High HREE Research Articles

Diversity of Siderian magmatic pulses revealed by intracrustal melting of oceanic crust in the Mineiro Belt, southern São Francisco Craton

The early Paleoproterozoic Era has been largely recognized as a period of global magmatic slowdown (∼2.3–2.2 Ga). The Lagoa Dourada Suite, located in the Mineiro Belt – southern São Francisco Craton - Brazil, is a juvenile high‑aluminum 2350 Ma TTG suite and spatially associated with a network of dykes that received little attention in the literature. Based on field features, petrography, whole-rock geochemistry, U-Th-Pb, and Hf isotopes in zircons, we propose that these dykes represent three different magmatic pulses produced via intracrustal melting of oceanic crust. Pulse 1 (<2347 Ma) is syn-plutonic with low-SiO2, high-Mg# and HREE, moderate [La/Yb]N, and low Ba+Sr contents. Pulse 2 (ca. <2330 Ma) shows high-SiO2, low-Mg# and HREE, moderate [La/Yb]N, and high Ba+Sr. Pulse 3 (ca. <2325 Ma) has intermediate-SiO2, low-Mg#, high HREE, moderate to high [La/Yb]N, and high Ba+Sr values. Their Hf compositions are broadly similar to those of the host rock, with positive εHf(t) values and the Hf-TDM model ages ranging from 2.65 to 2.46 Ga, attesting to their juvenile nature. These characteristics, coupled with their homogeneous Hf signature (176Hf/177Hf = 0.281382 to 0.281481), suggest that all these magmatic pulses came from a similar source. However, the whole-rock chemical diversity of the dykes and host rock suggests chemical heterogeneities within this source rock and/or the different magmatic pulses were generated by variable degrees of partial melting of a tholeiitic metabasaltic source throughout approximately 25 Ma. These findings reveal a wide compositional diversity of magmas that gave rise to a network of dykes crosscutting the Lagoa Dourada Suite and, consequently, to the framework of a juvenile continental crust on the southern margin of the São Francisco Craton. Furthermore, these rocks constitute a singular example of juvenile magmatism and contribute to a better understanding of the tectono-magmatic lull and potential petrological processes during this period.

Magmatic-hydrothermal evolution of the aquamarine-bearing Yamrang Pegmatite, Eastern Nepal: Insights from beryl, garnet, and tourmaline mineral chemistry

The Ikhabu Pegmatite Field (IPF) in the Nepal Himalaya is a newly discovered rare-element (REL) pegmatite field that hosts the Yamrang pegmatite, the primary source of the famous Nepalese aquamarines. In this contribution, we provide a comprehensive description of the geology of both the IPF and the Yamrang pegmatite, offering insights into the evolutionary stages of the Yamrang pegmatite based on its internal texture and the mineral chemistry of beryl, garnet, and tourmaline. The Yamrang pegmatite is classified as a beryl-columbite subtype of REL pegmatite, exhibiting axial symmetric zonation and is differentiated into five mineralogical-textural zones during magmatic and hydrothermal stages. The magmatic stage involves undercooling, disequilibrium crystallization, and fractional crystallization whereas the hydrothermal stage comprises alkali-enriched fluid exsolution, the formation of miarolitic cavities, and the crystallization of aquamarine. Yamrang aquamarines are enriched in FeO, up to 1.1 wt%, with Fe2+ as the primary chromophore, resulting in their blue color. Two types of beryl have been identified: magmatic beryl in zones 3 and 4 and hydrothermal beryl in miarolitic cavities (zone 5). Magmatic beryls are characterized by their alkali-poor to sodic composition, prismatic to columnar habit, homogeneous texture, and the presence of Fe-Al octahedral substitution. In contrast, hydrothermal beryls belong to the sodic to Li-Cs beryl group, with tabular to acicular habit, weak compositional zoning, and channel-tetrahedral substitution. Yamrang garnet belongs to the almandine-spessartine series, while the tourmalines are Mg-Ca rich dravite-schorl to Fe-Mn rich schorl variety. Alkali-poor to Li-Cs beryl with low REE contents, spessartine garnet (spessartine component, Xsps ∼ 52) with high HREE, and schorl with high Ti but low Li, Pb, and REE contents suggests that the Yamrang Pegmatite is an intermediate-fractionated REL pegmatite. Potential pegmatite evolution indicators for IPF include high alkali, Fe, Mn, and Rb contents in beryl, high Xsps in garnet, and an increase in Li, Fe, Mn, Zn, Ga, and Nb, alongside depletion of Mg, Ca, Ti, and Ta in schorl. The identification of a rare metal pegmatite field in the Nepalese Himalaya reinforces the general rare metal mineralization potential of the entire Himalaya region.

