Low HREE Research Articles

ِA new occurrence of rift-related damtjernite (ultramafic) lamprophyre, Gebel Anweiyib area, Arabian Nubian shield: Insights from bulk rock geochemistry and remote sensing data analysis

The occurrence of lamprophyre dikes is being reported first time in the Gebel Anweiyib area, south Eastern Desert, Egypt. The investigation of the ultramafic lamprophyre (damtjernite) dikes intruded into the crustal younger granites and metasediments based on their spectral characteristics, mineralogical and geochemical compositions. Three multispectral sensors of Landsat-8, ASTER and S2 have been used to detect and characterize the lithologic units and the lamprophyre dikes. The reflectance and absorption signatures of the granitic rocks and lamprophyre dikes were the basis for detecting them from the remotely sensed data using the application of advanced image processing algorithms including FCC, BR, PCA, MNF, and DS. Geochemical studies inferred that the damtjernite dikes are under-saturated with SiO2 (<38.89 wt%), alkaline with total alkalis (avg. 4 wt%), metaluminous with A/CNK (0.22–0.51), and ultramafic with MgO (13.88–19.23 wt%). They exhibit unique geochemical characteristics of mantle (with high MgO, Ni, Cr, and Mg#) coupled with crustal (high Sr, Ba, Zr, Ga and REEs) sources, similar to global ultramafic lamprophyres. High TiO2 and highly fractionated REEs with low HREEs (avg. 23 ppm) relative to LREEs (avg. 354 ppm) contents, yielding residual garnet in melts. Low partial melting of primitive garnet lherzolite can account the obtained REEs in the ultramafic lamprophyres. These features signify their derivation from a deeper enriched asthenospheric (plume-type) magma like OIB, which is attested by the high Nb/La (avg. 1.32), Nb/U (avg. 102), La/Yb (avg. 43), and low Y/Nb < 0.52, Rb/Sr < 1, Rb/Ba (<0.14), and Sr/Ba (<0.9), suggesting their derivation from a mantle-derived source rather than crustal rocks. High concentrations of TiO2 and Fe2O3 in the ultramafic lamprophyres, reflecting their formation from a fertile peridotite by mantle plume-derived melts with K-residual phlogopite rather than amphibole as obtained by low Ba/Rb and high Rb/Sr ratios. The Anweiyib lamprophyres are anorogenic and confined to being derived from rifting regions, and their magma is not contaminated with crustal sources during emplacement.

Deeply Derived Magma Controlling the Polymetallic Mineralization at Shuikoushan, South China: Constraints from Mineral U–Pb Dating and Whole-Rock Geochemistry

Various magmatic–hydrothermal activities have resulted in different styles of polymetallic mineralization in South China. Shuikoushan is a large Fe-Cu-Pb-Zn-Au-Ag orefield situated in fold-and-thrust belts within the South China Block. Two types of granodiorite have been identified in recent drilling work. The early-stage, coarse-grained granodiorite has developed magnetite-bearing skarns in the deep level. The late-stage, fine-grained granodiorite is associated with garnet-hematite–magnetite–pyrite–sphalerite–chalcopyrite-bearing skarns in its contact zone. Away from the garnet-bearing skarn are calcite–quartz–pyrite–sphalerite–galena veinlets in faulted breccia. Fieldwork has identified iron mineralization in both skarns, whereas copper mineralization was only discovered in the garnet-bearing skarns. Lead, zinc, gold, and silver mineralization were observed in the garnet-bearing skarns and faulted breccia. Zircon U–Pb analyses suggested the emplacement of two granodiorite at 167.8 ± 0.8 Ma (MSWD = 1.1, N = 31) and 163.6 ± 0.7 Ma (MSWD = 1.3, N = 32). Apatite and garnet U–Pb dating further indicated the magnetite-bearing skarns of 166.2 ± 1.9 Ma (MSWD = 4.5, N = 27), the hematite–magnetite–sulfide-bearing skarns of 158.6 ± 2.8 Ma (MSWD = 1.3, N = 34), and the calcite–quartz–sulfide veinlets of 159.5 ± 5.2 Ma (MSWD = 1.7, N = 24). The time–space relationship between the two intrusions and hydrothermal activities suggests that the fine-grained granodiorite is responsible for polymetallic mineralization. Whole-rock geochemistry analyses demonstrated the enrichment of LILEs and the depletion of Nb and Ta in two granodiorites, with a slight enrichment in LREEs and flat HREE patterns. These granodiorite bodies therefore belong to high-K calc-alkaline magma generated via the crust’s partial melting. The fine-grained granodiorite generally has a lower HREE and higher Dy/Yb, Sr/Y ratios than coarse-grained granodiorite, corresponding to the source of magma in garnet stable lower crust. The residual garnet keeps ferric iron in melts, leaving the fine-granodiorite more oxidized for copper and gold concentration. Through these analyses and our drilling work, a continuous skarn–hydrothermal–epithermal system has been identified for Cu-Pb-Zn-Au-Ag targeting in Shuikoushan.

