Crustal Anatexis Research Articles

Migmatite and leucogranite in a continental-scale exhumed strike-slip shear zone: Implications for tectonic evolution and initiation of shearing

AbstractPlutons within continental strike-slip shear zones bear important geological processes on late-stage plate transpression and continent-continent collision and associated lateral block extrusion. Where, when, and how intrusions and shearing along transpressional strike-slip shear zones respond to plate interactions, however, are often debated. In this study, we investigated migmatite associated leucogranite and pegmatite from the exhumed >1000-km-long Ailao Shan-Red River left-lateral strike-slip shear zone in Southeast Asia that was active during India-Eurasia plate convergence. Most zircons from the migmatites and leucogranitic intrusions present inherited core-rim structure. The depletion of rare earth element patterns and positive Eu anomalies suggest that leucosomes and leucogranites are the result of crustal anatexis. Zircon rims from the foliated migmatites and leucogranites record U-Pb ages of 41–28 Ma, revealing the timing of the Cenozoic crustal anatexis event along this strike-slip shear zone. Ages of the magmatic zircons from the unfoliated pegmatites provide the timing of the termination of a high-temperature tectono-thermal event and ductile left-lateral shearing at 26–23 Ma. The Cenozoic crustal anatexis along the Ailao Shan-Red River strike-slip shear zone indicates that thickened crust underneath the shear zone involved previously subducted crust. We propose that the Cenozoic thermal state has an important effect on the crustal anatexis and thus on the rheological behavior of the lithosphere by thermal weakening, which plays an essential role in localizing the initiation of the deep-seated lower-crustal shear zone.

The production of granitic magmas through crustal anatexis at convergent plate boundaries

Granite petrogenesis is associated with the formation and evolution of continental crust along convergent plate boundaries. While magmatic processes such as partial melting and fractional crystallization result in the compositional diversity of granites, the nature of magma sources is related to tectonic settings of granite generation. Although granites consist predominantly of magmatic minerals, they generally contain small amounts of peritectic and residual minerals. This results in the compositional difference between natural granites and experimental melts. In addition to the fractional crystallization of mantle-derived mafic magmas for the generation of granites during accretionary orogeny, a wide range of crustal rocks can be partially melted to form granites in both accretionary and collisional orogens. In the latter case, metasedimentary and metaigneous crustal sources lead to the dichotomy of S- and I-type granites. Geochemically, granites with negative εNd(t) and εHf(t) values are certainly derived from reworking of the ancient crust. On the other hand, granites with positive εNd(t) and εHf(t) values may be produced either by reworking of the juvenile crust or by fractional crystallization of asthenospheric mantle-derived magmas at actively convergent plate boundaries. In this case, major and trace elements may be of use in distinguishing between the crustal and mantle contributions. Granite, migmatite and granulite generally form lithological associations at previously converged plate boundaries, with migmatitic domes as the major component of metamorphic core complexes. Their formation is related to extensional heating subsequent to thinning of the thickened mantle lithosphere. As soon as the thickened lithospheric mantle was foundered into the asthenospheric mantle, it makes the space for asthenospheric upwelling to induce active rifting along the thinned lithosphere. This provides an external heat source for anatectic metamorphism at high thermal gradients, marking the tectonic transition in the property of orogenic systems into the rifting stage. Therefore, the active rifting is spatially and temporally dictated by thinning of the orogenic mantle lithosphere, and its operation is a geodynamic mechanism for crustal anatexis at convergent plate boundaries. Consequently, the lithospheric thickness has a profound effect on crustal anatexis for granitic magmatism, which is considerably later than plate convergence.

The Late Carboniferous to Early Permian high silica magmatism in the Southern Mongolia: Implications for tectonic evolution and continental growth

Tremendous granites were produced during the late-stage evolution of the Central Asian Orogenic Belt and important to understanding the tectonic processes from subduction to collision. Here, we present zircon U-Pb-Hf, and whole-rock geochemical, and Sr-Nd isotopic data for a suite of Carboniferous-Permian granites from the Khan-Bogd area of the southern Mongolia. U-Pb age dating of zircons indicates that the alkaline feldspar granites and quartz monzonite-monzodiorites on the southwest of the Khan-Bogd town were formed at ~315–310 Ma and the alkaline feldspar granites on its south at ~290 Ma. The ~315–310 Ma alkaline feldspar granites possibly result from crustal anatexis of the Devonian-Carboniferous juvenile arc crust there, evidenced by their intensive continental geochemical signals and roughly similar Nd and Hf isotopes to these arc crust. The coeval quartz monzonite-monzodiorites with limited outcrop show arc affinity. Thus, these Late Carboniferous granites indicate a sharp decrease of mantle-crust interaction from earlier arc accretion. Combined with previous work, this change was possibly triggered by a low-angle subduction. The ~290 Ma alkaline feldspar granites show A-type petrological and geochemical features. In addition, they have more variable Nb/Ta (~10–18), Nd and Hf isotopes indicative of various continental crust involvement in their magma source. The lack of adakite and arc-type magma deprives the possibility of lithosphericdelamination, ridge subduction and slab rollback. As a significant mantle input evidenced by coeval bimodal magmatism, this crust-derived granite was likely induced by a slab breakoff (~305–290 Ma), rather than backarc extension. Combined with previous work, this slab breakoff probably indicates a tectonic transition of post collision (~315–290 Ma) from subduction-accretion and marks the closure of a wide secondary ocean (North Tianshan-Hegenshan ocean) of the Paleo-Asian Ocean. Moreover, it plays an important role for the vertical crustal growth of the CAOB.

