west Dharwar craton (WDC) of age 2.7–3.36Ga, and the east Dharwar craton (EDC) of age, dominantly, 2.5Ga. The craton progressively transitions into the Southern Granulite Terrain (SGT) with age of metamorphism around 2.6Ga.The inversion and modeling of receiver function data reveal significant variation of Moho depth, viz., 38–54km in the WDC, and 40–46km in the SGT and 32–38km in the EDC. The average shear wave velocity (Vs) of crust beneath the WDC is ∼3.85km/s as compared to ∼3.6km/s in the EDC. We infer highly variable thickness (16–30km) of mafic cumulate (Vs≥4.0km/s and Vp≥7.0km/s) beneath the WDC, in contrast with a thin one (<5km) beneath the late Archean EDC. The 3.36Ga greenstone belt in the WDC has maximum basal layer thickness of ∼30km. These results suggest the intermediate-mafic composition and exceptional thickness of crust beneath the WDC (>50km) as compared to felsic to intermediate composition for the EDC crust with almost flat Moho (down to ∼38km). These results suggest preserved mafic crustal root beneath the middle Archean terrain in the WDC that has remained inert since then. On the other hand, felsic to intermediate composition of crust with a nearly flat Moho beneath the late Archean EDC could be a consequence of regional delamination of lower crust. The preserved distinct Moho topography across the Archean terrains suggests it as compositional boundary. Considering the surface exposure of 15–20km crust, based on P–T condition, in the granulite segment of the WDC, we speculate a Himalaya-like crustal thickness (50–70km) beneath the middle Archean crust pointing toward a plate tectonic-like scenario at ∼3.0Ga.

Deep electrical structure and geological significance of Tongling ore district

Abstract The Tongling ore district is one of the most important Cu-polymetal producers in the Lower Yangtze River metallogenic belt of China. To understand deep crust structure beneath the Tongling ore district more comprehensively and objectively and to explore important scientific issues, such as characteristics of lithospheric evolution, the relationship between the crustal structure and metallogenesis, it is necessary to study the variation of crust electrical structure in the study area. For this purpose, six magnetotelluric (MT) sounding profiles have been completed by SinoProbe-03 Project. Based on the imaging results of the six profiles, electrical conductivity structure of the crust has been analyzed. The result shows that the geology beneath the profiles is characterized by layering in vertical direction and block division in horizontal direction. The high-resistivity features are related to the transparent zones detected at the upper crust in reflection seismic surveys. The absence of reflectivity from the depths indicates a homogeneous zone, which could often be related to granitic intrusions. Combining the regional tectonic evolution, MT and other geophysical data, we suggest that the high-resistivity bodies crossing the Tongling uplift reveals the center of magma activity and the layering in vertical direction in the lower crust reveals a normal lower crust model and propose a small-scale underplating model which is different with both the “partial melting of a thickened or delaminated lower continental crust” model and the “oceanic ridge subduction”. The underplating might be related to small-scale lithospheric or lower crust delamination controlled by deep fault during the Early Cretaceous. The result of this MT survey is important for constraining lithospheric evolution and metallogenesis in the Lower Yangtze Metallogenic Belt, such as collision of North China Craton and Yangtze Craton, underplating models, deep structure, mechanisms of NE–SW trending rift as well as graben basin, and probable metallogenesis.

Partial melting of lower crust at 10–15 kbar: constraints on adakite and TTG formation

The pressure–temperature (P–T) conditions for producing adakite/tonalite–trondhjemite–granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10–15 kbar and 800–1,050 °C to investigate the major and trace elements of melts and residual minerals and further constrain the P–T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase + amphibole ± garnet) at 10–15 kbar and 800 °C, garnet granulite (plagioclase + amphibole + garnet + clinopyroxene + orthopyroxene) at 12.5 kbar and 900 °C, two-pyroxene granulite (plagioclase + clinopyroxene + orthopyroxene ± amphibole) at 10 kbar and 900 °C and 10–12.5 kbar and 1,000 °C, garnet pyroxenite (garnet + clinopyroxene ± amphibole) at 13.5–15 kbar and 900–1,000 °C, and pyroxenite (clinopyroxene + orthopyroxene) at 15 kbar and 1,050 °C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with <20 wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements (HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are >20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10–40 wt% melting of lower crust composition at 10–12.5 kbar and 800–900 °C and 15 kbar and 800 °C closely resemble adakite/TTG rocks. TiO2 contents of the 1,000–1,050 °C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10–12.5 kbar and 1,000 °C and 15 kbar and 1,050 °C have elevated HREE, whereas partial melts at 13.5–15 kbar and 900–1,000 °C in equilibrium with >20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P–T conditions for producing adakite/TTG from mafic lower crust are 800–950 °C and 10–12.5 kbar (corresponding to a depth of 30–40 km), whereas a depth of >45–50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.