Reworked Neoproterozoic sub-arc lithosphere beneath the Eastern Desert (Egypt): Evidence from geochemistry and geochronology of metabasites and granitoid gneisses, Gabal Um Gunud area

ABSTRACT High-grade granitoid gneisses (740–710 Ma) and elongate bodies of amphibolite and hornblende schist with cm-scale layers of garnet pyroxenite form an overturned fold in the Gabal Um Gunud area, South Eastern Desert of Egypt. Whole-rock geochemical data combined with zircon U-Pb-Hf isotope data suggest that the metabasites represent relics of oceanic crust with an N-MORB affinity, which has been derived from partial melting (2.0–1.4 GPa; ~65- ~ 45 km depth) of a depleted mantle source beneath an intra-oceanic, spreading forearc basin. Such increment of melting degree resulted in small basaltic melt batches with transitional tholeiitic to boninitic affinities. Progressive slab subduction and partial melting of the tholeiitic amphibolite produced high-Al and low HREE melts for trondhjemite at av. T = 854°C and low-Al and high HREE melts for tonalite at higher temperatures (av. 949°C). The low Mg# of the trondhjemite-tonalite rocks may be attributed to limited contamination of subduction components, as evidenced by the weak lanthanide tetrad effects (TE1,3~1) and near-chondritic Zr/Hf, Nb/Ta, and Y/Ho ratios, while the garnet pyroxenite was the residuum produced in equilibrium with the trondhjemite-tonalite melts.

Late Mesozoic Adakite Granites in the Northern Framing of the Eastern Flank of the Mongol–Okhotsk Orogenic Belt

The examination of original and available petrochemical, geochemical, and isotope (Sm-Nd, Rb-Sr) data showed that subalkaline granitoids in the northern framing of the eastern flank of the Mongol–Okhotsk orogenic belt belong to the adakite series. The granitoids are characterized by high Sr/Y, Sr, Al2O3, and LREE, extremely low HREE concentrations, and the absence of negative Eu anomalies. The results obtained and the analysis of the geodynamic setting of their formation made it possible to assume that the granitoids were formed at a depth of more than 45 km through melting of garnet-bearing (20–50% garnet) rocks, which are likely represented by lower crustal Precambrian complexes widespread in the southern framing of the Siberian craton. The source of parental melts involved both mantle and crustal matter. This likely occurred in a subduction setting through melting of frontal or lateral parts of oceanic slab in subduction “windows”. The Late Jurassic–Early Cretaceous Chubachin adakite complex (149–138 Ma) was distinguished. Its formation preceded the initiation of suprasubduction differentiated calc-alkaline magmatism (140–122 Ma) in the northern framing of the eastern flank of the Mongol–Okhotsk orogenic belt.

Monazite geochronology and geochemistry constraints on the formation of the giant Zhengchong Li-Rb-Cs deposit in South China

The giant Zhengchong Li-Rb-Cs deposit (ore resource: 270 k tonnes (t) Li2O @ 0.557 wt%, 110 kt Rb2O @ 0.225 wt%, and 6 kt Cs2O @ 0.013 wt%) in South China is associated with quartz-zinnwaldite-topaz rock, which is composed mainly of quartz, zinnwaldite and topaz with a porphyritic texture. However, the genetic link between these rocks and their Jiuyishan granite batholith host is unclear, and the magmatic-hydrothermal process involved in the rare-metal mineralization remains enigmatic. In this study, we analyzed the in-situ U-Pb age and mineral chemistry of monazite, a ubiquitous accessory phase at Zhengchong, to reconstruct the precise geochronological framework of this highly-fractionated felsic magmatic system, and the origin of the Li-Rb-Cs-rich quartz-zinnwaldite-topaz rock. Based on petrographic observations and back-scattered electron (BSE) imaging, monazite crystals in different rocks at Zhengchong have the following characteristics: (1) Mnz-I is mainly BSE-zoned monazite disseminated in granite porphyry, coexisting with zircon or included in zinnwaldite, (2) Mnz-II consists of relatively fine-grained BSE-zoned or homogeneous monazite distributed in feldspar phenocryst-bearing quartz-zinnwaldite-topaz rock, (3) Mnz-III is BSE-zoned monazite with embayed texture in the core, distributed in coarse-grained quartz-zinnwaldite-topaz rock, and (4) Mnz-IV is homogenous, BSE-gray monazite from fine-grained porphyritic quartz-zinnwaldite-topaz rock (as the main orebodies). All monazite crystals are characterized by high Th content, indicating magmatic origin. The LA-ICP-MS monazite U-Pb ages indicate granitic magmatism and mineralization cluster around 150 Ma, implying a close genetic link between the granite porphyry host and the quartz-zinnwaldite-topaz rock. We compared the chondrite-normalized REE distribution patterns and principal component analysis and partial least squares-discriminant analysis of the monazite analyzed, and suggested that the BSE-zoned Mnz-I, II and III are likely of the same type, significantly different from the homogenous Mnz-IV. The gray rims (under BSE) of Mnz-I, II and III are similar to Mnz-IV, and differ markedly from the bright core (under BSE) in their lower HREE and ThO2, but higher Eu and Eu/Eu* contents. When combined with the whole-rock composition data, the monazite compositional variation cannot be attributed to rapid changes in magma composition, depressurization, and increased water content, but rather to the increasing oxygen fugacity and the decreasing temperature and Si activity, accompanied by the elevated fractionation of granitic magma. Thus, the variation in monazite composition reflects the formation of Zhengchong Li-Rb-Cs deposit, which may be the result of high magmatic differentiation of fluorine-rich A-type magmas. Although multistage rare metal metallogenic events have been identified in the Nanling region, it is suggested that the Late Jurassic Zhengchong Li-Rb-Cs mineralization occurred within a short duration (<1 million years). Overall, the monazite chemical and geochronological signatures are good pathfinders for magmatic evolution and Li-Rb-Cs mineralization in the Jiuyishan complex, which could be used as a potential prospecting tool for highly-evolved granite-related rare metal deposits.