The Effects of Source Mixing and Fractional Crystallization on the Composition of Eocene Granites in the Himalayan Orogen

Abstract Granites are generally the final products of crustal anatexis. The composition of the initial melts may be changed by fractional crystallization during magma evolution. Thus, it is crucial to retrieve the temperature and pressure conditions of crustal anatexis on the basis of the composition of the initial melts rather than the evolved melts. Here we use a suite of ∼46–41 Ma granites from the Himalayan orogen to address this issue. These rocks can be divided into two groups in terms of their petrological and geochemical features. One group has high maficity (MgO + FeOt = 2–4 wt%) and mainly consists of two-mica granites, and is characterized by apparent adakite geochemical signatures, including high Sr concentrations and Sr/Y and La/Yb ratios, and low concentrations of heavy rare earth elements and Y. The other group has low maficity (MgO + FeOt <1 wt%) and consists of subvolcanic porphyritic granites and garnet/tourmaline-bearing leucogranites. This group does not possess apparent adakite signatures. The low-maficity group (LMG) has lower MgO + FeOt contents and the high-maficity group (HMG) has higher Mg# compared with initial anatectic melts determined by experimental petrology and melt inclusions studies. Petrological observations indicate that the HMG and the LMG can be explained as a crystal-rich cumulate and its fractionated melt, respectively, such that the initial anatectic melt is best represented by an intermediate composition. Such a cogenetic relationship is supported by the comparable Sr–Nd isotopic compositions of the two coeval groups. However, these compositions are also highly variable, pointing to a mixed source that was composed of amphibolite and metapelite with contrasting isotope compositions. We model the major and trace element compositions of anatectic melts generated by partial melting of a mixed source at four apparent thermobarometric ratios of 600, 800, 1000 and 1200 °C GPa–1. Modeling results indicate that melt produced at 1000 °C GPa–1 best matches the major and trace element compositions of the inferred initial melt compositions. In particular, a binary mixture generated from 10 vol% partial melting of amphibolite and 30 vol% melting of metapelite at 850 ± 50 °C and 8·5 ± 0·5 kbar gives the best match. Therefore, this study highlights that high thermobarometric ratios and subsequent fractional crystallization are responsible for the generation of the apparent adakitic geochemical signatures, rather than melting at the base of the thickened crust as previously proposed. The thermal anomaly responsible for the Eocene magmatism in the Himalayan orogen was probably related to asthenosphere upwelling in response to rollback of the subducting Neo-Tethyan oceanic slab at the terminal stage of continental collision between India and Asia. As such, a transition in dynamic regime from compression to extension is necessary for the generation of high thermobarometric ratios in continental collision zone. Therefore, after correcting for potential effects of fractional crystallization and crystal accumulation on melt composition, granite geochemistry coupled with thermodynamic modeling can better elucidate the petrogenesis of granites and the geodynamic mechanisms associated with anatexis at convergent plate boundaries.

A newly discovered Late Cretaceous metamorphic belt along the active continental margin of the Neo-Tethys ocean

Abstract High-grade metamorphic rocks and crustal melts provide crucial evidence for growth and differentiation of the continental crust, and are widespread in collisional orogens. However, their importance in the evolution of continental arcs remains poorly understood. Metamorphism and related anatexis in the preserved continental margin of the Neo-Tethys ocean serves as a key natural laboratory to investigate this process. Along the Neo-Tethyan arc margin, the Gaoligong shear zone, Yunnan region of China, is an important locality for linking Lhasa in the north with Sibumasu and Burma in the south. Here, Late Cretaceous granulite-facies metamorphism and crustal anatexis have been identified for the first time in the Gaoligong area. Zircon and monazite U-Pb dating indicates that S-type granites formed at 87–73 Ma, granites and buried pelitic sediments were simultaneously metamorphosed at 75–70 Ma during Neo-Tethyan subduction, and all lithologies were overprinted by a younger 40–30 Ma magmatic and strike-slip event related to India-Asia collision. Phase equilibria modeling of high-grade anatectic gneiss in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system indicates peak pressure-temperature (P–T) conditions of 780–800 °C and 6.5–7.5 kbar and defines a cooling and decompressional P–T path for the metapelites. This demonstrates that sediments within the Neo-Tethyan active continental arc were buried to >20 km depth at 75–70 Ma. In combination with the metamorphic record of the Lhasa, Burma, and Sibumasu blocks, an extensive Late Cretaceous metamorphic belt must have formed along the Neo-Tethyan subduction zone. This spatially correlates with coeval gabbro-diorite suites exposed in the Gangdese, Sibumasu and Burma terranes that were triggered by thinning of the lithospheric mantle. This prolonged Late Cretaceous mantle-derived magmatism and lithospheric thinning may have provided a regional-scale heat source for high-grade metamorphism and crustal anatexis along the active continental margin of the Neo-Tethys ocean.

