Partial Melting Of Amphibolite Research Articles

Silicon isotopes in an Archaean migmatite confirm seawater silicification of TTG sources

Unraveling ancient melting processes is key to understanding how the earliest, tonalite-trondhjemite-granodiorite (TTG)-dominated continental crust formed from partial melting of amphibolite. Application of silicon isotope analysis to ancient crust reveals that Archaean TTGs exhibit consistently high Si isotope values (δ30Si) compared to modern granitoids, attributed to seawater-derived silica introduced by either (a) partial melting of variably silicified basalts or (b) assimilation of authigenic silica-rich marine lithologies in the melt source. However, both mechanisms can introduce highly variable δ30Si, conflicting with the strikingly consistent δ30Si compositions of Archaean TTGs. This study investigates an alternative model, whereby the distinct mineralogy and chemistry of TTG melt sources impart a distinct silicon isotope composition to the melt, compared with “modern” granitoids. We measured δ30Si in component parts (melanosome and leucosome) of an Archaean (2.7 Ga) mafic migmatite and coeval amphibolites and mafic granulites from the Kapuskasing uplift, Canada, to explore how Si isotopes fractionate during incipient TTG melt formation. Our data reveal leucosome (i.e., melt) exhibits consistently high δ30Si values compared to a relatively isotopically lighter melanosome (i.e., residuum). We also derive inter-mineral silicon isotope fractionation factors for mineral separates that agree well with those of ab initio estimates for the same minerals and show that the magnitude of equilibrium fractionation between TTG source rock and melt replicates that in Phanerozoic granitoids. We conclude the effects of magmatic differentiation on δ30Si have remained consistent throughout Earth history, meaning that Archaean TTGs must require a source isotopically heavier than unaltered basalt, as reflected by our amphibolites and mafic migmatite components. The consistently heavy δ30Si of seawater through Earth history, and the high SiO2 content of amphibolites relative to coeval leucosome-free granulites in our study area, imply seawater silicification is the source of the observed high δ30Si. Thus, the consistently heavy Si isotope compositions measured in Archaean melt products define a unique aspect of ancient crust formation: that of the silicification of TTG source rock, implying the intrinsic involvement of a primeval hydrosphere.

Neoarchaean DTTGs from the Dunhuang Block, Tarim Craton: insights into petrogenesis and crust–mantle interactions

ABSTRACT Earth’s first continental crust is formed by Archaean and mainly consisted of tonalite–trondhjemite–granodiorite with a small amount of diorites (DTTGs), which has an essential role in probing early crust–mantle dynamic regime and in understanding the formation mechanism of continental crust. Here, we present zircon U‒Pb dating and Lu‒Hf isotopes, whole-rock geochemistry, and petrography on DTTGs rocks in the Dunhuang Block. Three episodes of DTTGs were emplaced circa 2.67 Ga, 2.60 Ga, and 2.50 Ga. The circa 2.67 Ga TTGs exhibit high SiO2 contents (68.14–71.70 wt%), low MgO contents (0.65–1.31 wt%), and high ratios of (La/Yb)N (146 on average), with their enriched Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t) = -5.77–0.53], indicating origination from partial melting of amphibolites at thickened lower crust. In contrast, the circa 2.60 Ga transitional TTGs exhibit relatively high MgO contents (2.80–3.39 wt%), flat REE (Rare earth element) patterns with moderate ratios of (La/Yb)N (20.49 on average), and dispersed Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t)= −3.99–3.08]. Accordingly, circa 2.60 Ga transitional TTGs melts were produced by partial melting of the shallower crust induced by mantle-derived magma upwelling. The circa 2.50 Ga diorites exhibit low SiO2 (55.72–59.11 wt%) but high MgO (3.51–4.52 wt%) contents with positive Nd-Hf isotopes [ƐHf (t) = -0.16–4.17 and ƐNd (t) = 2.00–4.45], suggesting that they originated from partial melting of mantle wedges metasomatized by fluid from subduction slabs. Combined with the detailed petrogenetic studies and crustal thickness variation, we conclude that the complex crust–mantle interaction may be an essential reason for the Neoarchaean diversity of DTTGs from the Dunhuang Block, which experienced prolonged arc accretion before Neoarchaean, followed by delamination between 2.67 and 2.60 Ga and subsequently transitioned to subduction.