The effect of crystal fractionation on the geochemical composition of syn-exhumation magmas: Implication for the formation of high δ56Fe granites in collisional orogens

Syn-exhumation magmatism in collisional orogens is an important process for crustal differentiation and crust-mantle interaction at convergent plate boundaries. It is intriguing to elucidate which factors control the geochemical composition of such magmatic products. To answer this question, a combined study of whole-rock major-trace elements and Nd-Fe isotopes, mineral O isotopes, and zircon U–Pb ages and trace elements was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure (UHP) metamorphic belt in east-central China. Several granitic plutons of Triassic age were newly found in this typical collisional orogen, with a total exposure area of ∼10 km2, suggesting large-scale partial melting of the deeply subducted continental crust in the terminal stage of continental collision. Field observations indicate that the massive granites show little deformation. The granites are high-K calc-alkaline and exhibit enrichment in LILE and LREE but depletion in HFSE relative to HREE in trace element distribution patterns. Zircons in these granites contain relict magmatic cores characterized by steep HREE patterns with strong negative Eu anomalies. These cores show middle Neoproterozoic U-Pb ages of 724–779 Ma, consistent with the protolith age of UHP metaigneous rocks in the Dabie-Sulu orogenic belt. Zircon rims exhibit oscillatory zoning, and are characterized by Triassic U-Pb ages of 210–216 Ma, low LREE contents and steep HREE patterns with significant negative Eu anomalies, suggesting their growth from granitic magmas during exhumation of the deeply subducted continental crust. The granites have low δ18O values of 2.0–6.1‰ for quartz, 1.1–5.2‰ for whole-rock, and 0.9–3.2‰ for zircon cores. The low δ18O values and Neoproterozoic U-Pb ages of zircon cores in the granites are characteristic feature of the deeply subducted continental crust in the northern margin of the South China Block. One coesite inclusion was found in the anatectic zircon rim, pointing to the deep origin of anatectic melts at a subarc depth of >80 km. Whole-rock SiO2 contents are correlated with major and trace elements, which are ascribed to crystal fractionation of mainly biotite and plagioclase during magma evolution. In particular, these granites exhibit highly variable δ56Fe values from 0.05 to 0.30‰, which are correlated with Fe3+/ΣFe, SiO2, Al2O3, Nb/Ta and Eu/Eu*. This suggests that the crystal fractionation of biotite would have controlled the Fe isotope variation in the granites. Therefore, the syn-exhumation granites experienced significant geochemical differentiation mainly through the fractional crystallization during the magma evolution. A compilation of syn-exhumation granites in the typical collisional orogens of the world shows that such granites were mainly formed through dehydration melting of hydrous minerals during decompressional exhumation. They fall into the magnesian group with relatively low REE and HFSE (like Ti, Nb and Zr) contents, distinct from the geochemical compositions of A-type granites. Therefore, syn-exhumation granites record crustal differentiation in the terminal stage of continental collision.

REE and cathodoluminescence features of scheelites from the Tongshankou Cu–Mo deposit in Eastern China

The Tongshankou porphyry-/skarn-type Cu–Mo deposit is located in the Edong ore district, the largest district of the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in Eastern China. From the ore-causative granodiorite porphyry and skarn at Tongshankou, three types of scheelite are recognized (Sch1 to Sch3): Sch1-subhedral–anhedral, occurring in the chalcopyrite ores within the porphyry; Sch2-granular, occurring inside garnet in the endoskarn and Sch3-subhedral–anhedral, occurring in calcite veins in the exoskarn. Sch2 and Sch3 have different internal texture according to CL images: Sch2-Dark core and bright rim, with the heterogeneous dark core; Sch3-Dark core and narrow dark rim, showing patches of different brightness in core. In addition, three REE types have been differentiated: 1) REE-Type1: LREE-enriched, HREE-depleted, and negative Eu anomalies patterns (Sch1), which is related to the porphyry system; 2) REE-Type2: No apparent fractionation between LREE and HREE, with positive Eu anomalies (Sch2), which is related to the endoskarn; 3) REE-Type3: Higher LREE and lower HREE with extremely positive Eu anomalies (Sch3), which is related to the exoskarn. The negative MoO3 vs. WO3 correlation for the Tongshankou scheelites suggests Mo-W substitution in the scheelite crystal lattice. No humped pattern (MREEs-enriched) of REE, lower Na content and the lack of correlation between Na and REE at Tongshankou excluded the Na substitute into the Ca site. The positive correlation with Nb and REE for Sch 1 and 3, suggests equation [2] and [3] should be more plausible mechanism to generate the Tongshankou scheelite REE patterns.