Compositional Evolution of the Variscan Intra-Orogenic Extensional Magmatism in the Valencia del Ventoso Plutonic Complex, Ossa-Morena Zone (SW Iberia): A View from Amphibole Compositional Relationships

The Ossa-Morena Zone (OMZ), SW Iberia, has numerous Lower Carboniferous compositionally zoned plutons that formed in a Variscan intra-orogenic extensional setting. This magmatism shows a wide compositional variation comprising alkaline, transitional, and calc-alkaline suites. The calc-alkaline suite was produced by hybridization of alkaline magmas with felsic melts generated by crustal anatexis related to the intrusion of mafic magmas in the middle crust. In this work, we present a textural and mineralogical study of the Variscan Valencia del Ventoso main pluton from the OMZ to track the compositional evolution of magmas during hybridization using constraints from amphibole compositions and to determine the P-T conditions of emplacement using amphibole-based thermobarometry. This pluton exhibits reverse zoning with an inner facies containing alkaline dolerites, gabbros, and quartz diorites, an intermediate facies with transitional diorites, and an outer facies with calc-alkaline quartz diorites to monzogranites. Magmas from the intermediate and border facies crystallized under oxidizing conditions at relatively low temperatures (range: 640–760 °C) and ca. 280–300 MPa, implying near H2O-saturated conditions. These rock facies show mineralogical evidence of hybridization between alkaline to mildly alkalic and calc-alkaline magmas. The former is inferred from the occurrence of antecrysts of labradorite-andesine, high-Ti pargasite-hastingsite, and biotite with deficiency in tetrahedral-site occupancy, a distinctive feature of biotite from the inner facies alkaline dolerites. This contrasts with later crystallization from the calc-alkaline magma of andesine-oligoclase, low-Ti magnesiohornblende-edenite, and biotite with full tetrahedral-site occupancy. Constraints from amphibole-melt compositional relationships in antecrystic high-Ti amphibole suggest that the alkaline magmatic component could have a high- to ultra-K affinity.

Late Triassic Orogenic Assembly of the Tibetan Plateau: Constraints from Magmatism and Metamorphism in the East Lhasa Terrane

Abstract The early Mesozoic evolution of the Lhasa terrane, which represents a major component of the Himalayan–Tibetan orogen, remains highly controversial. In particular, geological units and events documented either side of the eastern Himalayan syntaxis (EHS) are poorly correlated. Here, we report new petrological, geochemical and geochronological data for cogenetic peraluminous S-type granites and metamorphic rocks (gneiss and schist) from the Motuo–Bomi–Chayu region of the eastern Lhasa terrane, located on the eastern flank of the EHS. Zircon U–Pb dating indicates that these units record both Late Triassic magmatic (216–206 Ma) and metamorphic (209–198 Ma) episodes. The granites were derived from a Paleoproterozoic crustal source with negative zircon εHf(t) values (–5·5 to –16·6) and TDM2 model ages of 1·51–1·99 Ga, and are interpreted to have formed by crustal anatexis of nearby metasediments during collisional orogeny and crustal thickening. The gneisses and schists experienced similar upper amphibolite-facies peak metamorphism and associated partial melting, followed by decompressional cooling and retrograde metamorphism. These rocks were buried to lower-crustal depths and then exhumed to the surface in a collisional orogenic setting during plate convergence. From comparison of these data with those for other metamorphic belts with similar grades and ages, and association of coeval granitic magmatism widespread in the central–east Lhasa terrane, we propose that the studied cogenetic magmatism and metamorphism in the Motuo–Bomi–Chayu region records Late Triassic accretion of the North Lhasa and South Lhasa terranes, which represents the first evidence of the Paleo-Tethys Ocean closure in this part of Asia. These data provide new constraints on the spatial and temporal evolution of the Paleo-Tethyan Wilson Cycle and provide a ‘missing link’ to correlate the geology and tectonic history of the Lhasa terrane continental crust on either side of the EHS.

