Melting Of Amphibolite Research Articles

Petrogenesis of Eocene A-Type Granite Associated with the Yingpanshan–Damanbie Regolith-Hosted Ion-Adsorption Rare Earth Element Deposit in the Tengchong Block, Southwest China

The ion-adsorption-type rare earth element (iREE) deposits dominantly supply global resources of the heavy rare earth elements (HREEs), which have a critical role in a variety of advanced technological applications. The initial enrichment of REEs in the parent granites controls the formation of iREE deposits. Many Mesozoic and Cenozoic granites are associated with iREE mineralization in the Tengchong block, Southwest China. However, it is unclear how vital the mineralogical and geochemical characteristics of these granites are to the formation of iREE mineralization. We conducted geochronology, geochemistry, and Hf isotope analyses of the Yingpanshan–Damanbie granitoids associated with the iREE deposit in the Tengchong block with the aims to discuss their petrogenesis and illustrate the process of the initial REE enrichment in the granites. The results showed that the Yingpanshan–Damanbie pluton consists of syenogranite and monzogranite, containing REE-bearing accessory minerals such as monazite, xenotime, apatite, zircon, allanite, and titanite, with a high REE concentration (210–626 ppm, mean value is 402 ppm). The parent granites have Zr + Nb + Ce + Y (333–747 ppm) contents and a high FeOT/MgO ratio (5.89–11.4), and are enriched in Th (mean value of 43.6 ppm), U (mean value of 4.57 ppm), Zr (mean value of 305 ppm), Hf (mean value of 7.94 ppm), Rb (mean value of 198 ppm), K (mean value of 48,902 ppm), and have depletions of Sr (mean value of 188 ppm), Ba (mean value of 699 ppm), P (mean value of 586 ppm), Ti (mean value of 2757 ppm). The granites plot in the A-type area in FeOT/MgO vs. Zr + Nb + Ce + Y and Zr vs. 10,000 Ga/Al diagrams, suggesting that they are A2-type granites. These granites are believed to have formed through the partial melting of amphibolites at a post-collisional extension setting when the Tethys Ocean closed. REE-bearing minerals (e.g., apatite, titanite, allanite, and fluorite) and rock-forming minerals (e.g., potassium feldspar, plagioclase, biotite, muscovite) supply rare earth elements in weathering regolith for the Yingpanshan–Damanbie iREE deposit.

Eocene crustal thickening in the Tethyan Himalaya: Insights from Barrovian metamorphism and granite geochemistry from the Ramba area

Magmatism, structures, and metamorphism in the Ramba dome of the Tethyan Himalaya were investigated to shed light on orogenic processes during the early stages of the India-Asia collision. Deformed granite dikes in the dome envelope yield zircon U-Pb ages of ca. 45 Ma. These Eocene granites have adakitic, Na-rich compositions (K2O/Na2O = 0.20−0.61), weak to no Eu anomaly, enrichment in Sr, depletion in heavy rare earth elements and Y, and low MgO and Mg# contents. These characteristics contrast with the Miocene potassic granites in the core of the dome and suggest that the Eocene adakites were derived from the high-pressure melting of crustal amphibolites in a thick crust. The mica schists of the dome envelope have an early foliation (S1) that is overprinted by upright folds (F2). Phase-equilibria modeling of garnet and staurolite mica schists suggests a Barrovian-type, prograde P-T evolution in association with S1, with peak conditions of 6.7−7.2 kbar/590−605 °C and 7.3−7.8 kbar/650−670 °C, respectively, which are typical of crustal thickening metamorphism. Monazites from S1-dominated staurolite mica schists yield metamorphic ages of ca. 51−49 Ma, while those from the late foliation (S2) that transposed S1 give younger ages of ca. 10 Ma. The integration of geochemical, structural, metamorphic, and geochronological data suggests that peak Barrovian D1 metamorphism and adakitic magmatism occurred in the Eocene in response to crustal thickening. The results provide critical constraints for addressing the crustal shortening deficit of the region.