Generation of collision-induced Early to Middle Miocene adakitic magmas in Pertek (Tunceli) area from Eastern Anatolia postsubductional setting, Turkey

Early to Middle Miocene andesite-dacite porphyries are well exposed to the Pertek area of Tunceli, Eastern Anatolia, and represent an example of adakite-like magma generation in Eastern Anatolia (postsubductional) collisional setting. Mineral associations in these porphyries are composed of plagioclase (oligoclase-andesine-labradorite), amphibole (pargasite-ferropargasite), biotite, rare quartz, K-feldspar, and minor Fe-Ti oxides. Geochemically they are high-K calc-alkaline in nature and characterized by high SiO2 (>62 wt.%), Al2O3 (mostly >16 wt.%), Na2O/K2O ratio (1.3-1.7), and Sr (generally >400 ppm) contents. Volcanic rocks display depletion in HFSEs, Nb, Ta, and Ti, and slight negative Eu anomaly; have low HREEs, Y (

Geodynamic setting and ore formation of the Younusisayi thorium deposit in the Altyn orogenic belt, NW China

Thorium deposit related to alkaline/peralkaline magmatism is the dominant thorium source. Its ore-forming thorium mechanism has long been fascinating. The Younusisayi Th deposit, hosted in early Paleozoic alkali-feldspar granite in the South Altyn Complex, NW China, is an example to explore the Th mineralization processes in magmatic-hydrothermal systems. Zircon U-Pb dating shows that the ore-hosting alkali-feldspar granites are emplaced at 445.7–439.0 ± 1.3 Ma. The apatite that occurs concomitantly with thorite in the mineralized alkali-feldspar granites formed at are 442.0–438.0 ± 14 Ma, indicating Th mineralization temporally related to the magmatism. Four populations of fluorite exhibit distinct occurrences, the fluorite Ia and Ib are in intergrowth with thorite, but the fluorite IIa and IIb are not. Notably, four populations of fluorite are differentiated by their Th + U and HREE concentrations, fluorite Ia and Ib show higher Th + U contents and lower HREE contents than fluorite IIa and IIb. Th was extracted by magma during emplacement. As a strong incompatible element, thorium will not be enriched in rock-forming minerals. In the process of magmatic evolution, thorium is gradually enriched in the post-magmatic fluids which contains abundant volatile components. Thorium can be transported as F-Th complexes in F-rich fluids. As temperature decreased, thorium precipitated in the form of thorite, and fluorite Ia and Ib crystallized with a low HREE content. Fluorine from the decomposition of F-Th complexes, reacts with HREE in the fluids to form F-HREE complexes, which increases the HREE content in the fluid. Therefore, the fluorite IIa and IIb which formed in the late stage of mineralization have obviously higher contents of HREE. We propose that the Younusisayi deposit, with an average grade of 0.4 wt% ThO2, formed in a post-collisional extensional environment during the early Paleozoic. The Younusisayi deposit is best classified as a magmatic-hydrothermal deposit. Our results demonstrate the usefulness of geochemistry of fluorite for tracing the ore-forming process, thus establishing the ore formation model.

Petrography and geochemistry of the granitoids and associated volcanic rocks of the northern part of Kushaka and Birnin Gwari schist belts, NW Nigeria: implications for provenance and geological setting

The granitoids and the associated volcanic rocks of the northern part of Kushaka and Birnin Gwari schist belts were emplaced in the ca. 3.5 – 1. 0 Ga remobilized basement complex terrain composed of metasedimentary and metaigneous rocks that later underwent medium- to high-grade metamorphism during the Pan-African thermo-tectonic event. They comprise dominantly of diorite, granodiorite, granite, granite gneiss and basalt, and are product of metasomatism and injections. The diorite and granodiorite occur as paleosome and the granite as leucosome with the development of high temperature minerals, locally attaining granulite facie metamorphism. Plagioclase, biotite, hornblende, pyroxene and olivine fractionation played an important role during their genesis through fractional crystallization of basaltic magma and partial melting of older dioritic-granodioritic source rock in the deep crust which were themselves ultimately derived through fusion of mantle materials contaminated by continental crust and enriched by fluids derived from oceanic crust in an arc setting. Geochemical characteristics have revealed different chemical trends in granitoids and basalts. The granitoids are calc-alkaline, ferroan and magnesian, metaluminous and peraluminous in character. They also exhibit I- and S-type signatures with enrichment in LILE, radioelements (Th and U), depletion in Nb, Sr, P and Ti, high LREE fractionation factors (La/Yb) (1.05 to 77.20), and pronounced negative Eu anomalies (Eu/Eu* = 0.34 to 1.10). Similar patterns of spidergrams show that the rocks are genetically related and were emplaced in a volcanic arc and syn-collisional setting. The basalt is tholeiitic, metaluminous and high in Fe and Mg with relative enrichment in LILE, HFSE, low and near flat LREE and HREE, low fractionation [(La/Yb)N = 1.4] with Eu/Eu* value of 1.10. It is evidently a back arc cum mid-ocean ridge (MORB) basalt. The consistent decrease in the content of MgO, Fe2O3 MnO, CaO, Sc, Cr and V of the basalt, diorites, granodiorites, and granites indicates continuous igneous crystallization process. It seems that extrusion of basaltic magmas from the sub-circular Kushaka Complex derived from subduction of oceanic crust resulted in complete change in the genesis of the magmas at the time, in this region. The granitoids and the basalt may have formed behind subducted Pan-African plate due to effects of compressional and tensional forces caused by oceanic plate roll-back which resulted to a zone of extension, parallel to the island arc. The granitoids present similar chemical characteristics to those in the other areas underlain by the basement complex and schist belts in the north and eastern parts of the Pan-African mobile belt, while basalts are similar to ophiolites and amphibolites in other schist belts of Nigeria forming a lateral continuation of the same mobile belt.Â