Origin of highly fractionated peraluminous granites in South China: Implications for crustal anatexis and evolution

Voluminous Mesozoic intra-continental granitic rocks occur in South China, which provide insights into the processes and associated P-T conditions of crustal anatexis and evolution in an intra-continental setting. Most granitic plutons contain multiple phase intrusions, commonly with an early-stage main-phase granite and a late-stage minor-phase highly fractionated granite, and the latter is generally thought to result from extreme differentiation of the main-phase granite. We challenge this model based on our case study on the Tianmenshan composite pluton that is composed of an early-stage biotite granite and a late-stage two-mica granite. Zircon U-Pb dating yields similar emplacement ages of 157.3 ± 3.1 Ma for the biotite granite and 158.5 ± 3.4 Ma for the two-mica granite. The two rock types are peraluminous with A/CNK = 1.07 to 1.17 and show comparable Nd isotopic compositions, with εNd(t) = −9.4 to −11.5 for the biotite granite and −10 to −11.2 for the two-mica granite. The two-mica granite is characterized by high silica (SiO2 = 75.2–77.5%), low abundances of Sr (2.3–42 ppm) and Ba (10–114 ppm), large negative Eu anomalies in the chondrite-normalized REE patterns and typical REE tetrad effect, which, along with its high Ga/Al ratios, Fe-index and F contents, suggesting its affinity to highly fractionated granite. Nevertheless, the petrographic observations, zircon REE concentrations and biotite compositions consistently demonstrate that the late-stage two-mica granite formed under more water-deficient and reduced conditions than the early-stage biotite granite. This, along with a modeling study of trace elements (Rb, Sr, Ba,Y and Nb) and the lack of continuum in terms of whole-rock chemistry and biotite compositions between the two rock units, suggests that the two-mica granite cannot be formed through differentiation of the main-phase biotite granite. We suggest that the parental magma to the late-stage two-mica granite could have been derived from partial melting of a granulitic residue after extraction of granitic melts to form the biotite granite from the Neoproterozoic metavolcano-sedimentary rocks, followed by a significant feldspar-dominated fractionation during magma ascent. This model may apply to most other composite granite plutons in South China, and is of significance to better understand the process of crustal anatexis and evolution in an intra-continental setting.

Fluid-present and fluid-absent melting of muscovite in migmatites in the Himalayan orogen: Constraints from major and trace element zoning and phase equilibrium relationships

Partial melting of the continental crust may proceed through either influx of free water (hydration, fluid-present melting) or breakdown of hydrous minerals (dehydration, fluid-absent melting). The two types of physicochemical mechanism would be operated for crustal anatexis in nature, and their effects on the composition of anatectic products were closely examined by experimental approaches as well as the study of natural leucogranites from the Himalayan orogen. However, the leucogranites usually experienced several stages of evolution during magmatism, leading to difficulties in retrieving the composition of original melts. This issue is now addressed by studying Himalayan migmatitic metapelites because they generally share similar source rocks and P–T conditions with the leucogranites. In combined with field observations, an integrated study of whole-rock geochemistry, mineral chemistry, zircon geochronology, and phase equilibrium modelling was conducted for migmatitic metapelites from the Nyalam Valley in the Greater Himalayan Sequence. The results show that the migmatitic metapelites share similar clockwise P–T paths. They experienced an early prograde metamorphism through compressional heating to reach peak pressures of 7.3–9.0 kbar at ~700 °C before ca. 31 Ma. Afterwards, the migmatitic metapelites underwent decompressional heating to ~725 °C at 6.0 kbar for upper-amphibolite facies metamorphism at 26–21 Ma. Subsequently, they experienced decompressional cooling to a temperature below the solidus at ca. 18 Ma. The crustal anatexis was dominated by muscovite melting reactions during the decompressional heating from ca. 26.4 to 18 Ma. Despite the similar P–T paths, the migmatitic metapelites experienced different types of melting reactions involving muscovite (i.e., dehydration and hydration). Whereas the hydration melting resulted in a marked increase in An contents (~65 to 75 mol%) and the absence of K-feldspar, the dehydration melting led to relatively low-An (<30 mol%) plagioclase and abundant K-feldspar. The two types of muscovite melting reaction may be common in the Himalayan orogen in the Late Cenozoic. While the fluid-present melting would take place at a shallower depth, the fluid-absent melting would occur at a greater depth. This coupled dehydration-hydration relationship has important implications for the geodynamic mechanism of crustal anatexis in the continental collision zone.