The genesis of ca. 1.78 Ga granitoids in the Xiong’er large igneous province: Implications for continental crust generation

The genesis of intermediate-felsic rocks and the implications for crustal growth are highly controversial. These issues are particularly relevant to large igneous provinces, which are one of the most effective methods of crustal generation. Mafic magmas, even those with continental marks, have been considered as the contribution from mantle to crust, whereas the intermediate-felsic series in large igneous provinces has uncertain implications for crustal growth due to uncertainty about their modes of generation. In this paper, we focus on the intermediate-felsic granitoids, i.e., Gushicun diorites and granites, in the Xiong’er large igneous province (ca. 1780‒1750 Ma) through detailed study of their petrology, chronology, and geochemistry. Our new data show that both diorites and granites have the same emplacement age of ca. 1780 Ma and were synchronized with the Xiong’er large igneous province. The granitoids are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Li, Ba, and Rb) but depletion in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). They have almost the same ΣREE (rare earth element) contents and TTi-Zircon (∼850 °C), whole-rock Nd (εNd(t) from −8.24 to −5.76), zircon Hf (εHf(t) isotope ratios of −14 to −8 isotope ratios), and O-isotope compositions (average δ18O = 5.3‰). The fine-grained Gushicun diorites have low Sr/Nd (<10) and Eu/Sm (<0.3) but high Si/Al (>3.2) ratios and are similar to the basaltic andesites in the Xiong’er large igneous province, which suggests that the diorites are crystallization melts that differentiated along the liquid line of descent. The Rayleigh fractionation model based on the REE contents shows that the Gushicun diorites were formed by 40%‒50% fractional crystallization of Xiong’er basaltic melts. The characteristics of high K2O (3.8‒4.7 wt%) and Yb contents (4.0‒6.5 ppm), low Sr/Y (1.4‒3.3), and low metaluminous values (A/CNK = 0.8‒1.1) of the Gushicun granites with mantle-like O-isotope values rule out the melting genesis of low K2O (<0.5 wt%) amphibolite and trondhjemite−tonalite−granodiorites (TTGs) of the surrounding Taihua Complex. The Gushicun granites show a geochemical affinity with andesites in the Xiong’er large igneous province. The alphaMELTs simulation based on the major elements argues against the partial melting of amphibolite and TTGs but supports the melting genesis of Xiong’er intermediate igneous rocks with low water contents (<1 wt%). These findings shed light on the formation of the felsic series in large igneous provinces, which remelted the previous mafic-intermediate rocks via underplating basaltic magmas. Despite having enriched Nd- and Hf-isotope features, the Gushicun diorites represent juvenile crust, while the granites were formed by the reworking of juvenile crust instead of ancient crust. This paper also suggests that radiogenic isotopic ratios cannot be reliably used to assess types of continental crust.

Geological setting and genesis of the Kurmansky gabbro-trondhjemite massif (Middle Urals)

This paper reports the results of petrogeochemical studies of the Kurmansky gabbro-trondhjemite massif (eastern slope of the Middle Urals), lying in the western part of the large Reftinsky allochthonous block within the accretion East Uralian megazone. The relevance of this study is determined by the uncertainty in geodynamic setting and formation conditions of the rock massif and its role in the evolution of the Ural Mobile belt. We specified the countours of the massif. It is shown that the rocks were resulted from spatiotemporal convergence of partial melting in the mantle and lower crust at the island-arc stage of the Ural Mobile belt evolution. Partial melting of mantle peridotite, under the influence of an aqueous fluid rising from the subduction zone, initiated the occurrence of basite melts. The separation of the melt and its subsequent evolution to the compositions of gabbrodiorite and diorite took place at Ptot=10 kbar. Trondhjemites were formed as a result of partial melting of amphibolites at Ptot≥8 kbar, PH2O=0.1–0.2 kbars. The crystallization of trondhjemites in the crust was accompanied by the wollastonite skarns on contact with carbonate rock and xenoliths culminated at mesoabyssal level, Ptot=PH2O=1 kbar. The comparison between the composition of Kurmansky gabbro-trondhjemite massif and the island-arc- and collision-related magmatic suites in the region allowed us to assume that the Kurmansky massif belongs to the independent Early Devonian (?) gabbro-trondhjemite complex of island arc origin. The rock metamorphism conditions were evaluated, with the transformations supposedly related to the accretion of early island arc complexes at the Murzinsky-Aduysky microcontinent, which took place in the Devonian.

Petrogenesis of Dacites in a Triassic Volcanic Arc in the South China Sea: Constraints From Whole Rock and Mineral Geochemistry

Triassic volcanic rocks, including basalts and dacites, were drilled from Meiji Atoll in the South China Sea (SCS), which represents a rifted slice from the active continental margin along the Cathaysia Block. In this study, we present apatite and whole rock geochemistry of Meiji dacites to decipher their petrogenesis. Apatite geochronology yielded U-Pb ages of 204–221 Ma, which are identical to zircon U-Pb ages within uncertainty and thus corroborate the formation of the Meiji volcanic rocks during the Late Triassic. Whole rock major elements suggest that Meiji dacites mainly belong to the high-K calc-alkaline series. They display enriched patterns in light rare earth elements (LREE) and flat patterns in heavy rare earth elements (HREE). They show enrichment in large-ion lithophile elements (LILE) and negative anomalies in Eu, Sr, P, Nb, Ta, and Ti. The dacites have initial 87Sr/86Sr ratios of 0.7094–0.7113, εNd(t) values of -5.9–-5.4 and εHf(t) values of -2.9–-1.7, whereas the apatite has relatively higher initial 87Sr/86Sr ratios (0.71289–0.71968) and similar εNd(t) (-8.13–-4.56) values. The dacites have homogeneous Pb isotopes, with initial 206Pb/204Pb of 18.73–18.87, 207Pb/204Pb of 15.75–15.80, and 208Pb/204Pb of 38.97–39.17. Modeling results suggest that Meiji dacites can be generated by <40% partial melting of amphibolites containing ∼10% garnet. Therefore, we propose that the Meiji dacites were produced by partial melting of the lower continental crust beneath the South China block, triggered by the underplating of mafic magmas as a response to Paleo-Pacific (Panthalassa) subduction during the Triassic. Meiji Atoll, together with other microblocks in the SCS, were rifted from the South China block and drifted southward due to continental extension and the opening of the SCS.

