High HREE Contents Research Articles

The Val Biandino Intrusive Suite (central Southern Alps, N Italy): new geochronological and geochemical data on the Early Permian magmatic activity in the Southalpine Domain

The Early Permian in the present-day Europe area was a time when a major tectonic shift occurred, leading from the tectonic collapse of the Variscan orogeny to the crustal extension and thinning that characterized the Early Permian times. Crustal extension was associated with extensive magmatism at different crustal levels: from gabbro in the lower crust or at the mantle/crust transition to subaerial high-SiO2 volcanic activity. In the whole Southalpine Domain, the Early Permian intrusive bodies occur from the west (e.g. Ivrea-Verbano Complex and “Graniti dei Laghi”) to the east (Ifinger, Brixen and Cima d’Asta intrusive complexes). Among these, in the central Southern Alps (comprised between the Giudicarie Belt and the Lake Como), minor intrusive complexes also occur. The Val Biandino Intrusive Suite consists of two magmatic units: the Val Biandino Quartz-Diorite (VBQD) and the Valle di San Biagio Granite (VSBG). The first of them consists of gabbro-diorite to granodiorite bodies associated with leucocratic cordierite-bearing granitic dikes that intruded the pre-Permian basement. To the west, a W-dipping normal fault of Permian age represents the boundary between this unit and the Valle di San Biagio porphyric granite. All rock varieties of the Val Biandino Intrusive Suite display a high-K calc-alkaline affinity with metaluminous to peraluminous character. Field crosscutting relationships point to a late generation of the cordierite granites of the Val Biandino Quartz-Diorite unit with respect to the more mafic types. SHRIMP U–Pb zircon dating provided an age of 285.2 ± 1.9 Ma for a cordierite granite of the Val Biandino Quartz-Diorite unit and 283.2 ± 1.9 Ma for the porphyric Valle di San Biagio Granite. Geochemical data suggest that gabbro-diorite, quartz-diorite, granodiorite and leucogranite are not co-magmatic. The existing gaps in term of SiO2 wt% and the higher HREE contents in mafic and intermediate rocks with respect to granite coupled with similar LREE in all rocks support this hypothesis. The high Rb/Sr ratios (> 1) in leucogranite, together with the occurrence of white mica and cordierite point to a significant contribution of crustal partial melting in their genesis. The Val Biandino Intrusive Suite was likely formed through the interaction of magma genesis at the mantle/crust transition and partial melting of the heterogeneous pre-Permian basement of the Southalpine Domain. This scenario explains the strong heterogeneity displayed by the relatively small intrusive complex and the strong crustal signature exhibited by all the magmatic types of the Val Biandino Intrusive Suite.

Origin, dynamics, and chemical evolution of garnet-bearing leucogranitic magma, Eastern Desert of Egypt: Controls on the rare-metal enrichment in the A-type magmatism

Egypt hosts numerous rare-metal granites, i.e., highly evolved granites enriched in rare metals (Ta, Nb, Be, Sn, Zr, Th, and REE). However, the processes involved in the rare-metal enrichment are not fully understood. We present new data on the textural characteristics and chemical composition of rare-metal mineralization associated with microgranite dikes in the Ras Abdah area of the Egyptian Eastern Desert. These dikes are garnet-bearing leucogranites (GLG) composed of perthitic alkali-feldspars and quartz. When compared to other Egyptian A-type granites, microgranite dikes are alkaline rocks with particularly higher HREE contents. Zircon, huttonite, fergusonite (Y), and Fe-Ti-Zn oxides (magnetite, Zn-bearing ilmenite and pyrophanite) are largely associated with the altered domains, which are also enriched in Nb, Zr, Y, Ta, Th, and REE. However, similarities between the chondrite-normalized REE patterns of the altered and unaltered domains of the GLG dikes favor the hypothesis of a unique magmatic signature. Moreover, the chemical and textural features of rare-metal minerals indicate that the alteration of primary minerals was caused by deuteric fluids or aqueous residual melt exsolved from the parental granitic magma (autometasomatism). Garnet compositions are rich in the spessartine component (up to 84 %), which is typical of garnet in highly evolved pegmatitic rocks. Furthermore, garnet exhibits no major element zoning but shows chemical fluctuations in trace element concentrations, suggesting correspondingly abrupt changes in melt composition due to sequential magma pulsing. This magma emplacement may cause crystal nucleation and oscillatory crystallization followed by magmatic segregation. Overall, parental magma type, dike injection, and magmatic-hydrothermal processes all play a role in the unusual enrichments of rare metals.

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 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.

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.

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.

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.