Early Jurassic and Late Cretaceous plagiogranites in Nain–Baft ophiolitic mélange zone in Iran: remnants of rift–drift and SSZ evolution of a Neotethyan seaway

We have investigated the geology, geochemistry and geochronology of various plagiogranite intrusions that are spatially associated with ophiolitic subunits in a mélange terrain exposed within the Nain–Baft fault zone along the western boundary of the Central–East Iranian Microcontinent (CEIM). One group of plagiogranites is intrusive into foliated amphibolites, whereas the plagiogranites of the other group are intrusive into gabbros, dyke swarms and pillow lavas that are tectonically juxtaposed. U–Pb zircon dating of the first group of plagiogranites has revealed near-concordant crystallization ages of 176.94 ± 0.71, 179.84 ± 0.92 and 190.40 ± 2.1 Ma, indicating the early Jurassic timing of their emplacement. U–Pb zircon dating of the second group of plagiogranites has revealed a weighted average age of 92.34 ± 0.38 Ma, indicating that they were emplaced in the early Late Cretaceous. The Early Jurassic plagiogranites are low-K, sub-alkaline rocks with negative Eu anomalies, low Ti and high La and Ce contents. They display elevated ratios of large ion lithophile elements to high field strength elements. They were part of a magmatic system that facilitated the earliest rift–drift stages of a Neotethyan seaway development, and their magmas were produced by fractional crystallization of a mafic melt associated with asthenospheric upwelling beneath a rift system. The Late Cretaceous plagiogranites are low-K, sub-alkaline rocks with negative Eu anomalies, high Y and low Ta contents. They show light REE depletion relative to flat heavy REE patterns, and high Ba/Th ratios. Magmas of these plagiogranites were the products of partial melting of amphibolites in the subducting oceanic lithosphere in the same Neotethyan seaway. Thus, we posit that plagiogranite rocks in suture zones can be excellent trackers of geochemical, geochronological and magmatic evolution of various types of oceanic crust through rift–drift, seafloor spreading and subduction initiation stages within the same ocean basins in the past. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

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.

Voluminous magma formation for the 30-ka Aira caldera-forming eruption in SW Japan: contributions of crust-derived felsic and mafic magmas

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km3 of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ∼0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (∼39 vol%) and melt with a dacitic (∼70 wt% SiO2) composition. The felsic magma mixed with ∼10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting.

Paleoproterozoic crustal evolution of the Tarim Craton, NW China: Constraints from geochronology and geochemistry of orthogneisses and granitic veins in the Xingdi region of the Quruqtagh Block

The Archean–Paleoproterozoic basement rocks, exposed in the Xingdi region of the Quruqtagh Block, provide a great opportunity to characterize the early crustal evolution of the Tarim Craton and reconstruct the Columbia supercontinent. Here we report zircon U–Pb ages, Lu–Hf isotope compositions, and bulk-rock geochemical data of orthogneisses and granitic veins from the Xingdi region of the Quruqtagh Block in the northeastern margin of the Tarim Craton. The orthogneisses formed in the late Neoarchean to early Paleoproterozoic (2504 ± 33 Ma, 2419 ± 24 Ma and 2450 ± 27 Ma), and were metamorphosed in the late Paleoproterozoic (1789 ± 43 Ma and 1748 ± 35 Ma). The orthogneisses have relatively high Sr (312–710 ppm) contents, and low Y (5.3–13.8 ppm) and Yb (0.46–1.10 ppm) contents, resulting in high Sr/Y (32–118) and La/Yb (17–64) ratios that are characteristics of adakites. The relatively high Mg# (49–55) and depleted zircon Hf isotope characteristics (positive εHf(t), with TDM2 of ca. 2.5–2.8 Ga) indicate that the orthogneisses crystallized from partial melting of garnet-bearing amphibolite in a subduction environment. The granitic veins that intrude the orthogneisses formed during the late Paleoproterozoic (1760 ± 25 Ma), which is roughly contemporaneous with the metamorphic ages of the orthogneisses; the granitic veins are characterized by extremely high SiO2 (>80 %), enrichments in Rb, Ba and U, and depletions in Nb and Ta. It is inferred that the granitic veins formed by in situ anatexis of the orthogneisses during the late Paleoproterozoic. Based on the zircon ages with Hf isotope compositions, it is evident that the continental crust in the Xingdi region of the Tarim Craton largely formed between 2.5 and 2.8 Ga and was reworked at ca. 1.8 Ga; this evolutionary history is similar to that of the Dunhuang Block. Comparison of the Paleoproterozoic tectono-thermal events in the different terranes of the Tarim Craton, it is suggested that the Tarim Craton is composed of discrete terranes that detached from different ancient cratonic nuclei prior to the late Paleoproterozoic, and assembled within the Columbia supercontinent respectively.