Iron Isotope Composition of Adakitic Rocks: The Shangcheng Pluton, Western Dabie Orogen, Central China

There has been little research on the metal isotopic composition of adakitic rock. The main objective of our investigation was to obtain more knowledge on the iron isotopic composition of adakitic rocks and provide new evidence for the genesis of Shangcheng pluton from an iron isotope perspective. The Dabie orogen is divided into eastern and western areas by the Shangcheng-Macheng fault, and the Shangcheng pluton is located in the western Dabie orogen area. The iron isotopic composition of these rocks ranges from 0.08‰ to 0.20‰ (2SD, n = 3). The δ56Fe values of two rocks from the SGD (Sigudun) unit are relatively low (0.11 ± 0.03‰ and 0.08 ± 0.04‰), while the δ56Fe values of the other samples are basically consistent (0.18–0.2‰). Evidence from elemental geochemical characteristics and petrogenesis defines the Shangcheng pluton as adakitic rocks. Our investigation on the elemental and isotopic compositions hints that the enrichment of heavy iron isotopes cannot be explained by weathering/alteration and fluid exsolution. Fractional crystallization of magnetite may account for the enrichment of light iron isotopes in two rocks from the SGD unit, while the fractional iron isotope trend in the other five samples can be explained by Δ56Fecrystal-melt = ~0.035‰. Two investigated rocks from SGD units may have been derived from the partial melting of amphibolite, while the other five samples may have been derived from the partial melting of eclogite containing 10–15% garnet.

Early Paleozoic and Late Mesozoic crustal reworking of the South China Block: Insights from Early Silurian biotite granodiorites and Late Jurassic biotite granites in the Guangzhou area of the south-east Wuyi-Yunkai orogeny

The South China Block (SCB) is considered to have undergone extensive reworking of continental crust. However, the processes and mechanisms of this reworking remain uncertain. Here, we investigated Early Silurian (442 Ma) biotite granodiorites and Late Jurassic (164–159 Ma) biotite granites in the Guangzhou area of SE Wuyi-Yunkai orogen (WYO), South China. The biotite granodiorites and biotite granites show major and trace element characteristics similar with crustal-derived melts. They have enriched whole-rock Sr–Nd ((87Sr/86Sr)i = 0.7108–0.7210; εNd(t) = − 11.7 to −7.6) and zircon Hf (εHf(t) = −17.2 to −1.7) isotopic compositions. Early Silurian biotite granodiorites have relatively homogeneous zircon δ18O values (8.6‰–9.3‰) while Late Jurassic biotite granites show a wide range of zircon δ18O values (7.4‰–10.6‰). We suggest that the Early Silurian biotite granodiorites were formed by partial melting of amphibolites and that the Late Jurassic biotite granites were formed by partial melting of a hybridized crustal source containing metasedimentary rocks with subordinate juvenile crustal rocks. Combining our results with those of previous studies in the SCB, we suggest that Early Paleozoic intracontinental reworking of the SCB can be divided into three stages: crust double-thickening, orogenic collapse, and post-orogenic lithospheric extension. During the Late Ordovician to Early Silurian, Guangzhou area of the SE WYO underwent reworking mainly of middle–lower-crustal rocks in response to crust double-thickening. In addition, during the Late Jurassic, the area underwent reworking mainly of middle–lower-crustal metasedimentary rocks, which was induced by heating from mantle-derived mafic magmas.

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.

Early Mesozoic crustal evolution in the NW segment of West Qinling, China: Evidence from diverse intermediate–felsic igneous rocks