Geodynamics of the eastern Yilgarn Craton

Over the last decade there have been significant advances in our understanding of the stratigraphy, magmatism, deformation, metamorphism and timing of mineralisation, in the eastern Yilgarn Craton (EYC) of Western Australia. The integration of these disciplines has enabled a holistic review of the tectonic history of the EYC which favours a paraautochthonous tectonic model. A significant advance has been the recognition of a ∼2810 Ma rifting event off the eastern margin of the Youanmi Terrane which set up the north-northwest trending architecture of the EYC. Rifting was followed by a ∼100 Myr long phase of tectonic activity initiated by the establishment of a convergent margin at 2715 Ma characterised by a west-dipping subduction zone to the east of the EYC. Subduction was associated with the deposition of 2715–2670 Ma volcanic stratigraphy and the emplacement of voluminous tonalite-trondhjemite-granodiorite magmatism, which resulted in magmatic thickening of the crust. Volcanism was terminated by a ∼5 Myr pulse of east-northeast contraction followed by mid-orogenic extension possibly linked to lithospheric and lower crustal delamination. The lack of ultra-high pressure metamorphism and the presence of high geothermal gradients preclude this contractional event from recording continent–continent collision. Mid-orogenic extension at 2665 Ma resulted in the introduction of metasomatised mantle melts (mafic-granites and syenites), deposition of siliciclastic basins (which record metamorphic anticlockwise PTt paths) and the start of significant economic gold mineralisation in the EYC. High stretching factors and/or delamination associated with this extension event resulted in significant heat input into the base of the crust, which eventually led to the emplacement of Low-Ca (crustal melt) granites and cratonisation of the EYC. Major gold mineralisation postdates mid-orogenic extension, and was associated with renewed compression-transpression (∼2650 Ma) and the development of steep predominantly north-northwest striking sinistral and later north to north-northeast striking dextral strike–slip faults during Low-Ca granite emplacement.

Formation of High-Mg Diorites through Assimilation of Peridotite by Monzodiorite Magma at Crustal Depths

The 134^130 Ma Han-Xing diorite^monzodiorite complex intrudes the central part of the North China block (NCB).The high-Mg diorites have intermediate SiO2, high MgO, Cr and Ni contents, elevated Mg-numbers (Mg#), and are characterized by high Sr and Ba, highly fractionated rare earth elements (REE), and strong Nb and Ta depletions. These geochemical signatures are similar to those of high-Mg adakite and sanukitoid magmas that form through complex crust^mantle interactions. We have investigated felsic magma^peridotite interactions in the Han-Xing diorite complex by undertaking a detailed petrological and bulk-rock and mineral trace element study. The high-Mg diorites contain olivine xenocrysts and dunite xenoliths that have been partly resorbed by the quartz-bearing matrix. These textures, coupled with the high Cr and Ni, and low Ca, Al, P, Y, Sc and Zr contents, provide evidence that the olivines were derived from a depleted peridotite. The assimilation of olivine produced diorite with highly variable MgO, Cr, Ni and Mg# values. However, all rocks have similar incompatible element patterns, indicating that they originate from a common pristine magma.The relative timing of olivine assimilation is constrained by reversely zoned clinopyroxene and amphibole crystals. The cores of the reversely zoned minerals have low Mg# values (clinopyroxene, � 75; amphibole, � 60) and REE characteristics indicating an origin by crystallization from a felsic magma compositionally similar to the monzodiorites. Inclusions of andesine, K-feldspar and biotite are present in the low-Mg# cores of reversely zoned clinopyroxenes, and are further proof of the felsic character of the pristine magma.The high-Mg (up to 90) midsections of the reversely zoned minerals have high Cr and Ni contents and lower but subparallel REE, indicating assimilation of peridotite by the monzodiorite magma. An eclogite residue for the pristine magma is precluded by the presence of plagioclase inclusions, and low Na2 Oa nd Al2O3 and unfractionated heavy REE in the low-Mg# cores of the reversely zoned clinopyroxenes. The maximum depth of the peridotite assimilation is estimated to be � 20 km, using the Al-inamphibole geobarometer for the high-Mg# midsection of a reversely zoned amphibole. This provides further evidence that the interaction of felsic magma with peridotite occurred at crustal rather than mantle depths. We suggest that the peridotite was emplaced at middle or upper crustal levels by previous orogenic processes.The original monzodiorite magma was probably produced in an extensional setting by partial melting of lower crust. Our observations indicate that the delamination model cannot be applied to the petrogenesis of the Han-Xing high-Mg diorite. The interaction of felsic melt with ultramafic rocks at crustal pressures has not been considered as a mechanism for the generation of high-Mg adakites and sanukitoids so far. This might have important implications for crustal evolution. Whereas in previous models sanukitoids have been related to either crustal growth by partial melting of enriched mantle peridotite or crust destruction through lower crust delamination, in the case reported here, they document crustal fractionation through anatexis of lower crustal rocks.