Middle Permian granitoids in the western section of the northern Qaidam Block, NW China: Petrogenesis and tectonic implications

Origin and tectonic setting of late Late Paleozoic to Early Mesozoic igneous rocks in the northern Qaidam Block (NQ) have long been a matter of debate. In this paper, we report new zircon U-Pb ages and Hf isotopic compositions and bulk rock major and trace elemental compositions for granitoids of the Yanchangbeishan area in the NQ. The granodiorite, tonalite, granodiorite, and syenogranite yielded zircon U-Pb ages of 271.9 Ma, 262.0 Ma, 272.4 Ma and 261.3 Ma, respectively, corresponding to εHf(t) values are 13.04–14.32, 8.14–10.77, 8.66–11.88, and 8.22–10.43, respectively. These granitoids are compositionally I-type granites and exhibit varying degrees of enrichment in Rb, Th, U, and K, and depletion in Nb, Ta, and Ti. According to field relationships, petrology, geochronology, and geochemistry, the granitoid types can be divided into three groups: (1) ca. 272 Ma granodiorites, which are formed through partial melting of metagreywackes and amphibolites derived from depleted mantle; (2) ca. 262 Ma tonalite, which represents a typical high-silica adakite derived from partial melting of subducted oceanic crust during the evolution of depleted mantle; and (3) ca. 261 Ma syenogranite, which is formed by the partial melting and high-degree fractional crystallization of mixed crustal materials. Considering our findings and the regional geological background, we propose a five-stage tectonic evolution model of subduction slab roll-back and subduction zone retreat for the late Late Paleozoic to Early Mesozoic of NQ.

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.

Origin of the high conductivity anomalies in the mid-lower crust of the Tibetan Plateau: Dehydration melting of garnet amphibolites

High-conductivity layers of 0.1–1 S/m distributed in the mid-lower crust of the Tibetan Plateau are traditionally explained as the fluid/melt related crustal flow. Dehydration of amphibole-bearing rocks may play an important role in explaining these anomalies. To survey the anomalies' origin, the electrical conductivities of amphibole-bearing samples are measured at 1.5 GPa and 600–1300 K. Our experiments show that dehydration melting of amphibole occurs at about 1100 K. Before dehydration melting of amphibolites, proton conduction together with small polaron conduction dominate the conduction process, whereas ionic conduction plays a more important role after the dehydration melting of amphibolites. The dehydration melting of polycrystalline amphibole yields a conductivity of up to 1 S/m under the lower-crust conditions beneath the Tibetan Plateau. Combining the calculated thermal gradient in the crust of Tibetan Plateau with the petrology proof and seismic data, dehydration melting of garnet amphibolites likely contributes to the high conductivity anomalies within the Tibetan crust. MW and cube model simulation indicates that the melt fraction is 3.8–36 vol% in the partial molten region.

ZIRCON ANATOMY FROM THE ROCKS ASSOCIATION OF THE OSTRIVSKY QUARRY (ROS-TIKYCH MEGABLOCK OF THE UKRAINIAN SHIELD)

Granitoids play a key role in the geological structure of the Ros-Tikych megablock. Supercrustal rocks of the Ros-Tikych series have been preserved in the granitoids only in the form of isolated fragments such as elongated remains, small skialites and even smaller "melted" xenoliths. In particular, in the Ostrivsky quarry, located on the right bank of the Ros River east of Bila Tserkva, granitoids are found (even-grained, porphyry-like granites) among which, as a rule, small bodies of granodiorites, plagiogranites and amphibolites occur. In order to determine the source of the parent magmas of rocks the properties of zircon crystals and the isotopic composition (87Sr/86Sr ratio) of apatite were studied. An analysis of the zircon crystals of the crystalline rocks exposed at the Ostrivsky quarry allows us to propose that the and plagio- and difeldspar granites were formed from one protolith. This is because they contain similar virtually identical zircon relics as nucleus. In addition, none of the granitoids contain zircon crystals whose internal structure is similar to zircon crystals found in amphibolite. This suggests that the granitoids were not derived by melting of amphibolites. Most likely, amphibolites are relicts of the protolith that were not assimilated during granite formation. The occurrence of heterogeneous zircon crystals (relic zircon cores of the protolith) in the protolith of the various studied granitoids indicates that they formed from volcanic-sedimentary rocks. Apatites in plagiogranitoids and porphyry granite contain strontium of similar isotopic composition. Their 87Sr/86Sr isotopic ratio is 0.70680 in apatite granodiorite and 0.70822 in granite. A high ratio of 87Sr/86Sr = 0.77940 was measured for apatite from monazite-bearing granite, thus indicating a different source for its parent magma.