Geochemistry, origin and anatexis temperature of monzogranite formation in Mount Khalaj (Mashhad, Iran)

Geochemistry, origin and anatexis temperature of monzogranite formation in Mount Khalaj (Mashhad, Iran)

Tectonic evolution of the Paleoarchean to Mesoarchean Badampahar-Gorumahisani belt, Singhbhum craton, India – Implications for coexisting arc and plume signatures in a granite-greenstone terrain

An association of tonalite-trondhjemite-granodiorite (TTG), other granitoids and greenstones constitute the Paleoarchean-Mesoarchean Singhbhum craton in India. The deformed and metamorphosed greenstone succession of Badampahar-Gorumahisani belt (BGB) in eastern Singhbhum consists of mafic–ultramafic volcanics, intercalated chemical sediments in the lower part and clastic sediments in the upper part. Geology, structure and geochemistry of BGB mafic volcanics, contemporaneous TTG and slightly younger intrusive granite have been integrated to constrain the tectonic setting of Paleoarchean felsic crust generation and coupled deformation of granite-greenstone. Subalkaline, low-K BGB metabasalts show distinct tholeiitic trend of differentiation from komatiitic basalt parent magma. BGB metabasalts with LILE enrichment, negative Nb anomaly, and Ti/V ratio in the range 10–20, are comparable to island arc tholeiites, though the associated komatiites and high-Mg basalts suggest plume related origin. Limited crustal contamination of a komatiitic basalt magma cannot be ruled out altogether. Singhbhum granite batholith with older tonalite-trondhjemite and younger granite-granodiorite plutons were derived from low- to high-K mafic source, together with some TTG reworking for younger granites. With low HREE, higher primordial mantle normalized La/Yb ratio, insignificant Eu anomaly, and coupled depletion of Y and enrichment of Sr, older strongly deformed plutons compare well with the Archean TTG suite. The younger intrusive granites show weak positive Eu anomaly (Eu/Eu* ~ 1.3–1.7) and strongly positive Y anomaly. Geochemical discrimination (e.g. Rb vs Y + Nb) suggests comparison with volcanic arc granites, which is also supported by enrichment of LILE (K, Rb and Ba) and LREE, and relative depletion of HREE and HFSE. However, structural geometry of BGB and adjoining granitoid plutons, association with komatiite, absence of typical signatures of derivation from subduction wedge, and heterogeneity of source for the BGB mafic volcanics, suggest Paleoarchean crustal accretion by mantle plume activity as well as local shallow subduction or subcretion under a transitional tectonic regime.

Garnet Geochemistry of Reduced Skarn System: Implications for Fluid Evolution and Skarn Formation of the Zhuxiling W (Mo) Deposit, China

A newly discovered tungsten ore district containing more than 300,000 tons of WO3 in southern Anhui Province has attracted great attention. The Zhuxiling W (Mo) deposit in the district is dominated by skarn tungsten mineralization. This paper conducted in suit EPMA and LA-ICPMS spot and mapping analysis of the skarn mineral garnet to reveal the evolution of fluids, metasomatic dynamics, and formation conditions of skarn. Two generations of garnet have been identified for Zhuxiling W (Mo) skarn: 1) Gt-I generation garnet is isotropic, Al-rich grossular without zoning. As a further subdivision, Gt-IB garnet (Adr19-46Grs49-77 (Sps+Pyr+Alm)4-5) contains significantly high content of Ti and Mn compared with Gt-IA garnet (Adr3-42Grs53-96 (Sps+Pyr+Alm)1-5). 2) Gt-II generation garnet is anisotropic, Fe-rich andradite with oscillatory zoning. Gt-II garnet displays compositional changes with a decrease of Fe and an increase of Mn from proximal skarn (Gt-IIA) to distal skarn (Gt-IIB) with the presence of subcalcic garnet for Gt-IIB type (Sps+Pyr+Alm = 56–68). The presence of pyrrhotite associated with subcalcic garnet indicates a relatively reduced skarn system. Gt-I grossular is overall enriched in Cr, Zr, Y, Nb, and Ta compared with the Gt-II andradite, and both W and Sn strongly favor Fe-rich garnet compared with Al-rich garnet. Gt-IA grossular garnet presents a REE trend with an upward-facing parabola peaking at Pr and Nd in contrast to low and flat HREE, and Gt-IB grossular garnet has a distinct REE pattern with enriched HREE. Gt-IIA andradite garnet displays a right-dipping REE pattern (enriched LREE and depleted HREE) with a prominent positive Eu anomaly (Eu/Eu* = 3.6–15.3). In contrast, Gt-IIB andradite garnet shows depleted LREE and enriched HREE with a weak positive Eu anomaly (Eu/Eu* = 0–6.0). The incorporation and fractionation of REE in garnet are collectively controlled by crystal chemistry and extrinsic factors, such as P–T–X conditions of fluids, fluid/rock ratios, and mineral growth kinetics. Major and trace elements of two generations of garnet combined with optical and textural characteristics suggest that Gt-I Al-rich grossular garnets grow slowly through diffusive metasomatism under a closed system, whereas Gt-II Fe-rich andradite represent rapid growth garnet formed by the infiltration metasomatism of magmatic fluids in an open system. The Mn-rich garnet implies active fluid–rock interaction with Mn-rich dolomitic limestone of the Lantian Group in the district.