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.

Apatite as a record of ore-forming processes: Magmatic-hydrothermal evolution of the Hutouya Cu–Fe–Pb–Zn ore district in the Qiman Tagh Metallogenic Belt, NW China

Apatite is a common accessory phase in magmatic rocks and associated hydrothermal deposits that holds information about the evolution from magma to ore-forming fluid. In this study, we systematically studied the geochronological and geochemical characteristics of apatites hosted in the Fe skarn ores and associated intrusions of syenogranite and monzogranite in the Hutouya polymetallic Fe–Cu–Pb–Zn ore district, Qiman Tagh Metallogenic Belt (QTMB), NW China, in order to gain insight into the metallogenic processes from magmatic crystallization to hydrothermal fluid exsolution. The apatite U–Pb dating results of skarns and related intrusions are 229–228 Ma, and the monzogranite emplacement is slightly later than the syenogranite emplacement and skarn formation. The REE concentrations of the apatites from the skarn-related intrusions are characterized by LREE-enriched patterns and weak negative Eu anomalies. However, the apatites from the skarn ores have high HREE contents, and are characterized by flat chondrite-normalized patterns and strong negative Eu anomalies. These data, combined with variations of element concentrations and ratios in the apatites (eg., Sr, SO3, F, Cl, REE, (La/Yb)N, (Sm/Nd)N, (La/Sm)N, (Eu/Eu*)N, (Ce/Ce*)N), record the transition of ore-forming processes from the magmatic crystallization to hydrothermal exsolution, and indicate that the skarn-related magmas with moderately reduced signatures may have been derived from partial melting of crustal materials potentially affected by the limited mantle input under an extensional environment.

Late Neoarchean plate subduction in Western North China Craton: Evidence from ca. 2.51 Ga to 2.46 Ga basement rocks in Northern Ordos Basin

The most poorly exposed and least understood in the North China Craton (NCC) lies largely hidden beneath the Ordos Basin, severely limiting the cognition of the Precambrian tectonic evolution in the western NCC and the tectonic division. Diverse orthogneiss samples collected from four boreholes drilled into the basement of the north side of Hangjinqi Fault Zone (HFZ) in the North Ordos Basin (NOB) are investigated by integrating zircon U-Pb dating, Hf-Nd isotopes, and whole-rock geochemical analyses. The TTG gneiss, quartz-monzonite gneiss, granodiorite gneiss, and amphibolite were emplaced at ca. 2.51–2.46 Ga, of which the amphibolite also recorded the metamorphic age of ca. 1.95 Ga. The TTG gneisses display low MgO, Mg#, Cr, and Ni, high HREE and Rb/Sr, enrichment of LILEs and depletion of Nb, Ta, and Ti, and positive εNd(t) (2.04–4.06) and εHf(t) (3.04–7.59) values, suggesting that they were mainly originated from partial melting of the tectonically thickened mafic island-arc crust. In contrast, the quartz monzonite gneisses and granodiorite gneisses show the geochemical features of Archean sanukitoids with higher MgO, Mg#, Cr, and Ni, and similar εNd(t) (1.34–3.82) and εHf(t) values (3.94–7.32), suggesting that they were derived from the partial melting of the mantle metasomatized by fluids and melts of the subduction. The amphibolites exhibit a mixed characteristic of E-MORB and arc-like geochemical affinities resembling the Mariana back-arc basin, which was derived from the partial melting of spinel lherzolite. The arc and arc-back lithological assemblages suggest that the north margin of the NOB might be in a geodynamic transition period from a compressional regime correlated with plate subduction to an extensional regime correlated with slab rollback during ca. 2.51–2.46 Ga. Combined with previous analytical results, the northern margin of the NOB was most likely in a long-term active continental margin setting during 2.51–1.95 Ga and involved in the collision event between the Yinshan Block and Ordos Block at ca. 1.95 Ga.

Geochemical Signatures of Felsic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Greenstone Belts