Continental convergent margins are commonly considered as significant sites for understanding crustal reworking and growth, but the geodynamic processes involving crustal growth remain highly debatable. Triassic intermediate–felsic intrusive rocks are widespread in the northwestern segment of the West Qinling orogenic belt on the NE Tibetan Plateau. These rocks well document the evolution of one branch of the Paleo-Tethys oceans (i.e., the A'nyemaqen Ocean) in East Asia. Here we present a comprehensive study of zircon UPb and LuHf isotopic data, along with whole-rock elemental and SrNd isotopic data to decipher the origins of intermediate–felsic intrusions and crustal evolution of the West Qinling in the Triassic. Two episodes of magmatism were detected in this study, i.e., the early high-Mg diorite porphyry–granodiorite–granodiorite porphyry assemblage (Episode #1, ~244–230 Ma), followed by a later monzogranite–granodiorite–granite porphyry assemblage (Episode #2, ~228–203 Ma). These two episodes of magmatism are interpreted as products related to oceanic subduction and continental collision, respectively. In Episode #1, the mafic magmatic enclaves were mainly mantle-derived and were indicative of magma mixing. The high-Mg diorite porphyries were likely derived from partial melting of the mantle wedge metasomatized by sediment-derived melts, whereas the coeval granodiorites and granodiorite porphyries mainly originated from partial melting of amphibolites with significant addition of mantle-derived components. By contrast, the granodiorite porphyries in Episode #2 probably originated from the lower crust-sourced melts with insignificant mantle contribution. The contemporaneous granodiorites and monzogranites were generated by partial melting of the crustal amphibolites or metagreywackes. The proportion of positive zircon εHf(t) values decreased from Episode #1 to Episode #2, indicating a decreasing tendency of the mantle or juvenile component contribution. Our study suggests that the addition of mantle-derived magmas to the continental crust during oceanic subduction processes played an important role in crustal growth.

Dehydration melting of amphibolite at 1.5 ​GPa and 800–950 ​°C: Implications for the Mesozoic potassium-rich adakite in the eastern North China Craton

Mesozoic intermediate-felsic magmatic rocks in the eastern North China Craton commonly show geochemical similarity to adakites. However, the lack of direct constraints from partial melting experiments at high pressures and temperatures fuels a debate over the origin of these rocks. In this work, we performed partial melting experiments at 1.5 ​GPa and 800–950 ​°C on amphibolite samples collected from the vicinity of the Mesozoic potassium-rich adakitic rocks in the Zhangjiakou area, northern margin of the North China Craton. The experimental melts range from granitic to granodioritic compositions, with SiO2 ​= ​56.4–72.6 ​wt.%, Al2O3 ​= ​16.1–19.3 ​wt.%, FeO∗ ​= ​2.4–9.6 ​wt.%, MgO ​= ​0.3–2.0 ​wt.%, CaO ​= ​0.6–3.8 ​wt.%, Na2O ​= ​4.7–5.3 ​wt.%, and K2O ​= ​2.6–3.9 ​wt.%, which are in the ranges of the surrounding Mesozoic potassium-rich adakitic rocks, except for the higher Al2O3 contents and the data point at 1.5 ​GPa and 800 ​°C. Trace element compositions of the melts measured by LA-ICP-MS are rich in Sr (849–1067 ​ppm) and light rare earth elements (LREEs) and poor in Y (<10.4 ​ppm) and Yb (<0.88 ​ppm), and have high Sr/Y (102–221) and (La/Yb)n (27–41) ratios and strongly fractionated rare earth element (REE) patterns, whereas no obvious negative Eu anomalies are observed. The geochemical characteristics show overall similarity to the Mesozoic potassium-rich adakitic rocks in the area, especially adakites with low Mg#, again except for the data point at 1.5 ​GPa and 800 ​°C. The results suggest that partial melting of amphibolite can produce potassium-rich adakitic rocks with low Mg# in the eastern North China Craton under the experimental conditions of 1.5 ​GPa and 850–950 ​°C. The experimental restites consist of hornblende (Hbl) ​+ ​plagioclase (Pl) ​+ ​garnet (Grt) ​± ​clinopyroxene (Cpx), a mineral assemblage significantly different from that of the nearby Hannuoba mafic granulite xenoliths which consist of Cpx ​+ ​orthopyroxene (Opx) ​+ ​Pl ​± ​Grt. Chemically, the experimental restites contain higher Al2O3 but lower MgO and CaO than the Hannuoba mafic granulite xenoliths. We therefore argue that the Hannuoba mafic granulite xenoliths cannot represent the direct products of partial melting of the experimental amphibolite.

Role of titanite in the redistribution of key trace elements during partial melting of meta-mafic rocks: An example from Namche Barwa migmatite