Diagenetic history and provenance of Devonian terrestrial sandstones at the margin of Gondwana: Padeha Formation, Eastern Alborz, Iran

• The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean. • The Padeha sandstones were sourced by the late Neoproterozoic granitic gneisses of Iran. • Further mesodiagenesis was terminated by the Eocimmerian collision. The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean, the details of which are obscured by later collision. A record of the early phase of opening of Paleotethys is preserved in the 600-m thick Padeha Formation, a Middle Devonian terrestrial rift basin succession. The sandstones are quartz arenite, sub-arkose and arkose and contain minimal detrital matrix (~1%). Bulk-rock geochemistry, according to major elements, suggests a source to the sandstones that was dominated by I-type granite, but also, based on trace elements, included small amounts of intermediate or mafic rocks. Low Zr and HREE contents suggest little or no sandstone in the source area and thus low abundance of fine-grained heavy minerals (<1%). The sandstones were sourced by the widespread granitic gneisses of central and northern Iran that formed at the northern margin of Gondwana in the late Neoproterozoic–Ediacarian. Diagenesis of the sandstones is dominated by compaction and fracturing of framework quartz without widespread stylolites in the sandstones, and cementation by silica, carbonates and Fe-oxides. Fractures within quartz grains are filled mostly by silica, but locally by carbonate and authigenic K-feldspar. Albitization of plagioclase, K-feldspar overgrowths and partial dissolution of detrital K-feldspar post-date silica cements and suggest temperatures between 70 and 120 °C. The particular diagenetic assemblage of altered K-feldspar and limited quartz stylolites was a consequence of a high geothermal gradient of ~70 °C/km at a burial depth of <1.5 km. Further mesodiagenesis was terminated by the Eocimmerian collision.

Petrogenesis of Archean TTGs and potassic granites in the southern Yangtze Block: Constraints on the early formation of the Yangtze Block

The southern Yangtze is underlain by Mesoarchean igneous rocks that were emplaced in two pulses, including trondhjemites at ca. 3092–3071 Ma followed by potassic granites at 2920 Ma. These represent the oldest TTG suites identified in the southern Yangtze and the oldest potassic granites in the entire Yangtze Block. Hf isotopic analysis reveals εHf(t) values of −2.49–7.49 and Hf crustal model ages (TDMC) of 3601–3000 Ma for zircons from the trondhjemites, whereas those from the potassic granites show εHf(t) values of −1.79–4.18 and TDMC ages of 3453–3043 Ma. The trondhjemites have high SiO2, Mg# and low Al2O3 and K2O/Na2O, and are highly evolved with low Ni, Cr and Co. The trondhjemites are characterized by low Sr/Y, Eu/Gd and (La/Yb)N, and high Y and HREE, typical of high-HREE and low-pressure TTGs. Their geochemical and isotopic compositions indicate that they are derived from melting of juvenile mafic crust with little contribution from older crustal materials, consistent with a plagioclase-bearing basaltic source. The multielement patterns of potassic granites are similar to that of the trondhjemites, albeit with lower HREE and Y contents. Their high K2O values, strongly fractionated REE patterns and low Y contents indicate the presence of garnet and/or amphibole as residual phases in the source region, which indicates they formed at higher pressure than the trondhjemites. The isotopic and geochemical data imply that the formation of the potassic granites could be related to reworking of older TTGs. The increasing K2O/Na2O of the 3092–3071 Ma trondhjemites to potassic 2920 Ma granites corresponds with stabilization, maturation and thickening of the Archean continental crust in the southern Yangtze. The geochemical data of the TTGs in the Kunming region show important differences from that of contemporaneous TTGs from the Kongling Complex, suggesting that southern Yangtze is composed by an independent old nucleus, which is exotic to the northern Yangtze. A synthesis of petrogenesis, geochronology, geochemistry and isotope data of Archean granites of the Yangtze Block provides important insights into the architecture of its Archean crust. We propose that the Yangtze Block can be subdivided into discrete northwestern, northeastern and southern domains, but which are all affected by late Paleoproterozoic tectonothermal events, suggesting that they had been assembled together by then to form the united Yangtze Block.