Abstract Felsic volcanic rocks are abundant in ancient greenstone belts and important host rocks for volcanogenic massive sulfide (VMS) deposits. About half of all VMS deposits are hosted by dacite or rhyolite, an association that reflects anomalous heat flow during rifting, partial melting of basaltic crust, and fractional crystallization in high-level magma chambers. For over 30 years, geochemical signatures of these rocks (e.g., F classification of Archean rhyolites) have been widely used to identify possible hosts for VMS deposits in ancient greenstone belts. However, comparisons with modern oceanic settings have been limited, owing to a lack of samples of felsic volcanic rocks from the sea floor. This is changing with increasing exploration of the oceans. In this study, we have compiled high-quality geochemical analyses of more than 2,200 unique samples of submarine felsic volcanic rocks (&amp;gt;60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hot-spot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical diversity spanning the full range of compositions of rhyolites found in ancient greenstone belts. This diversity is interpreted to reflect variations in crustal thickness, the presence or absence of slab-derived fluids (dry melting versus wet melting), and mantle anomalies. Highly variable melting conditions are thought to be related to short-lived microplate domains, such as those caused by diffuse spreading and multiple overlapping spreading centers. Systematic differences in the compositions of felsic volcanic rocks in the modern oceanic settings are revealed by a combination of principal components analysis, unsupervised hierarchical clustering, and supervised random forest classification of the compiled data. Dacites and rhyolites from midocean ridge settings have moderately depleted mantle signatures, whereas rocks from ridge-hot-spot intersections and ocean islands reflect enriched mantle sources. Felsic volcanic rocks from arc-back-arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry (strong large ion lithophile element enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements [HREEs]). This contrasts with felsic volcanic rocks in Archean greenstone belts, which show high field strength element and HREE enrichment (so-called FIIIb-type) due to a less depleted mantle, a lack of wet melting, and variable crustal contamination. The differences between modern and ancient volcanic rocks are interpreted to reflect the lower mantle temperatures, thinner crust, and subduction-related processes in present-day settings. We suggest that the abundance of FIIIb-type felsic volcanic rocks in Archean greenstone belts is related to buoyant microplate domains with thickened oceanic crust that were better preserved on emerging Archean cratons, whereas in post-Archean tectonic settings most of these rocks are subducted.

Late Cretaceous adakitic intrusive rocks in the Laimailang area, Gangdese batholith: Implications for the Neo-Tethyan Ocean subduction

Abstract The Gangdese batholith, emplaced from the Cretaceous to the Eocene in the southern Lhasa terrane of Tibet, provides critical constraints on the tectonic-magmatic evolution of the Neo-Tethyan Ocean and the India-Asia continental collision. In this article, we report new data for the Laimailang monzogranite in the southern Gangdese, including major and trace element analyses, zircon U–Pb dating, and in situ Hf isotope analyses. In situ zircon U–Pb dating of sample yielded Late Cretaceous ages (ca. 81 Ma). The monzogranite is characterized by relatively high-silica (≥56 wt% SiO2), Na-rich, and high-Al granitoids that are characterized by high Sr, low Y and HREE contents and strongly fractionated REE patterns, with no significant Eu anomaly, indicating that they are consistent with the definition of adakite. These monzogranite have high K2O (2.92–6.5%) and negative ε Hf(t) (−2.1 to −5.4), suggesting that these rocks were likely derived from the partial melting of the lower continental crust. We conclude that the Laimailang rocks of adakitic affinity were derived due to the melting of the thickened lower continental crust in the late Cretaceous.

Pressure–temperature–time constraints on metamorphism in the southeastern Taltson Domain, Saskatchewan, Canada

The southeastern Taltson Domain is located on the southern margin of the Rae Craton in northern Saskatchewan, Canada, and encompasses the region south of the Athabasca Basin and west of the Snowbird Tectonic Zone. There has been relatively little metamorphic geochronology and no quantitative pressure–temperature (P–T) constraints from the southeastern Taltson Domain, which hinders attempts to link its metamorphic history to surrounding regions. New in situ LA-ICP-MS U–Pb monazite and zircon ages from rare metasedimentary rocks from the southeastern Taltson Domain reveal a polymetamorphic history. A cordierite–sillimanite–spinel-bearing migmatite from Fournier Lake records high thermal gradient metamorphism at 1988 ± 21 Ma, reaching P–T conditions of 4–6.4 kbar and > 900 °C. The cordierite–sillimanite–spinel assemblage overprints an earlier biotite–sillimanite-bearing assemblage that formed at 2075 ± 31 Ma. An orthopyroxene–garnet gneiss from within the Snowbird Tectonic Zone has a dominant monazite population at c. 1.90 Ga, but contains rare monazite ages between c. 2.19–2.01 Ga that support a phase of metamorphism prior to 2.0 Ga. The main metamorphic event to have affected the southeastern Taltson Doman occurred at c. 1.94–1.90 Ga. Phase equilibria modelling suggests peak P–T conditions were 8.3–11.2 kbar and 800–930 °C. Monazite with high HREE contents yields weighted mean ages of 1926–1918 Ma, interpreted to reflect prograde metamorphism prior to significant garnet growth. Zircon from an extremely garnet and zircon rich sample (now silicified and chloritized as a result of younger alteration) formed at c. 1918 Ma. REE compositions of the zircon indicate that it formed in equilibrium with garnet at temperatures of ∼ 830 °C, suggesting peak metamorphism occurred around c. 1.92 Ga. HREE-poor monazite with weighted mean ages of c. 1910–1895 Ma formed after the majority of garnet growth and either reflect the timing of melt crystallisation or recrystallisation during retrogression and fluid flow along major shear zones in the region. The P–T conditions and c. 1.93–1.90 Ga ages from the southeastern Taltson Domain are very similar to other rocks in the Dodge–Snowbird Domain of the Rae Craton, and demonstrate that the footprint of mid-P granulite facies metamorphism extends south and west into the southeastern Taltson Domain.