Pure sediment-derived granites in a subduction zone

Abstract The Earth is unique in the Solar System due to significant volumes of granite in the lithosphere. However, the origins of granites are still highly debated, especially sediment-derived granites, which are often treated as a geochemical end-member of the continental crust. In the Yunnan region of South China, we identify the occurrence of pure sediment-derived granite in a subduction system. The suite of strongly peraluminous granite reported herein is interpreted to represent pure metasedimentary melts based on their whole-rock elemental and Sr-Nd-B and zircon Hf-O isotopic compositions. These Late Permian–Early Triassic (ca. 254–248 Ma) granites are characterized by radiogenically enriched Sr, Nd, and Hf isotopic signatures. They show δ11B and δ18O signatures akin to those of continental shales. Geochemical modeling indicates no contributions from the mantle that can be detected. Considering the regional tectonic evolution, these granites are suggested to be formed in a subduction zone by decompression melting of rapidly exhumed back-arc sediments. We posit that decompression melting was triggered by widespread extension and thinning of the crust prompted by rollback of the subducting oceanic crust. These granites thus provide evidence that granite formation in subduction zones does not necessarily contribute to crustal growth. These subduction-related pure sediment-derived granites have different elemental ratios and contents (e.g., Al2O3/TiO2 and Yb) from the Himalayan leucogranites. Considering their source compositions (e.g., pelitic rocks), which are similar to those of the Himalayan leucogranites, these differences are likely due to their higher formation temperature and lower pressure despite a great similarity in isotopic compositions. Identification of pure sediment-derived, strongly peraluminous granites (SPGs) in subduction systems provides an important geodynamic mechanism for crustal anatexis, which can both geochemically and tectonically complement their collisional counterparts identified in the Himalayas.

The role of mantle and the ancient continental crust in the generation of post-collisional high-K calc-alkaline and alkaline granites, with main reference to the Arabian-Nubian Shield

Continental-scaled magmatic activity occurs after the end of collision-orogenic stages of the Wilson cycle. This post-collisional magmatism is widely exhibited, from 635 to 540 Ma, in a series of large Neoproterozoic (Pan-African) orogenic belts throughout North Africa. Special reference is given to the Arabian-Nubian Shield (ANS) and the LATEA metacraton (Tuareg shield). For comparison, two Late Paleozoic-Early Mesozoic transcontinental belts, the Variscan in Western Europe and the east Central Asian Orogenic Belt, are briefly discussed. Two main types of consecutive magmatic suites characterize the Late Neoproterozoic (Ediacaran) and Late Paleozoic-Early Triassic (Variscan) post-collisional magmatic events despite their different formation age and geographic location: (a) Early post-collisional suites composed of dominant HKCA monzogranite, with secondary quartz monzonite, granodiorite, and related high-K calc-alkaline gabbro, diorite; (b) Late post-collisional to intra-continental suites of Alk syenogranite, alkali feldspar granite and quartz syenite, in places with Na-rich pyroxene and amphibole; volcanic activity and numerous dike swarms are abundant. The mafic rocks related to Alk granite suites are monzogabbro and monzonite (shoshonitic series). Granitoids of the same type, HKCA or Alk, are very similar in mineral and chemical composition throughout the belts and in different belts. This similarity is observed in regions with very distinct types of crust, e.g. the dominant juvenile oceanic island arc crust in the ANS, and Archean-Paleoproterozoic continental crust in the LATEA metacraton. The isotope U-Sr-Nd-Hf-O signatures suggest that HKCA granite magmas in these regions were produced from different crustal sources with varying contribution of mantle-derived basic materials to the granitoid magma generation. For HKCA granites, a model of crustal anatexis, accompanied with a limited influx of H2O, K, LILE, HFSE from underplated basalt magmas (convective diffusion in the contact of silicic and mafic liquids) is suggested. The most likely model of the Alk silicic magma generation is partial melting of related mantle-derived mafic rocks (monzogabbro, monzonite in the lower crust. Data on the enhanced temperature of A-type silicic magmas, more than 1000 °C, suggest that magma-generation could occur even at the depth of uppermost lithospheric mantle.

Petrogenetic characterization of pegmatites and their host rocks in southern Akwanga, North-Central Basement Complex, Nigeria

The pegmatites in southern Akwanga occur within the reactivated belt of the basement complex of Nigeria. The pegmatites consist of dominantly albite–muscovite pegmatites (rare-metal), southern parts of the map areas and biotite-microcline pegmatites (barren) central parts of the map. The pegmatites intruded gneiss-migmatitic complex consist of metasedimentry rocks; granitic gneisses and biotite gneisses and rarely meta-igneous, amphibolites. The rare metal pegmatites are composed of quartz, albite and muscovites and tourmaline. Garnets, ilmenites and minor tin–columbite–tantalite mineralization constitute accessory minerals in contrast to the biotite-microcline pegmatites. The host rocks are composed of quartz, plagioclase (An5–21; albites–oligoclase), microcline and muscovite. Minor constituents include biotites, cordierites and hornblendes. Ilmenite occurs as opaques. The pegmatites and their host rocks are corundum and hypersthene normative, highly peraluminous, exhibiting similar geochemical signatures; however, the rare metal pegmatites are more fractionated than the host rocks and the biotite-microcline pegmatites. The rare metal pegmatites are relatively enriched in Rb, Li, Cs, B, Be, Nb and Ta, low in K/Rb and Al/Ga ratios than the biotite–microcline–pegmatites and their host rocks. The pegmatites are products of crustal anatexis of sedimentary origin. This indicates that the rare metal pegmatites are source rock controlled (product of post-collision activities) rather than fractional crystallization.