Petrogenesis of the Late Triassic Sanshilipu monzogranites in North Qinling orogenic belt

The Sanshilipu monzogranites is located in the northern margin of the South Qinling Block, with an outcrop of about 10 km2, and as an intrusion into Palaeozoic Danfeng ophiolitic mélange. The samples consist mainly of plagioclase, K‐feldspar, quartz, biotite, and minor titanite, apatite, magnetite, with contents of SiO2 (61.73–68.49%), Al2O3 (14.58–16.61%), K2O + Na2O (5.79–8.87%), MgO (1.07–2.91%), and low TiO2 (0.55–0.83%), similar to those of I‐type granite. They also show high concentrations of LREE, Pb, and Sr, slightly negative Eu, but low HREE, Nb, Ta, and Y. High Sr and Sr/Y, low Y characteristics are similar to those of adakite. High precision LA‐ICP‐MS U–Pb dating and trace element analysis of zircon in the Sanshilipu granite yielded a concordant age of 205 ± 1 Ma. This age is apparently younger than the closure age of the Mian‐Lue Ocean (242–221 Ma). We propose that the Sanshilipu monzogranites formed by magma mixing of thickened lower crust and limited mantle‐derived mafic magma during post‐collision of the South China and North China blocks.

1860-Ma I-Shaped Mafic Sills in the Murmansk Craton, Eastern Fennoscandia: Petrology and Tectonic Setting of Within-plate Mafic Events during Nuna Assembly

1860-Ma within-plate mafic rocks of the Eastern Murman Sill Province were studied in the northern part of the Murmansk Craton, where they compose sheet bodies (sills) up to 100 m thick in Archean granitoids and gneisses. The sills have a simple inner structure, show no macroscopic signs of in situ differentiation even in large bodies, and consist of homogenous medium-grained poikilophitic dolerites weakly varying in composition. Minor variations in mineral and chemical composition of the rocks allow us to regard the mafic sills as I-shaped sills. Their homogeneity is likely caused by the rapid crystallization of aphyric magmas. The studied sills are close to N-MORB in major-element composition, but significantly differ in trace element signatures. The low Cr and Ni concentrations in the dolerites indicate the high degrees of fractional crystallization, while the low HREE and HFSE contents suggest that primary komatiite melts were formed through the high-degree melting of a depleted mantle source. The absence of phenocrysts in chilled dolerites prevented the use mineralogic and petrographic criteria for estimating the crystallization conditions in intermediate magma chambers. AlphaMELTS 1.9 and PRIMELT3 MEGA modelling results indicate that the primary melts could be formed through a high-degree melting of ascending high-temperature mantle plume, with subsequent contamination and fractional crystallization in mid-crustal intermediate magma chambers. The Nd and Sr isotopic composition of the dolerites and constituent minerals suggests that the crustal contamination included several stages. The first, most significant crustal contribution (up to 10%) was related to the contamination by Archean tonalites in the intermediate mid-crustal chambers. This provided negative eNd values and radiogenic Sr isotopic composition (Sri = 0.702–0.704) in the dolerites. The second stage included the upper-crustal contamination during lateral propagation of melts along gentle weakened zones, which resulted in the wide variations of Sr-Nd isotopic composition in rocks and minerals. The third stage was related to the contamination by 87Sr-enriched fluid that is supported by the sharp increase of Sri (up to 0.708) at constant Nd isotopic composition in gabbro-pegmatites. Such fluid could be formed via biotite dehydration caused by heating of host granites by large volume of emplaced mafic melts. The age, volume, and composition of the studied dolerites and similarity with coeval mafic rocks in the Superior Craton indicate that the I-shaped mafic sills in the Murmansk Craton could be formed on the periphery of the Circum Superior large igneous province. It suggests that the Murmansk Craton of the Fennoscandian Shield and the Superior Craton of the Canadian Shield at 1860 Ma were joined into a single consolidated lithospheric block that host within-plate mafic rocks and likely represented an early core of the Paleoproterozoic Nuna supercontinent.

Geochemistry and zircon trace elements composition of the Miocene ore‐bearing biotite monzogranite porphyry in the Demingding porphyry Cu‐Mo deposit, Tibet: Petrogenesis and implication for magma fertility