Geochemical, geochronological, and isotopic constraints for the Archean metamorphic rocks of the westernmost part of the Carajás Mineral Province, Amazonian Craton, Brazil

The Archean nucleus of the Amazonian Craton, represented by the Carajás Mineral Province (northern Brazil), crops out as a broad exposition of igneous, sedimentary, and metamorphic rocks showing a complex polyphase geologic history. A succession of orthogneisses (metagranitoids), amphibolites, and metavolcano-sedimentary rocks that show greenstone belt characteristics occur in Tancredo Neves Village district, northeast of São Félix do Xingu city (state of Pará), is the focus of this study. Detailed field, petrographic, and stratigraphic data reveal that these metamorphic rocks underwent two different tectono-metamorphic events: (i) the regional metamorphism (>2.8 Ga), which occurred under greenschist-to mid-upper amphibolite-facies conditions; and (ii) the dynamic metamorphism (>2.6 Ga) which reached amphibolite-facies conditions and overprinted previous metamorphic textures. Associated regional strike-slip NW–SE to EW-trending ductile shear zones generated mylonitic, ultra-mylonitic, and migmatitic zones. Whole-rock geochemistry discriminates four groups of metagranitoid protoliths. G1 group is composed of peraluminous trondhjemitic rocks samples, calc-alkaline, weakly fractionated, and enriched in HREE. G2 group comprises metaluminous and alkaline monzogranite samples with high Zr, Y, Nd, HREE, and intermediate fractionation. U–Pb (LA-MC-ICP-MS) dating of G2 zircons yields a crystallization age of 2745.6 ± 5.0 Ma. Groups G3 and G4 encompass peraluminous, calc-alkaline to alkali-calcic, anorogenic, fractionated monzogranitic and granodioritic rocks. G3 and G4 groups are further distinguished by high and low La/Yb(N) ratios, respectively. The results obtained for Tancredo Neves Village district very much resemble the Canaã dos Carajás (Carajás Mineral Province) area characteristics, comprising intense magmatism (2.7–3.0 Ga) and reworking related to at least two distinct metamorphic events in the Archean. As Canaã dos Carajás, Tancredo Neves Village can be a promising area for mining exploration.

Archean to Proterozoic (3535–900 Ma) crustal evolution of the central Aravalli Banded Gneissic Complex, NW India: New constraints from zircon U-Pb-Hf isotopes and geochemistry

The central Aravalli Banded Gneissic Complex, a Proterozoic high-grade polymetamorphic terrane with Archean components in northwest India, is mainly composed of granulites, granitoids, TTG gneisses and metasedimentary rocks. Combined sets of zircon U-Pb-Hf isotopes and whole-rock geochemical data reveal that the gneissic complex is made up of Neoarchean TTGs, Paleoproterozoic A-type granitoids, and Mesoproterozoic nepheline syenite, which intruded at ca. 2550, 1720–1705 and 1440 Ma, respectively. Zircon xenocrysts additionally point to Paleoarchean-Neoarchean magmatic episodes at ca. 3535 and 2665 Ma. The discovery of a ca. 3535 Ma inherited zircon with superchondritic εHft provides the first direct evidence for the existence of an early Paleoarchean crust in the Aravalli orogen, which was formed by the partial melting of a depleted mantle reservoir. Ages from zircon rims suggest Grenville-age metamorphic overprints at ca. 970 Ma and ~ 900 Ma. High HREE contents and subchondritic εHf2.55Ga = -7.3 to 0.7 of the Neoarchean TTG suggest partial melting of a heterogenous mafic crust derived from depleted mantle sources during the Paleo- to Mesoarchean. The Paleoproterozoic A-type granitoids with subchondritic εHft between −6.1 and −12.5 originated mainly from internal reworking of a heterogenous, predominantly Neoarchean felsic crust, between 1720 and 1705 Ma. Near chondritic Hf isotope composition of the ca. 1440 Ma Kishangarh nepheline syenite reflects derivation of the parental magma from enriched mantle sources.