南迦巴瓦地区广泛出露的中下地壳变基性岩部分熔融形成的层状混合岩和淡色花岗岩，为研究部分熔融过程中榍石的地球化学行为对熔体的微量元素组成的影响提供了良好的机会。相对于源岩或熔融残留体，淡色体亏损Ti、V、REE、Y、Nb、Ta、U等元素，与混合岩中榍石的微量元素特征互补。混合岩、淡色体和榍石微量元素特征表明南迦巴瓦角闪岩部分熔融形成的淡色体的微量元素特征主要受控于榍石的地球化学行为。角闪岩脱水部分熔融过程中，由于长英质熔体的低Ti溶解度，榍石以未熔残留体形式存在于暗色体中，导致熔体亏损Ti、REE、Nb、Ta、V、U等元素和Sr/Y比值相对升高。关键元素在榍石和熔体之间的配分系数受熔体成分影响明显。角闪岩中变质榍石D_Nb/Ta<1，因此变质榍石残留导致熔体Nb/Ta相对于源岩升高；而高Si-Al花岗质熔体中榍石D_Nb/Ta>1，因此与高Si-Al熔体平衡的榍石的分离（转熔或结晶分异）将导致熔体Nb/Ta比值相对源岩降低。榍石在部分熔融过程中的微量元素效应为理解变基性岩部分熔融产生熔体的地球化学特征提供新的认识。;Layered migmatite formed by partial melting of amphibolites from mid to lower crust levels is widespread in the Namche Barwa. Titanite, as a key metamorphic residual and magmatic phase, occurs in the migmatite as well as in the leucosome, which provides a good opportunity to investigate the role of titanite in regulating the trace element compositions in the amphibolite-derived melts. Compared with migmatites, the leucosomes are depleted in elements such as Ti, V, REE, Y, Nb, Ta and U, which are enriched in titanite. Such characteristics indicate that the trace element compositions in the anatectic melts are mainly controlled by the geochemical behavior of titanite. During partial melting of amphibolite, due to low solubility of TiO₂ in felsic melts, titanite grains commonly retain in the residue and in turn result in the depletion of those key elements compatible in titanite structure. The trace element partition coefficients between titanite and silicic melts are significantly affected by the melt's compositions. Residual metamorphic titanite with D_Nb/Ta<1 will result in higher Nb/Ta in melt than that in the protolith. However, when equilibrating with high Si-Al melts, magmatic titanite preferred Nb over Ta with D_Nb/Ta>1. Separation of such titanite grains will decrease the Nb/Ta ratio in the residual melts. Data and results presented in this study demonstrate that titanite, similar to other accessory phases, could strongly affect the geochemical characteristics of melts produced by melting mafic rocks.

Petrogenesis and tectonic implications of late Oligocene highly fractionated leucogranites in the Ailao Shan-Red River shear zone, SW China

We report for the first time the occurrence of leucogranite bodies that intruded the Ailaoshan metamorphic complex along the Ailao Shan-Red River (ASRR) shear zone. Leucogranites in Yuanyang, Southeast Yunnan province, are composed of quartz (~30%), alkali feldspar (~45%), plagioclase (~5%), muscovite (~13%) and biotite (~3%) and other minor silicate minerals (~3%, garnet, tourmaline). U–Pb dating of zircon from leucogranites yields a weighted mean age of 27.4 ± 0.5 Ma, suggesting a late Oligocene anatexis event. These leucogranites are strongly peraluminous with A/CNK values of 1.01–1.19 and have high SiO2 (73.3–77.0 wt%) and K2O + Na2O (7.87–10.08 wt%), and low CaO (0.41–0.99 wt%), MgO (0.04–0.21 wt%), TiO2 (0.01–0.06 wt%) and Fe2O3T (0.44–0.86 wt%) contents. They are geochemically similar to Himalayan leucogranites and are enriched in large-ion lithophile elements (e.g. Rb), and depleted in high-field-strength elements (e.g. Nb, Zr, and Hf). They have low Zr/Hf (12.3–22.5) and Nb/Ta (2.54–12.73) with negative Eu anomalies (Eu/Eu* = 0.05–0.45), indicative of extensive fractional crystallization of plagioclase and zircon. In addition, these rocks have low 87Sr/86Sr (t) (0.7072–0.7111), high εNd(t) (−1.53 to −3.61). They contain zircon grains with εHf (−5 to +1.8) and old two-stage Hf model ages of 1.24–1.02 Ga, implying that they were probably derived from partial melting of amphibolite (old crustal material) within the lower crust. In combination with previous studies of metamorphism of the ASRR shear zone, our study demonstrates that the leucogranites may have been generated in a highly dynamic setting involving both extrusion and extension that defined the beginning of left-lateral slip-strike movement of the ASRR shear zone at 27 Ma.

Diverse magma sources for the Himalayan leucogranites: Evidence from B-Sr-Nd isotopes

The Himalayan orogen is featured by widespread S-type leucogranites (ca. 45 to 9 Ma) formed primarily from the partial melting of metapelites (800–480 Ma), and minor from the partial melting of amphibolite with subordinate metapelites. The Ama Drime gneiss and Mabja leucogranite pluton are both located at the footwall of the NS-trending Xainza–Dinggye normal fault in the central part of the Himalayan orogen, with published εNd(t) values of −21.0 to −19.6 and − 19.3 to −18.3 and 87Sr/86Sr(t) values of 0.90954–0.92574 and 0.84853–0.85474, respectively. In this study, we presented new SHRIMP zircon U-Pb ages and laser ablation-multi-collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) tourmaline boron isotope data on the metamorphic rocks and leucogranites of the Himalayan orogen. The weighted mean 207Pb/206Pb age of the zircon cores of the Ama Drime gneiss (T0446-2-3) is 1854 ± 4 Ma. The tourmaline δ11B values of the Ama Drime gneiss (T0446-1-6) are −17.6 to −14.3‰, similar to those of the Mabja leucogranite (T0436-4, −18.9 to −17.4‰). In contrast, the tourmaline δ11B values of the Quedang metapelite (T0389-18) and its partial melting product – Malashan leucogranite (T0659-12A-3) are substantially higher, i.e., −15.3 to −12.5‰ and − 16.2 to −8.0‰, respectively. The tourmaline δ11B values of the Yadoi leucogranite (T0321-4), derived from partial melting of amphibolite, range from −8.4 to −5.4‰. Therefore, the tourmaline B-isotopes and Sr-Nd-isotopes results are consistent, and may have reflected an E-W extension along the southern Tibetan rift system, indicating a late Miocene anatexis in the North Himalaya region. The melts derived from the partial melting of metapelites and mature crustal materials (e.g., Paleoproterozoic Ama Drime gneiss) ascended along the N-S trending Xainza-Dinggye normal fault, and were subsequently emplaced in the Tethyan Himalaya sequence.