Early–Middle Jurassic magmatic rocks along the coastal region of southeastern China: Petrogenesis and implications for Paleo-Pacific plate subduction

• The 200–160 Ma arc magmatic belt is identified along the coastal region of SE China. • This study firstly reports the ancient basement-derived granitoids in SE China. • The Paleo-Pacific Plate subduction underneath southeast China started at ca. 200 Ma. This study presents new zircon U–Pb ages and whole-rock geochemical data for the Fanyindong (FYD) and Jincheng (JC) granitoid intrusions in coastal Zhejiang and Fujian Provinces, respectively, southeast China, to investigate their petrogenesis and provide new constraints on the regional tectonic evolution and crustal growth. Zircon U–Pb ages indicate that the FYD and JC intrusions were emplaced at c. 170 Ma and 193 Ma, respectively. Both granitoids belong to the high-K calc-alkaline series, and display arc-like trace element patterns, with enrichment of large ion lithophile elements and Nb-Ta depletion. The FYD granitoids show adakitic geochemical characteristics, with high radiogenic Sr isotope ratios, and the most enriched bulk-rock Nd and zircon Hf isotopic compositions (crustal Nd–Hf model ages = ca. 2.35–2.75 Ga) ever reported in the Mesozoic granitoids within southeast China. Geochemical characteristics suggest that the FYD granitoids were generated by partial melting of mostly Paleoproterozoic basement rocks in a thickened lower crust. However, whole-rock Sr–Nd and zircon Hf isotopes of the JC granites show depleted source signatures. Geochemical signatures suggest the JC granites originated from the partial melting of a mixed source, consisting of the pre-existing, ancient, lower crust and juvenile, mantle-derived, basaltic rocks. The data from this study, in conjunction with previously published data, indicates that there exists a NE-trending, Early–Middle Jurassic, arc-related, magmatic rock belt along the southeast coast of China, and the Paleo-Pacific Plate subduction most likely started at c. 200 Ma. The early stage subduction of the Paleo-Pacific plate and the induced reactivation and extension of pre-existing E–W striking faults in the Nanling region of southeastern China gave rise to the generation of the JC granites. Ongoing subduction of the Paleo-Pacific plate resulted in a thickening of the continental margin, and subsequent crustal anatexis of basement materials produced the FYD granitoids.

Petrogenesis and tectonic implications of the ca. 2.5 Ga granitoid assemblage in the eastern Zhongtiao region, southern North China Craton

The key to unraveling the crustal evolution and extensive magmatism of the North China Craton (NCC) at the Archean–Proterozoic boundary (ca. 2.5 Ga) is to investigate the petrogenesis of the volumetrically dominant granitoids. In this contribution, we report new petrological, mineralogical, and geochemical data for the Huping and Luojiahe trondhjemites, which are ca. 2.5 Ga granitoid assemblages from the eastern Zhongtiao region of the southern NCC. The Huping trondhjemites have high SiO2 and Na2O contents and high (La/Yb)N and Sr/Y values, which are consistent with most Tonalite–trondhjemite–granodiorite (TTG) rocks worldwide. Evolved whole-rock Nd and Hf isotopic compositions indicate that the Huping trondhjemites were derived from partial melting of preexisting ca. 2.7 Ga mafic crust, and high MgO, Cr, and Ni contents suggest the possible involvement of depleted mantle in their source region. Integrated petrological, mineralogical, and geochemical data indicate that the Huping trondhjemites were likely formed by partial melting of delaminated lower crust. In contrast, the Luojiahe trondhjemites are Na- and Si-rich with negative Eu and Sr anomalies. Their geochemical characteristics suggest that they were likely formed by partial melting of low-K mafic crust that was shallower than the source of the Huping trondhjemites. The higher LREE content of allanite from the Luojiahe trondhjemites relative to that from the Huping trondhjemites indicates the Luojiahe trondhjemites formed at a higher melting temperature. The occurrence of the two types of coeval granitoids indicates that ca. 2.5 Ga crustal anatexis occurred at different depths. Furthermore, a strong heat flux at the base of the southern NCC at ca. 2.5 Ga is evident, which might have been introduced by mafic magma underplating. Based on our new data and the records of coeval magmatism in the region, this study suggests that lithospheric delamination and large-scale mantle-derived magma underplating beneath the thickened crust could have played important roles in the extensive magmatism of the southern NCC at the Archean–Proterozoic boundary.