Monazite recorded Paleoproterozoic granulite-facies metamorphism and Triassic fluid modification of the Weihai pelitic granulite, Sulu orogen

Monazite is an important accessory mineral in the pelitic granulites (Weihai area, Sulu orogen), and is also a powerful monitor for understanding the metamorphic evolution of the granulites. The pelitic granulites incongruously occur as lenses in granitic gneisses. The lithologies of the lenses gradually change from core to margin: The undeformed coarse-grained granulite, the foliated fine-grained granulite, the garnet-biotite-gneiss, and the migmatized granulite. The lens cores mostly preserve a peak granulite-facies metamorphic mineral assemblage of garnet + plagioclase (antiperthite) + quartz + sillimanite + biotite with accessory minerals of rutile, zircon, and monazite. The lens margins display a fluid-induced retrogression. In order to decipher the metamorphic processes of the pelitic granulites, a combined study of BSE imaging, U-Pb dating, and trace element composition for the monazites from the metapelitic lens were conducted. Monazites from the undeformed coarse-grained granulite only record a Paleoproterozoic age (1832±7 Ma, n=40). Monazites from the other lithologies yield the inherited Paleoproterozoic age and Triassic overgrowth age. For example, monazites from the migmatite yield intercept ages of 1818±10 and 211±22 Ma (n=56) with Triassic concordant age of 223.8±2.9 Ma. The Paleoproterozoic monazites are characterized by remarkable depletion in HREE and Y with obviously negative Eu anomalies, indicating their formation equilibrated with garnet and feldspar under granulite-facies conditions. During Triassic fluid modification, the monazite bright rims assimilated Th and Si but released U, HREE, Y, and P. This process resulted in the Triassic overgrowth monazites have higher HREE and Y contents, and lower Th and U contents with relatively lower Th/U ratios. Thus, the monazites in the pelitic granulites recorded a Paleoproterozoic metamorphic event and Triassic fluid modification. The Weihai pelitic granulites might have a tectonic affinity with the North China Craton. Therefore, the Paleoproterozoic pelitic granulites were mechanically drawn into the orogen during the Triassic continental collision, and subsequently were remoulded by the fluids during its exhumation.

Geochemistry of late paleoproterozoic Anjana and Amet granites of the Aravalli craton with affinities to sanukitoid series granitoids: Implications for petrogenetic and geodynamic processes

The Mangalwar Complex of the Aravalli craton is marked by the presence of late Paleoproterozoic granites referred to as Anjana Granite and Amet Granite. These granites occur as 1.64 Ga old plutons intruding greenstone sequences and migmatitic gneisses of Mangalwar Complex which comprises parts of BGC of the Aravalli craton. In the present contribution major, trace and REE data of these granites along with associated microgranular mafic enclaves (MMEs) are presented and discussed. Geochemically these granites are quartz monzonite, metaluminous, sub-alkaline and high-K calc-alkaline rocks. The most important characteristics of Anjana and Amet granites are low SiO2, high MgO, Mg#, K2O, Ba, and low Na2O/K2O ratios. In addition, the REEs show moderate to high fractionation, with (La/Yb) ratios up to 22 and 23 of the Anjana and Amet granites respectively, with no or positive europium anomalies. In the primitive mantle-normalized trace element diagrams both granites show depletion in high-field strength elements (HFSE) such as Nb, Ta, P, Ti and enrichment in LILEs. Most of these features are comparable to those of sanukitoid series rocks. Geochemically both granites are distinguished as high-Ti sanukitoids. Geochemical characteristics of MMEs suggest that they are similar to Anjana and Amet granites and in turn to sanukitoids with lower SiO2 content. They display LREE enriched patterns with low values (avg. 13) of (La/Yb)N, negative Eu anomalies and high HREE contents (58 ppm). It is suggested that the parental magma of Anjana and Amet granitic plutons originated through a four stage process (1) Generation of magmatic melts produced by partial melting of terrigeneous sediments of subducting slab in an arc setting; (2) interaction of those melts with the overlying mantle wedge, and total consumption of slab-derived melts during the reaction resulting in production of a metasomatized mantle; (3) tectonothermal event, possibly related to the slab break-off, causing asthenospheric mantle upwelling. This may have induced the melting of the metasomatized mantle and the generation of sanukitoid magmas. The parental magmas of Anjana and Amet granites and their mafic enclaves were generated at lower and higher lithospheric levels respectively (4) Granitic magma ascended due to viscosity and gravity instabilities and interacted with enclave magma at higher mantle level. Both magmas ascended towards upper crust and evolved through fractional crystallisation. Existing data suggest that in the Mangalwar Complex, the formation of sanukitoid magma started even during Mesoarchaean times and continued till late Paleoproterozoic. Formation of sanukitoid magma during this time indicates that in northern Indian shield the multi-stage subduction- accretionary orogenic processes continued for a protracted geological period and played a major role in the origin and evolution of early continental crust.