The petrogenesis of the Neoarchean Kukuluma Intrusive Complex, NW Tanzania

The Kukuluma Intrusive Complex (KIC) is a late Archean igneous complex, dominated by monzonite and diorite with subordinated granodiorite. The monzonite and the diorite suites have low silica content (SiO2 ≤ 62 wt%), moderate Mg# (Mg#average = 49), high Sr/Y (Sr/Yaverage = 79) and high La/Yb (La/Ybaverage = 56) ratios, and strongly fractionated (Lan/Ybn = 9–69) REE patterns. Their moderate Ni (Niaverage = 50 ppm), Cr (Craverage = 85 ppm), variable Cr/Ni ratio (0.65–3.56) and low TiO2 (TiO2average = 0.5 wt%) indicate little to no interaction with the peridotitic mantle. For most major elements (Al2O3, FeOt, Na2O, TiO2 and P2O5) the monzonite and the diorite suites display subparallel trends for the same SiO2 content indicating they represent distinct melts. Intrusions belonging to the diorite suite have high Na2O (Na2Oaverage = 4.2 wt%), Dy/Ybn (Dy/Ybn-average = 1.6), a positive Sr anomaly and uncorrelated Nb/La and Zr/Sm ratios suggesting derivation from partial melting of garnet-bearing amphibolite. Intrusions belonging to the monzonite suite have higher Na2O (Na2Oaverage = 5.61 wt%), Dy/Ybn (Dy/Ybn-average = 2.21), a negative Sr anomaly and correlated Nb/La and Zr/Sm ratios consistent with derivation from partial melting of eclogite with residual rutile. Small variations in the Th/U ratio and near chondritic/MORB average values (Th/Umonzonite = 3.65; Th/Udiorite = 2.92) are inconsistent with a subducting slab signature, and it is proposed that the monzonite and the diorite suites of the KIC formed by partial melting of garnet-bearing, lower mafic crust of an oceanic plateau. The granodiorite suite has lower Mg# (Mg#average = 41), moderately fractionated REE, low Sr/Y (Sr/Yaverage = 20), La/Yb (La/Ybaverage = 15), Dy/Ybn (DyYbn-average = 1.24) and small negative Eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation. Near-MORB Th/U (Th/Uaverage = 2.92) and Zr/Sm (Zr/Smaverage = 30.21) ratios are consistent with intracrustal melting of amphibolite.Archean rocks with an “adakitic” geochemical signature have been used to argue in favour of a plate tectonics regime in the Archean. The results presented here suggest that tectonic models for the Tanzania Craton, which invoke a subduction-related setting for all greenstone belts may need revision.

Petrogenesis and metallogenic setting of porphyries of the Duobuza porphyry Cu–Au deposit, central Tibet, China

Given that the Duobuza deposit was the first porphyry Cu–Au deposit discovered in central Tibet, the mineralization and mineralized porphyry in this area have been the focus of intensive research, yet the overall porphyry sequence associated with the deposit remains poorly understood. New geological mapping, logging, and sampling of an early granodiorite porphyry, an inter-mineralization porphyry, and a late-mineralization diorite porphyry were complemented by LA–ICP–MS zircon dating, whole-rock geochemical and Sr–Nd isotopic analyses, and in situ Hf isotopic analyses for both inter- and late-mineralization porphyry intrusions. All of the porphyry intrusions are high-K and calc-alkaline, and were emplaced at ca. 120Ma. The geochemistry of these intrusions is indicative of arc magmatism, as all three porphyry phases are enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy rare earth elements and high field strength elements. These similar characteristics of the intrusions, when combined with the relatively high (87Sr/86Sr)i, negative εNd(t), and positive εHf(t) values, suggest that the magmas that formed the porphyries were derived from a common source region and shared a single magma chamber. The magmas were generated by the mixing of upwelling metasomatized mantle-wedge-derived mafic magmas and magmas generated by partial melting of amphibolite within the lower crust.The inter-mineralization porphyry has the lowest εNd(t) and highest (87Sr/86Sr)i values, suggesting that a large amount of lower-crust-derived material was incorporated into the melt and that metals such as Cu and Au from the enriched lower crust were scavenged by the parental magma. The relative mafic late-mineralization diorite porphyry phase was formed by the residual magma in the magma chamber mixing with upwelling mafic melt derived from metasomatized mantle. The magmatic–hydrothermal evolution of the magma in the chamber released ore-forming fluid that was transported mainly by the inter-mineralization porphyry phase during the mineralization stage, which ultimately formed the Duobuza porphyry Cu–Au deposit.These porphyritic intrusions of the Duobuza deposit have high Mg# and low (La/Yb)N values, and show some high LILE/HFSE ratios, indicating the magma source was enriched by interaction with slab-derived fluids. Combined with age constraints on the regional tectonic evolution, these dating and geochemical results suggest that the Duobuza porphyry Cu–Au deposit formed in a subduction setting during the final stages of the northward subduction of the Neo-Tethyan Ocean.