Demingding is a poorly studied post‐collisional porphyry Cu‐Mo deposit, located in the eastern part of the Gangdese porphyry copper belt. In this study, we present LA‐ICP‐MS zircon U‐Pb dating, whole‐rock geochemistry, and zircon trace elemental data for the mineralization related biotite monzogranite porphyry in Demingding. Zircon U‐Pb dating shows that the weighted mean 206Pb/238U ages of the Miocene biotite monzogranite porphyry is 20.36 ± 0.46 Ma (MSWD = 2.6, n = 21). The Miocene biotite monzogranite porphyry is characterized by high SiO2, K2O, and Al2O3 contents and shows adakite‐like signatures. These rocks have high Sr/Y ratios, fractionated REE patterns with low HREE abundances, enrichment in LILE compared with HFSE, and have (87Sr/86Sr)i values of 0.7059 to 0.7062, εNd(t) values of −2.35 to −1.67 and (206Pb/204Pb)i ratios of 18.50–18.55. These features are similar to other Miocene adakite‐like intrusions in the Gangdese belt. We propose that the biotite monzogranite porphyry shares the same petrogenesis with other Miocene ore‐bearing adakite‐like intrusions in the Eastern Gangdese belt and was derived from remelting of the subduction‐modified, eclogitized Tibetan lower crust and mixing with hydrous mafic magmas from metasomatized Tibetan mantle. The zircons from biotite monzogranite porphyry have only minimal negative Eu anomalies (EuN/EuN* &gt; 0.3), and exhibit large ratios of Ce4+/Ce3+(average value of 113), 10,000*(EuN/EuN*)/Y (3.94–8.14, &gt;1), Ce/Nd (9–58, Average value of 30), (Ce/Nd)/Y (0.008–0.127, &gt;0.003), and low value of Dy/Yb (0.15–0.24, &lt;0.3), indicating that the biotite monzogranite porphyry is relatively hydrous and oxidized. The biotite monzogranite porphyry, therefore, is believed to be a relatively fertile intrusion with certain exploration potential.

Water-rock interactions in the Acoculco geothermal system, eastern Mexico: Insights from paragenesis and elemental mass-balance

Geochemical mass balances, supported by mineral characterization and whole-rock elemental analyses, were used to decipher the alteration paragenesis and to understand the elemental mobility related to the interaction of hydrothermal fluids with volcanic rocks (ignimbrites and lavas ranging from rhyolite to dacite) from the Acoculco geothermal system, in the eastern portion of the Trans-Mexican Volcanic Belt. Three extensive events of hydrothermal alteration were distinguished, from earliest to latest: (1) selective silicification and argillization, (2) hydraulic brecciation by silica-rich fluid, and (3) acid-sulfate (vuggy and argillic advanced) alteration. According to the Gresens-Grant mass-balance method, the altered volcanic rocks of Acoculco are generally enriched in Ti and Si and depleted in the rest of major elements; this chemical behavior is striking in the case of vuggy and silicic altered rocks. Small changes in some element concentrations calculated by the Gresens-Grant method may be due to chemical heterogeneity between the rock chosen to be the reference (“fresh rocks”) and the protolith of the altered ones. The chondrite-normalized REE gave different patterns for distinct types of alteration; vuggy and silicified rocks present low LREE and high HREE in opposition to the advanced argillic alteration. Boiling, primarily related to the last alteration event, may have happened at variable depths (825, 325 and 102 m below present surface); it produced changes fluid composition, resulting in acidic pH surface alterations.

Geology and geochemistry of Gunung Subang gold deposit, Tanggeung, Cianjur, West Java, Indonesia

The Gunung Subang gold deposit is located in the Sunda arc volcanic belt in West Java, Indonesia. The ore bodies are associated with volcanic rocks from the Miocene Koleberes and Bentang Formation. To constrain the significance of the ore genesis and mineralization, this study was carried out with the aim of examining the deposit using methods such as detailed field observation, petrography, petrochemistry, zircon U-Pb dating, Hf isotope, fluid inclusion analysis, and sulfur and in-situ Pb isotope analyses. The results suggest the following. (1) The volcanic rocks in Gunung Subang belong to the basalt-andesite group, with 48.66–79.17% (average 61.04%) SiO2. The trace element patterns of the volcanic rocks exhibit relatively low HREE content, flat HREE patterns, and weak negative or positive Eu anomalies, which are similar to those of typical arc magmatic rocks. (2) Most zircons selected from the volcanic rocks are magmatic, and only a few are inherited. The magmatic zircons have a weighted mean U-Pb age of 17.0 ± 0.4 Ma, and the inherited zircons are from the Precambrian, dated at 887–2379 Ma. The magmatic zircons have εHf(t) values ranging from −1.24 to 13.59, indicating that the magmatism may be contaminated, predominantly with juvenile crust but also with a small percentage of ancient crust. (3) The sulfur isotopic compositions are fairly uniform, and the δ34S values of pyrite, galena, and sphalerite range from 0.53 to 2.97‰, suggesting a magmatic origin. (4) The Pb isotopic compositions of pyrite and galena are clustered, and some include relatively high amounts of radiogenic lead. The mean ratios of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb are 18.59 ± 0.007, 15.65 ± 0.007 and 38.89 ± 0.017, respectively, indicating that the ore-forming material is highly related to Miocene to Pliocene volcanic rocks. (5) Fluid inclusions in the quartz from the ores are liquid and vapor-liquid phases, with components of the NaCl-H2O system with homogenization temperatures of 240–320 °C and salinities of 0.2–6.2 wt% NaCl. The coexistence of vapor and liquid-rich fluid inclusions suggests that boiling occurred throughout the deposition of the ore. (6) The results of Electron Probe Microanalysis (EPMA) indicate that pyrite is the major Au-bearing mineral. Combined with the observation of scanning electron microscope (SEM), gold is primarily present as Au-Ag-telluride minerals, and there may also be native gold (Au0). Finally, we consider the Gunung Subang is an epithermal gold deposit related to the Mid-Miocene magmatism.