Evolution of the Kolyvan-Tomsk granitoid magmatism (Central Siberia): Insights into the tectonic transition from post-collision to intraplate settings in the northwestern part of the Central Asian Orogenic Belt

Late Paleozoic – early Mesozoic geological evolution of the Kolyvan-Tomsk Fold Zone (KTFZ) located in the northwestern part of the Central Asian Orogenic Belt is significant to understanding the tectonic transition from post-collision to intraplate settings. We present new whole-rock geochemical and Sr-Nd isotopic data, and in situ zircon U-Pb ages for granitic rocks from the KTFZ basement to constrain their petrogenesis and to characterize changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal extended multi-stage emplacement of the KTFZ granitoids for about 20 Ma from ~260 to ~240 Ma, that can be divided into two stages: the Permian-Triassic (~260 to 250 Ma) and the Triassic (~250 to 240 Ma). The Permian –Triassic granitic rocks with SiO2 between 63% and 74% have high LREE, HFSE (except Nb and Ta), Sr, Mg#, Cr, Ni concentrations, Sr/Y and (La/Yb)N ratios and remarkably low Y and Yb contents and 87Sr/86Sr(T) values (0.704682 and 0.703368). They are magnesian and show metaluminous to weakly peraluminous natures, indicating their formation through high degree fractionation of mantle-derived magmas. The Triassic granitoids exhibit extremely high SiO2 (73–77%), HFSE, HREE, Rb contents, Rb/Sr ratios and elevated negative Eu and Sr anomalies, low Mg#. They are ferroan and have a peraluminous nature with the absence of aluminium-rich minerals, all of which indicate the presence of the crustal substance in the magma source. Isotopic Nd compositions show that the both the Permian –Triassic and Triassic granitoids have uniform magma's, which were derived from partial melting of juvenile Neoproterozoic sources.These two stages of granitoid magmatism are interpreted as reflecting the transition from post-collisional magmatism, related to the final closure of the Paleo-Asian Ocean due to collision between the Baltica and Siberian paleocontinents during the Late Permian – Early Triassic and hence, to the intraplate magmatism. The formation of the KTFZ granitic rocks was induced by an upwelling asthenosphere in an extensional geodynamic setting. During the post-collisional process, the large-scale lithospheric delamination plays a key role in the genesis of the Permian-Triassic granitoids and provide the magma generation for them. In the Triassic, we propose additional source of heat from mantle plume beneath the KTFZ, which promote extending of the granitoid magmatism in intraplate setting.

Neoproterozoic tectonic switch on the southwestern Yangtze Block: evidence from zircon U-Pb-Hf isotopes and geochemistry of the A- and I-type granites

ABSTRACT The petrogenesis and tectonic affinity of late Mesoproterozoic to early Neoproterozoic magmatic records in the Yangtze Block is important to understand its tectonic evolution within the context of the Rodinia supercontinent’s reconstruction. In this paper, our new SHRIMP zircon U-Pb dating results demonstrate that the Luonie granodiorites and Gonghe granites were emplaced at 1019.2 ± 5.3 Ma and 925.0 ± 4.9 Ma, respectively. The Luonie granodiorites show high HFSE and HREE contents and Ga/Al ratios, resembling A-type granites. They exhibit negative εHf(t) values (−1.6 to −3.9) with TDM2 ages of 1980–2125 Ma, suggesting that they were derived from partial melting of pre-existing continental crust. The Gonghe granites are calc-alkaline, slightly peraluminous and have low P2O5 contents, which are typical features of I-type granites. Zircons from Gonghe granites possess positive εHf(t) values (+10.8 to +13.1) with TDM2 ages of 969–1113 Ma, indicating juvenile mafic lower crust source. In conclusion, Luonie granodiorites were formed in an intra-plate rifting environment, whereas Gonghe arc-related intrusion was emplaced into an arc-setting. On the basis of these results, we propose that the rifting-related igneous rocks and subduction-related magmatism in the southwestern Yangtze Block record a tectonic transition from a continental-rifting basin in the passive plate margin to a collisional setting in an active continental margin in the early Neoproterozoic.