Geochronology and geochemistry of the Niujuanzi ophiolitic mélange, Gansu Province, NW China: implications for tectonic evolution of the Beishan Orogenic Collage

The Niujuanzi ophiolitic melange (NOM), located in the Beishan Orogenic Collage, marks the termination between the Huaniushan arc and Mingshui-Hanshan Massifs. The NOM is mainly composed of gabbros, diabases, plagiogranites, basalts, and greywacke. Two gabbros have ages of 433.8 ± 3.1 and 354.0 ± 3.3 Ma, two plagiogranites have ages of 429.8 ± 2 and 448.7 ± 2.0 Ma, and a diabase has an age of 433.4 ± 3.2 Ma. The gabbros and diabases are calc-alkaline and tholeiitic, with high Al2O3, CaO, and TiO2 contents and low FeOT contents. The gabbros have high Mg# values (49–82), while the diabases have relatively low Mg# values (46–61). The plagiogranites are calc-alkaline and metaluminous, with high SiO2 and Na2O contents and low Al2O3 and K2O contents. The gabbros and diabases are enriched in large iron lithophile elements and slightly depleted in high field strength elements relative to N-MORB and their trace element characteristics are similar to E-MORB. With respect to rare earth element (REE), they have slightly enriched LREEs relative to HREEs. The majority of the plagiogranite trace elements approximate those of the volcanic arc granite. The plagiogranites have obviously enriched LREEs relative to HREEs, with a slightly to strongly negative Eu anomaly, which is similar to ORG but distinct from volcanic arc and within plate granite. The NOM was formed from the Ordovician to the Carboniferous, representing the expansion period of the Niujuanzi Ocean. The gabbros, diabases, and plagiogranites were formed in a mid-ocean ridge environment. The gabbros and diabases were generated by different degrees of partial melting of the mantle, and the plagiogranites derived from both the crystallization differentiation of basaltic magma and the partial melting of amphibolites in the crust.

Eocene magmatism in the Tethyan Himalaya, southern Tibet

Abstract Recent studies in the Yardoi gneiss dome of the Tethyan Himalaya, southern Tibet demonstrated that the Himalayan orogenic belt experienced a major episode of crustal melting in the Mid-Eocene ( c. 44–40 Ma), represented by the genesis of a suite of high Sr/Y two-mica granites (TMGs) accompanied by leucogranites. Geochemical and U–Pb age data indicate that (1) both the leucogranite and the porphyritic leucogranite formed at c. 44–41 Ma are characterized by similar Sr–Nd–Hf isotope systematics to those in the less evolved TMGs; (2) these three suites of rocks might represent a Mid-Eocene igneous complex derived from partial melting of amphibolite under crustal thickened conditions; and (3) in contrast with the TMGs, leucogranites experienced hydrothermal-magma interactions as well as plagioclase fractional crystallization as revealed from pronounced negative Eu anomalies, much lower Sr concentrations, and elevated common Pb and U concentrations in the latest magmatic zircon overgrowth. Our data suggest that magmatic differentiation of more primitive magmas could have played a critical role in the generation of leucogranites. Documentation of partial melting of deep crustal rocks during the earliest phase of the tectonic evolution of the Himalayan orogen implies that the large-scale collisional orogenic belt could experience intensive partial melting under contraction and thickening conditions, which could profoundly affect the geophysical and geochemical properties as well as rheological properties and facilitate the initiation of movement or flow of deep crustal rocks.