Adakite genesis and plate convergent process: Constraints from whole rock and mineral chemistry, Sr, Nd, Pb isotopic compositions and U-Pb ages of the Lakhshak magmatic suite, East Iran

The genesis of Eocene to Early Oligocene plutons and dykes emplaced in the suture zone between two colliding continental blocks in southeastern Iran has been the subject of debate concerning the processes that formed magmas and influenced their intrusive history. We report geological, geochemical, mineralogical and isotope evidence that constrains the origin of the Lakhshak plutonic complex. SiO 2 contents in the granodioritic pluton and dacite dykes average 67 wt%, and in the monzodiorite dykes 53 wt%. Based on (La/Yb) N vs. Yb N the rocks correspond to adakites, originating from 10 to 25% melting of garnet amphibolite. The high A1 2 O 3 (14.95–17.15 wt%), Sr (442–1385 ppm) and Ba (601–1935 ppm) contents and relatively high La/Yb (25–45) and Sr/Y (35–57), but low Yb (mostly <1.6 ppm) and Y (mostly <18.2 ppm) are also features typical of adakitic rocks. Zircon and titanite from two samples of the pluton yield U-Pb ages of 29.98 ± 0.08 and 29.90 ± 0.05 Ma, a monzodiorite dyke yields 29.02 ± 0.10 Ma, and a dacite dyke 29.39 to 27.96 Ma. These ages confirm that the magmatism was post-collisional. The monzodiorite and dacite dykes have ( 87 Sr/ 86 Sr) i = 0.7053–0.7056 and εNd i = −1.1 to 2.2 (except one sample with εNd i = −8.6), which limit the extent of crustal assimilation and plot along a mixing array between Sistan ophiolites and the country rocks of the pluton. This is consistent with an origin from oceanic slab melts variously mixed with melts from flysch sediments. Isotope compositions of Sr, Nd and Pb, xenocrystic zircons, and magma mixing and mingling textures support such a process. Thermobarometry on various minerals indicates a continuum of crystallization stages during magma ascent, with pyroxene formed at 8 kbar, followed by amphiboles, which record conditions decreasing from 1166 °C / 7.8 kbar to 933 °C / 4.0 kbar. Tectonically, the Lakhshak adakites were most likely the consequence of post-collisional delamination that led to melting of Sistan oceanic lithosphere and its interaction with melts from pelagic sediments. • Lakhshak suite includes a granodiorite pluton and dacitic and monzodioritic dykes. • Intruded at 30 to 28 Ma after closing of Sistan Ocean and continental collision • It corresponds to adakites, originating from 10 to 25% melting of garnet amphibolite. • Caused by delamination, melting of oceanic lithosphere and some melted sediments

ПЕТРО-ГЕОХИМИЯ СУБВУЛКАНИЧЕСКИХ И ЭКСТРУЗИВНЫХ ОБРАЗОВАНИЙ КЕДРОВО-КОРГОНЧИКОВСКОГО РАЙОНА ГОРНОГО АЛТАЯ

The paper provides data on petro-geochemistry, petrology and ore content of subvolcanic and extrusive formations of the Korgon Devonian complex of the Kedrovo-Korgonchikovsky district of the Gorny Altai. Low-alkali rhyodacites, sodium-potassium rhyodacites, trachyrhyodacites, trachyrhyolites and rhyolites are characterized. They are attributed to the aluminous series and mainly to magnesian varieties. Increased gold contents and decreased silver contents are observed in rocks. Ratios of light to heavy lanthanoids show a large range of values – from 2.1 to 26.2 and light ones to medium ones – from 1.94 to 11.8. The tetrad effect of W-type REE fractionation, varying from 0.29 to 0.68 and indicating an abundance of volatile components, including H2O, CO2, S or H2S, was revealed in rocks, that served as the formation of quartzites and argillizites. Volcanites contain signs of mantle formations and contamination by crustal material due to melting of greywackes and amphibolites of the lower crust. The ore content of the district is realized in the formation of promising epithermal gold-silver and copper-molybdenum-gold-porphyry occurrences. The district is thought to be the object of the process manifestation of the fluid mantle-crustal ore genesis during formation of largevolume epithermal gold-silver, copper-molybdenum-gold-porphyry mineralization.

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.

Data for paper entitled "Origin of the high conductivity anomalies in the mid-lower crust of the Tibetan Plateau: Dehydration melting of garnet amphibolites"

Data for paper entitled "Origin of the high conductivity anomalies in the mid-lower crust of the Tibetan Plateau: Dehydration melting of garnet amphibolites"

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 &amp;lt;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.