Elemental and multiple isotopic evidences of enriched lithospheric mantle origin of the Xiadian gold deposit in the Jiaodong Peninsula, East China

The Jiaodong Peninsula is the largest gold concentration district in China and the third largest one in the world. It covers no >0.2% of China's total landmass yet accounts for 30% of the total national gold reserves with total proved resources near 5000 t Au. One wonders where all this gold comes from. Here we present detailed whole-rock element and Sr-Nd-Pb isotope, quartz H-O isotope and pyrite He-Ar isotope geochemical data for the Xiadian gold deposit, a large Jiaojia-type gold deposit in the Jiaodong district, which furnishes new constrains on the sources of ore-forming fluids and metals. The results indicate that the ore and alterated rock have much higher HREE (e.g. Yb and Y), HFSE (e.g. Nb, Ta, Zr and Hf) and transition elements (e.g. Sc, Ti, V, Cr, Co and Ni) than the host granites but similar to the lithospheric mantle-derived lamprophyres, suggesting an enriched lithospheric mantle origin. The H-O isotopes (δDV-SMOW = −93.9‰ to −102.3‰, δ18OH2O = −0.2‰ to 1.6‰) of fluid inclusions in quartzs collected from auriferous pyrite–sericite–quartz altered rocks are distinctly different from either primary magmatic water and metamorphic water or meteoric water in the Jiaodong district but similar to the local enriched mantle-derived fluids. The He-Ar isotopes [3He/4/He = 0.58 × 10−6 to 1.90 × 10−6 (0.42–1.36 Ra), 40Ar/36Ar = 725–1358] of fluid inclusions in pyrites collected from auriferous pyrite–sericite–quartz altered rocks are also similar to the local enriched mantle-derived fluids. The ore and alterated rock have distinct Nd-Pb isotopic compositions either from the host Linglong granites or from the asthenosphere-derived mafic dikes but similar to the lithospheric mantle-derived mafic dikes. Their Sr isotopic compositions show a mix trend between the lithospheric mantle-derived mafic dikes and Neoarchaean basement. All the elemental and multiple isotopic features support a metallogenic model that the ore-forming fluids and metals were derived from the local previously enriched lithospheric mantle, and such deeply sourced fluids may have interacted with Neoarchaean basement along their flow path and mixed with a little meteoric water when they arrived high levels in the crust.

Petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites of the Songnen Block, NE China: Implications for the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans

The petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites in NE China offers new insights into the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans. This study reports new LA–ICP–MS zircon UPb ages, geochemical data, and zircon Hf isotopic compositions of syenogranites and quartz diorite porphyries exposed on the northern margin of the Songnen Block, NE China. Zircon UPb age data indicate that the syenogranites and quartz diorite porphyries were emplaced at ca. 319 and 120 Ma, respectively. The syenogranites have high K2O contents of 4.15–4.79 wt% and (K2O + Na2O)/CaO ratios of 33–102, and low MgO (0.06–0.20 wt%) and P2O5 (0.01–0.02 wt%) contents. Their remarkably high HREE contents and Y/Nb ratios (2.07–2.40), significantly negative Eu anomalies (Eu/Eu⁎ = 0.15–0.28), and low Sr contents (29.7–85.8 ppm) indicate that the syenogranites are A-type granites. They have positive zircon εHf(t) values of 8.4–12.1 and Neoproterozoic Hf TDM2 ages of 795–561 Ma, suggesting a juvenile lower crustal source. The quartz diorite porphyries have relatively high Na2O and MgO, and moderate K2O contents. Their A/CNK values (0.95–1.18) indicate a transition from being metaluminous to weakly peraluminous. They are enriched in LILEs (e.g., Rb, K, and Ba) and depleted in HFSEs (e.g., Nb, Ta, Ti, and P). Their low Y (11.1–14.4 ppm) and Yb (0.98–1.34 ppm) contents and high Sr (589–1131 ppm), Cr, and Ni contents and Sr/Y ratios indicate that they were generated by partial melting of oceanic crust. Based on these data and regional geological investigations, we propose that the syenogranites on the northern margin of the Songnen Block were formed in a post-collision extensional setting, likely due to closure of the Paleo-Asian Ocean between the Xing'an and Songnen blocks. Formation of the Early Cretaceous quartz diorite porphyries may have been triggered by partial melting of the Paleo-Pacific flat slab. Apart from rollback of the Paleo-Pacific oceanic slab, the main reason for the eastward migration of magmatism during the Cretaceous was the trench-ward dehydration, eclogitization, and sinking of the residual Paleo-Pacific flat slab.

Grenville-age metamorphism in the Larsemann Hills: P-T evolution of the felsic orthogneiss in the Broknes Peninsula, East Antarctica

ABSTRACT The gneisses outcropping in the Larsemann Hills of East Antarctica are ideal for studying the composition and evolution of the Prydz tectonic belt. In this study, detailed petrological analyses, phase equilibrium modelling, and zircon analyses were performed on a felsic orthogneiss from the Broknes Peninsula in the Larsemann Hills. The study results show that the protolith of the felsic gneiss is granite, which is further identified to have been an S-type granitoid. The felsic orthogneiss intruded into the basement complex at ~976 Ma. The magmatic zircons in the felsic orthogneiss have lower LREE and higher HREE contents than those of the metamorphic zircons in the same rock. The felsic gneiss consists of the mineral assemblage garnet +ilmenite+ plagioclase +K-feldspar + sillimanite + quartz and underwent granulite facies metamorphism in the Grenvillian period (～899 Ma), with peak conditions of ～870°C and ～9.5 kbar. The Larsemann Hills should be categorized as the orogenic process from arc-continent collision to continent-continent collision during the Grenvillian period, crust thickening(~899 Ma) and subsequent collapse(550–500 Ma).