Neoarchean to Paleoproterozoic continental growth in the southeastern margin of the North China Craton: Geochemical, zircon U–Pb and Hf isotope evidence from the Huoqiu complex

The Huoqiu complex in the southeastern margin of the North China Craton (NCC) is dominated by Neoarchean gray gneisses, amphibolites and voluminous metasediments. Here we report the occurrence of Neoarchean to Paleoproterozoic rocks from drill-core samples. The gneisses are similar to TTG (tonalite–trondhjemite–granodiorite) in composition and show close spatial association with amphibolites. Geochemical characteristics such as high Sr/Y and (La/Yb)N with steep REE patterns and trace element modeling suggest that these rocks were generated by partial melting of hydrous meta-basalts (amphibolites) at the base of a thickened mafic continental crust, leaving a rutile-bearing eclogite residue. LA-MC-ICP-MS U–Pb age data from magmatic zircon grains show protolith emplacement ages of 2.76–2.71Ga. Subsequently, widespread migmatization took place at 1.91–1.82Ga, generating voluminous migmatites and high-K granites. Hf isotopic compositions of zircon grains from the amphibolite and gneiss show εHf(t) values of 2.4–15.5 and −3.0–1.5, respectively. The tDM2(Hf) model ages of the gneisses range from 2.87 to 3.14Ga, and are identical to the tDM1(Hf) ages of amphibolites (2.84–3.16Ga) within analytical uncertainty, suggesting that the gneisses formed by partial melting of amphibolite, and attest to large-scale reworking of the ancient continental crust during Neoarchean. The zircon grains from the granites define two groups with regard to their Hf isotopic composition. The older group (1916±42Ma) has εHf(t) values and tDM2(Hf) ages of −10.5–2.4 and 2.40–3.20Ga, respectively, whereas the younger one (1823±41Ma) shows a large variation in εHf(t) values ranging from −18.1 to 12.5, with tDM2(Hf) model ages of 1.70–3.59Ga. A couple of zircon grains from the younger group display consistent U–Pb ages and tDM2, indicating accretion of juvenile crust from depleted mantle sources during 1.82 to 1.91Ga. However, the dominant Hf isotope features are consistent with the reworking of preexisting continental crust. We therefore infer that only limited accretion of juvenile crust occurred during this time, and that the Paleoproterozoic (1.82 to 1.91Ga) tectonics in the southeastern margin of the NCC witnessed extensive reworking of older continental crust.

Constraining the process of Eoarchean TTG formation in the Itsaq Gneiss Complex, southern West Greenland

We present new major and trace element, high-precision high-field-strength-element, hafnium and neodymium isotope data for well preserved Eoarchean TTGs within the Itsaq Gneiss Complex (IGC) of southern West Greenland. These data are combined with thermodynamic model predictions of partial melting and fractional crystallization to gain new insights into continental crust formation in the Archean. Our results show that the observed compositional range of Eoarchean TTGs can be explained by a combination two processes: (1) 5–25% partial melting of amphibolite within thickened mafic crust and (2) subsequent fractional crystallization processes. The Eoarchean TTG suite of SW Greenland probably formed through mixing of melt batches that originally formed at different source depths between 10 and 14 kbar and ponded as plutons at mid-crustal levels. The trace element compositions of some TTGs point to subsequent fractional crystallization processes involving plagioclase, clinopyroxene, amphibole and garnet. Our model is consistent with recent studies proposing that the Eoarchean Itsaq Gneiss Complex TTGs from the IGC formed by re-working of mafic protocrust that stabilized as accreted juvenile crustal terranes in the Eoarchean. The model is also in good agreement with field observations from the area.

Petrogenesis and tectonic evolution of metaluminous sub-alkaline granitoids from the Takab Complex, NW Iran

AbstractThe Takab complex is composed of a variety of metamorphic rocks including amphibolites, metapelites, mafic granulites, migmatites and meta-ultramafics, which are intruded by the granitoid. The granitoid magmatic activity occurred in relation to the subduction of the Neo-Tethys oceanic crust beneath the Iranian crust during Tertiary times. The granitoids are mainly granodiorite, quartz monzodiorite, monzonite and quartz diorite. Chemically, the magmatic rocks are characterized by ASI&lt;1.04, AI&lt;0.87 and high contents of CaO (up to ~14.5 wt%), which are consistent with the I-type magmatic series. Low FeOt/(FeOt+MgO) values (&lt;0.75) as well as low Nb, Y and K2O contents of the investigated rocks resemble the calc-alkaline series. Low SiO2, K2O/Na2O and Al2O3accompanied by high CaO and FeO contents indicate melting of metabasites as an appropriate source for the intrusions. Negative Ti and Nb anomalies verify a metaluminous crustal origin for the protoliths of the investigated igneous rocks. These are comparable with compositions of the associated mafic migmatites, in the Takab metamorphic complex, which originated from the partial melting of amphibolites. Therefore, crustal melting and a collision-related origin for the Takab calc-alkaline intrusions are proposed here on the basis of mineralogy and geochemical characteristics. TheP–Tevolution during magmatic crystallization and subsolidus cooling stages is determined by the study of mineral chemistry of the granodiorite and the quartz diorite. Magmatic crystallization pressure and temperature for the quartz-diorite and the granodiorite are estimated to beP~7.8±2.5 kbar,T~760±75°C andP~5±1 kbar,T~700°C, respectively. Subsolidus conditions are consistent with temperatures of ~620°C and ~600°C, and pressures of ~5 kbar and ~3.5 kbar for the quartz-diorite and the granodiorite, respectively.