Th–U powered metamorphism: Thermal consequences of a chemical hotspot

AbstractThe Arkaroola region of the northern Flinders Ranges, South Australia, records high geothermal gradient mineral assemblages that are not spatially or temporally associated with intrusive magmatism. Cordierite‐bearing schists from the base of a ~12 km thick Neoproterozoic sedimentary sequence known as the Adelaide Rift Complex directly overlie Mesoproterozoic metasedimentary and granitic rocks with regional heat production values of ~7.9 µW/m3 at 580 Ma, two to three times greater than global average values for granitic rocks. We integrate in‐situ U–Pb monazite geochronology, Y+HREE‐in‐monazite thermometry and mineral equilibria modelling to show that rocks at the base of the sedimentary succession record amphibolite facies metamorphism at c. 580 Ma while the overlying sediments were still accumulating. Metamorphism took place under average geothermal gradient conditions in excess of 180°C/kbar (&gt;60°C/km) that propagated to depths of at least 12 km. These thermal gradients persisted for upwards of 150 Ma, maintained by a lack of crustal erosion, and are documented by long‐lived crustal anatexis. This system may be the archetypal example of Th–U powered metamorphism, recording the interplay between chemically extreme basement and thermally insulating sedimentary cover.

Boron and molybdenum isotopic fractionation during crustal anatexis: Constraints from the Conadong leucogranites in the Himalayan Block, South Tibet

Partial melts derived from crustal anatexis exhibit large variations in field characteristics, composition, and isotope geochemistry. The relative influence of melting process and magmatic differentiation on such heterogeneity is a subject of ongoing debate. Boron and molybdeum isotopes have the potential to shed light on the genesis of crustal melts, but the behaviors of B and Mo isotopes during crustal anatexis and differentiation are unclear. Here, we present geochemical and isotopic data, including whole-rock Sr–Nd–B–Mo and mineral B–Mo data, for the Conadong leucogranites in the Himalayan Block, to provide new insights into these issues. The studied samples include two-mica granites (biotite + muscovite), muscovite leucogranites (muscovite ± garnet), and biotite-rich granites. The two-mica granites were derived from muscovite dehydration melting of metasedimentary rocks. Their initial Sr and Nd isotopic compositions show positive correlation, implying non-modal partial melting associated with dissolution of accessory phases. The B contents (3.44–21.2 ppm) of the two-mica granites decrease but their δ11B (−15.80‰ to −13.25‰) and δ98Mo (−0.15‰ to 0.34‰) values increase with increasing Sr and Ba contents. This may have been controlled by different phases involved in non-modal melting (i.e., muscovite and feldspar). The muscovite leucogranites display features of residual liquids. Their δ11B (−15.38‰ to −11.90‰) and δ98Mo (−0.15‰ to 0.17‰) values negatively correlate with Sr and Ba contents, which can be ascribed to shallow-crustal crystal–liquid separation and melt–fluid interaction. The biotite-rich granites are least evolved, with large variations in their initial Sr–Nd isotopic compositions. They show relatively heavy whole-rock B isotopic compositions of −11.97‰ to −9.00‰. These may indicate that biotite-rich granites may be contaminated by entrained restitic minerals (e.g., biotite, feldspar and accessory minerals). Overall, this study demonstrates that compositional diversity of anatectic melts is controlled primarily by melting processes in the source and secondarily by emplacement-level crystal–liquid separation. Both non-modal partial melting and crystal–liquid separation could result in B–Mo isotopic fractionation in anatectic melts.

Miocene-Pleistocene crustal anatexis in the Namche Barwa massif, southern Tibet

位于喜马拉雅东构造结的南迦巴瓦地块经历了复杂的构造变形、强烈的变质和深熔作用，是研究碰撞造山过程中地壳深熔作用的重要对象。完整地厘定新生代晚期岩浆作用期次对于揭示南迦巴瓦地区的构造演化历史和深部过程具有重要意义。南迦巴瓦地块3件淡色花岗岩样品的锆石U-Pb定年结果显示该地块经历了11.30±0.16Ma和2.59±0.04Ma两期地壳深熔作用，可能与南迦巴瓦地块晚新生代快速隆升和剥蚀相关。南迦巴瓦地块保存了大量的~11Ma变质作用和地壳深熔作用记录指示该时间段为构造活动剧烈期。上新世晚期的淡色花岗岩表明，穹窿的隆升和剥蚀所导致的岩浆作用至少持续到了~2.59Ma，代表了南迦巴瓦地区一次年轻的构造岩浆事件。;The Namche Barwa massif, located in the eastern tectonic junction of Himalayan Orogen, has experienced complex tectonic deformation, intensive metamorphism and anatexis which resulted in various types of leucogranites to investigate the nature of crustal anatexis during the tectonic evolution of collision orogenic belts. Knowledge of the timing of such crustal anatexis in rapidly exhumed high grade metamorphic terranes like the Namche Barwa is important to understand the feedback of rapid near-surface processes (erosion and denudation) and those relatively deep processes (high-T metamorphism and partial melting). Zircon U-Pb dating of three leucogranite samples from the Namche Barwa yields that this block has experienced two episodes of young crustal anatexis (11.30±0.16Ma and 2.59±0.04Ma, respectively), which may be related to rapid denudation and uplift of the Namche Barwa in the Late Cenozoic. A number of studies have demonstrated that various rock types in the Namche Barwa have experienced a major episode of metamorphism and crustal anatexis at~11Ma, indicating intense tectonic activities in this period. Occurrence of leucogranite of late Pliocene in age further suggests that the anatectic processes caused by rapid denudation and uplift in this massif lasted at least to~2.59Ma.