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.

Early cretaceous igneous activities in the north flank of the North China Craton: the Shouwangfen complex example

ABSTRACT The Early Cretaceous igneous complexes in the north flank of the North China Craton (NCC) provide a window to investigate the Mesozoic magmatism in the NCC. Here we report the precise timing of Early Cretaceous magmatism and magma petrogenesis of the different rock types in the north flank of the NCC based on petrology, mineral chemistry, geochemistry, zircon geochronology and Sr-Nd isotopes. Zircon U-Pb dating reveals that the Shouwangfen complex was crystallized at 130–128 Ma. The rocks display enrichment in large ion lithophile elements (LILE) and LREE, and depletion in high field strength elements (HFSE) and HREE, with relatively high Sr/Y and La/Yb values, typical of subduction-related magmatic rocks. The mafic microgranular enclaves show typical igneous textures, acicular apatites, sieve-texture of plagioclase phenocrysts and overgrowth of amphibole around the pyroxene, corresponding to magma mixing and mingling and thermal exchange. The quartz monzonites display normal zoning of the plagioclase, low and homogeneous εNd(t) values and linear co-variations in two-component diagrams. The quartz monzodiorites are characterized by high and homogeneous εNd(t) values. Petrologic feature and geochemical data suggest that the quartz monzonites represent lower crustal magma with minor contribution of enriched melt of the sub-continental lithospheric mantle (SCLM), followed by fractional crystallization. The mafic microgranular enclaves were sourced from enriched SCLM, followed by mixing and mingling with the host quartz monzonitic magma. The quartz monzodiorites are mainly SCLM-derived and subsequently mixed with lower crust melts. The high water contents (≥3%) of these Early Cretaceous igneous complexes suggests a hydrous SCLM beneath the NCC. The Early Cretaceous igneous complexes formed within an extensional tectonic setting which were related to the retreat and dehydration of the paleo-Pacific slab. The hydration of the lithosphere induced extensive crust-mantle interaction and large-scale water-rich magmatism, leading to lithospheric thinning in the NCC during the Mesozoic.

Early Paleoproterozoic magmatism in the Yangtze Block: Evidence from zircon U-Pb ages, Sr-Nd-Hf isotopes and geochemistry of ca. 2.3 Ga and 2.1 Ga granitic rocks in the Phan Si Pan Complex, north Vietnam

Our understanding of the early evolution of the Yangtze Block is limited by the sparsely dispersed nature of pre-Neoproterozoic exposures. New, integrated petrographic, zircon U-Pb age and Hf-Nd isotope analyses, and whole-rock geochemical data for early Paleoproterozoic granites in the Phan Si Pan Complex provides new insights into the evolution of the Yangtze Block as well as its role in the Pre-Nuna supercontinent. LA-ICP-MS zircon U-Pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207Pb/206Pb ages of 2306 ± 12 Ma and 2096 ± 15 Ma, respectively. Zircons from the quartz monzonite have εHf(t) values ranging from −4.1 to −2.1, corresponding to TDM2 model ages of 3002–2890 Ma, whereas zircons in the gneissic granite have εHf(t) values between −0.95 and +1.72 and corresponding TDM2 model ages of 2660–2516 Ma, which are consistent with their whole-rock Nd isotope values. Geochemically, the quartz monzonites are I-type granites. Combined with their relatively high Sr/Y ratios and low Y concentrations, as well as fractionated REE patterns with relatively high LREE but low HREE concentrations, they were probably generated by partial melting of the thickened middle-lower crust under elevated temperature. Geochemical and isotopic signatures suggest that the ca. 2.1 Ga gneissic granites are high-K calc-alkaline, ferroan A-type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material. The Phan Si Pan complex has a distinct early Paleoproterozoic crustal evolution history compared with the other crustal provinces of the Yangtze Block, suggesting independent histories that were not unified until the late Paleoproterozoic during the assembly of Nuna. Moreover, the magmatism and tectonic evolution of the north Vietnam region is broadly similar to that of the Arrowsmith Orogen of the Rae craton in Laurentia suggesting a potential spatial linkage. The geologic record of the Yangtze Block does not support an early Paleoproterozoic shutdown of plate tectonics.