Contrasting geochemistry of the Cretaceous volcanic suites in Shandong province and its implications for the Mesozoic lower crust delamination in the eastern North China craton

The Qingshan volcanic sequences occur in the Mengyin and Jiaozhou basins, west and east of the Tan-Lu fault zone, respectively, were formed at 128±2 and 106–98Ma. Lithologically the Mengyin succession comprises unimodal andesite (53–65wt.% SiO2), whereas the Jiaozhou succession consists of bimodal andesite (56–59wt.% SiO2) and rhyolite (69–77wt.% SiO2). High-Mg (Mg# >60) and low-Mg (Mg# <60) andesitic suites are recognized in the Mengyin volcanic rocks. They show tholeiitic and calc-alkaline trends, respectively. Though the two suites share common trace element features of LILE enrichment and HFSE depletion, higher incompatible element contents in the high-Mg andesite relative to the low-Mg andesite rule out their genetic connection by magmatic differentiation. Similarly, the Jiaozhou andesites also possess higher incompatible element contents compared to their interbedded rhyolites, suggesting their distinct source rocks. The Qingshan mafic volcanic suites are characterized by radiogenic Sr and unradiogenic Nd isotopic compositions, of which the Mengyin high-Mg andesitic rocks display the lowest εNd(t) and highest initial Sr ratio. By contrast, the Jiaozhou rhyolites possess a positive correlation between Sr–Nd isotopes and relatively unradiogenic Sr isotopic ratios.The Qingshan mafic rocks exhibit geochemical characteristics distinct from those of mantle peridotite- or pyroxenite-derived melt as well as from Fangcheng basalts, the only basalts documented in the Cretaceous volcanic suites in Shandong province. They are inferred to be crust-derived melts. However, a clear decreasing trend in ACNK with Mg# increasing and the elevated MgO, Cr and Ni contents relative to basalt-derived melts indicate assimilation with mantle peridotite during their pristine magmatic evolution. Such a two-step process can be best explained by the Archean lower crust delamination of the North China craton with a ~35% partial melting followed by a variable extent of metasomatic interaction with convecting mantle peridotite, which thus resulted in the low-Mg and high-Mg andesitic suites. Nd–Sr isotopic and HFSE features imply that the convecting mantle was mixed with the foundered lithospheric peridotite, which previously experienced subduction-related melt metasomatism during the collision of the Yangtze-North China cratons in the early Triassic. The Qingshan volcanic suites in western Shandong are synchronous with the Tan-Lu strike-slip fault, inferring that the lower crust delamination was initially triggered by sinistral motion of the Tan-Lu fault at ~130Ma. Due to an eastward development of regional lithospheric delamination, the Jiaozhou bimodal volcanic succession was formed owing to the regional geothermal gradient increase and lithosphere thinning. The recognition of contrasting andesitic suites in the Qingshan volcanic succession and their temporal trend provide additional evidence for lower crust delamination during the early Cretaceous in the North China craton and new constraints on its lithosphere thinning model.