ВОЗРАСТ И ПЕТРОГЕНЕЗИС ВУЛКАНИТОВ КИСЛОГО СОСТАВА АЛГАНСКИХ ГОР КОРЯКСКОГО НАГОРЬЯ (СЕВЕРО-ВОСТОК РОССИИ)

The isotopic dating of U-Pb by the zircon method of volcanic acid zircons of the Konachan complex (Algansky mountains, northwestern part of the Koryak highlands) confirmed their Late Eocene age (34.6 ± 0.5-38.8 ± 0.4 Ma, Priabon). It has been shown that silicic rocks are represented by moderately potassium, peraluminous (ASI = 1.23–1.30), moderately and high magnesian differences, often with a high content of Cr and Ni. The contents of Sr, Y and Yb are adakite. High ratios of 143Nd/144Nd and low 87Sr/86Sr in the rocks of the complex, the distribution pattern of rare and rare-earth elements suggest that the source of salite melt, to some extent contaminated by sedimentary material, was metabasites. The formation of the Konachan complex is caused by magmatism during accretion of terranes of the Olyutor-East Kamchatka island-arc system [27]; compression processes led to the blocking of the upwelling front of the continental asthenosphere, initiating intense crustal anatexis and silicate magmatism.

Behavior of apatite in granitic melts derived from partial melting of muscovite in metasedimentary sources

Fluid-absent and fluid-fluxed melting of muscovite in metasedimentary sources are two types of crustal anatexis to produce the Himalaya Cenozoic leucogranites. Apatite grains separated from melts derived from the two types of parting melting have different geochemical compositions. The leucogranites derived from fluid-fluxed melting have relict apatite grains and magmatic crystallized apatite grains, by contrast, there are only crystallized apatite grains in the leucogranites derived from fluid-absent melting. Moreover, apatite grains crystallized from fluid-fluxed melting of muscovite contain higher Sr, but lower Th and LREE than those from fluid-absent melting of muscovite, which could be controlled by the distribution of partitioning coefficient (<i>D</i>_Ap/Melt) between apatite and leucogranite. <i>D</i>_Ap/Melt in granites derived from fluid-absent melting is higher than those from fluid-fluxed melting. So, not only SiO₂ and A/CNK, but also types of crustal anatexis are sensitive to trace element partition coefficients for apatite. In addition, due to being not susceptible to alteration, apatite has a high potential to yield information about petrogenetic processes that are invisible at the whole-rock scale and thus is a useful tool as a petrogenetic indicator.

Geochemical characteristics of Precambrian Basement rocks from southeastern Nigeria: an approach to their petrogenesis and tectonic setting.

Trace and Rare-Earth element geochemical study of twenty samples of migmatitic banded gneisses, garnet biotite schists, dolerites, granites and rhyolites was carried out in a bid to determine their petrogenetic and tectonic significance in the evolution of the southeastern Basement complex of Nigeria. The data shows that partial melting (crustal anatexis) of migmatitic gneisses and schists played a significant role in the evolution of the granitic intrusions. This is supported by the high incompatible (Rb/Sr = 0.16 to 1.31 and Ba/Sr = 0.75 to 6.21) elements ratio in the granitic intrusions than that of the migmatitic gneisses and schists (Rb/Sr, 0.051 to 0.824; Ba/Sr, 0.7 to 5). High ratios of Ba/Sr and Rb/Sr and lesser values of Ba/Rb ratios in some granitic intrusions than in others suggests increasing fractionation during the anatexis. The role of partial melting is also evident in the smooth REE patterns shown by most of these rocks and the negative Eu anomaly as indicated by the values of Eu/Eu* (0.097 to 0.7). LREE enrichment is evident in the high values of Ce/YbN (12.08-174.5), La/YbN (15.2-228.4) and La/SmN (2.6-7.2) in the granitic intrusions. Tectonic discrimination diagrams of the rocks indicate that the basement rocks were most probably formed in a post-collision orogenic setting while the dolerite and the rhyolite were formed in within-plate anorogenic setting.