Ca. 110 Ma adakite-like magmatism along the Bangong–Nujiang suture zone: Implications for crustal thickening and early uplift in the central Tibetan Plateau

The suggested Early Cretaceous timing of early formation of the Tibetan Plateau remains controversial. This study investigates this uncertainty by examining newly identified Early Cretaceous adakite-like magmatic rocks in the Rutog and Nyima areas of the northern–central Tibetan Plateau. Zircon U Pb dating indicates that these rocks formed at ca. 110 Ma and have negative to positive zircon ε Hf (t) values. They contain high concentrations of Sr and have high Sr/Y values, but contain low concentrations of Y and Yb and have low (La/Yb) N , suggesting they have an adakite affinity. They also have ε Nd (t) values that range from −4.5 to +1.3. These geochemical and isotopic data indicate that the adakite-like rocks probably formed from magmas generated by the partial melting of garnet-bearing amphibolite within a thickened region of juvenile and ancient crustal material. This magmatism also most likely occurred during the Lhasa–Qiangtang collisional event. Crustal thicknesses along the Bangong–Nujiang suture zone (BNSZ) were > 40 km in places at ca. 110 Ma, and most likely had a significant impact on the initial uplift of the Tibetan Plateau. In summary, the Early Cretaceous adakite-like magmatic rocks in the Rutog and Nyima areas suggest that the Tibetan Plateau started to form during the Early Cretaceous. • Early Cretaceous adakite-like rocks occur in the north-central Tibet. • Adakite-like rocks were derived from the thickened crust. • The Lhasa–Qiangtang collision happened before ca. 110 Ma. • The significant crustal thickening led to the early uplift in the Early Cretaceous.

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.

Oligocene Leucogranites of the Gangdese Batholith, Southern Tibet: Fractional Crystallization of Felsic Melts from Juvenile Lower Crust

Abstract The formation of high-silica leucogranites and related detailed evolution of granitic crystal mush in southern Tibet bear significant information on the tectonic and magmatic evolution of the Asian–Indian continent–continent collisional zone. Here, we first report an integrated investigation of the Oligocene (ca. 30 Ma) leucogranites and main body granitoids exposed within the Gangdese Batholith in Gyaca County, southern Tibet. The Gyaca leucogranites can be divided into two groups in terms of field observation (gradational contacts vs. dykes), petrography and geochemistry (plagioclase fractionation vs. accumulation trends), and are characterized by their formation through different stages of evolution from (early) fractionation to (later) accumulation (up to 30%) of plagioclase for Group I and II leucogranites, respectively. Overall, the two groups of leucogranites are both characterized by high SiO2 (71.4–75.7 wt.%), Na2O/K2O (&amp;gt;1.0) and Sr/Y (58–629), and low Rb/Sr (0.02–0.27). The Gyaca main body granitoids resemble the published Gangdese granitoids and most of them also have high Na2O/K2O and they generally show varied SiO2 (64.4–76.1 wt.%) and other major and trace elements. The Gyaca leucogranites and main body granitoids have very similar Sr–Nd–Hf–O isotopic compositions, with initial 87Sr/86Sr ratios from 0.7054 to 0.7064, ɛNd(t) values from −3.40 to +0.65, zircon ɛHf(t) values from −3.0 to +5.2, and zircon δ18O values from 5.59‰ to 6.84‰. These leucogranites and main body granitoids are interpreted to have a same magma source and can be formed by water-present melting of garnet amphibolites from juvenile lower crust plus minor materials from felsic ancient crust beneath the southern Lhasa Terrane. The genetic association of the Oligocene Gyaca leucogranites and main body granitoids and their geochemical diversity reveal an evolved magmatic system. The two types of leucogranites are probably formed by crystal-melt fractionation and plagioclase accumulation at different stages during the solidification of the magma chamber. The discovery of ca. 30 Ma leucogranites in the Gangdese Batholith, in combination with the Oligocene–Miocene high Sr/Y Gangdese granitoids and coeval Himalayan leucogranites (HLGs), indicate the coexistence of diverse granitic rocks in southern Tibet may principally result from partial melting of local deep crustal materials. A new petrogenetic model which illustrates the evolution and multiple emplacements of crystal mush in a granitic magma chamber is proposed for the formation and magmatic evolution of leucogranites in melts from juvenile lower crust in Tibet.