Monazite geochronology in melt-percolated UHP meta-granitoids: An example from the Erzgebirge continental subduction wedge, Bohemian Massif

Coupled age and trace element mapping in monazite grains using the Laser-Ablation Split-Stream ICP-MS technique was carried out for a sequence of samples from (U)HP crustal section in the Eger Crystalline Complex interpreted as a Variscan continental subduction wedge. The sampled section comprises banded orthogneiss with monazite protolith ages of 486 ± 3[10] Ma. The orthogneiss was heterogeneously transformed to migmatite, HP felsic granofels and U(HP) fine-grained Ky-Kfs granulite during coupled deformation and percolation of silicate melts in a subduction channel. Age of the (U)HP metamorphism and related melt percolation in the granulite is dated by Th-rich and HREE-poor monazite cores at 353.6 ± 2.7[8] Ma, that are in equilibrium with HREE-rich garnets. Th-rich cores of monazites in the migmatite, granofels and granulite that lack the (U)HP signatures show younger ages of 347.8 ± 3.4[8] Ma, 345.8 ± 2.4[8] Ma and 339.3 ± 1.3[7] Ma, respectively. The monazite rims with lower Th and higher HREE contents than the monazite cores are likely a result of (re)crystallization in equilibrium with melt that partly dissolved the HREE-rich prograde garnet during decompression. This (re)crystallization is constrained by ages of 344.2 ± 1.6[8] Ma in (U)HP granulites and 338–340 ± [8] Ma in migmatites, granofelses and granulites without the (U)HP signature. The groups of ages from 353 Ma to 338 Ma, are significantly different when only their in-run uncertainties (7–10 Ma) are compared and indicate a 15 ± 3 Ma period of monazite (re)crystallization from burial/peak to exhumation.

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).

Lithospheric transformation of the northern North China Craton by changing subduction style of the Paleo-Asian oceanic plate: Constraints from peridotite and pyroxenite xenoliths in the Yangyuan basalts

Mechanisms of lithospheric transformation beneath the North China Craton (NCC) were widely discussed, but the northern NCC was less studied compare to the eastern part. In this research, In-situ major and trace element and Sr isotopic compositions of spinel peridotite and pyroxenite xenoliths carried by the Yangyuan Tertiary basalt were analyzed to constrain subduction style of the Paleo-Asian oceanic plate (PAOP) and its influence on the northern NCC. These xenoliths were classified into dunite, pyroxenite, and two groups of lherzolites according to their petrography and chemical features. The petrography and mineral chemistry suggest that the dunite, pyroxenite, and Group 1 lherzolites are residues of partial melting which experienced subsequent hydrous metasomatism. Correlations between 87Sr/86Sr with the Zr/La and Zr/Nb ratios in clinopyroxenes from these xenoliths suggest that the metasomatic agents should be fluids derived from the subducted PAOP. Mineral RbSr isochron of the pyroxenite show that the minimum age of the metasomatism event should be 295 ± 38 Ma. Major element compositions of minerals in the Group 2 lherzolites are more fertile compare to the dunite and Group 1 lherzolites. Clinopyroxenes in these xenoliths exhibit LREE-depleted REE patterns with relatively high HREE contents and 87Sr/86Sr ratios (0.70403–0.70482). Chemical zonations are preserved in several clinopyroxenes with incompatible elements depleted from core to rim, which should be the results of melt depletion after metasomatism. Patches of silica-rich glasses were present along the grain boundaries. These glasses have sediment-like Pb isotopic compositions and trace elements distribution patterns and thus are supposed to represent the melt from recycled sediments. Combined with geophysical and tectonic evidences, we propose that roll-back of the southward low-angle subducted PAOP in the late stage of subduction contributed to remelting of the PAOP subduction-modified mantle wedge beneath the northern NCC.

Ca. 1050 Ma intra-continental rift-related A-type felsic rocks in the southwestern Yangtze Block, South China

Late Mesoproterozoic igneous rocks have been traditionally thought to mark Grenvillian orogens and are thus widely used for the reconstruction of Rodinia. In the southwestern Yangtze Block, South China, ∼1050Ma dacites and rhyolites in Huili and granites in Julin are rich in large ion lithophile elements (LILE; e.g., Rb, Ba, Th and U) and poor in Nb, Ta, and Ti relative to the primitive mantle. These felsic rocks contain similarly high Zr, Hf, Ga, and HREE contents and Ga/Al ratios, and have high calculated zircon saturation temperatures (∼900°C in average), thus resembling A-type granitoids. However, the dacites have whole-rock εNd(t) (−7.2 to −4.5) and zircon εHf(t) values (−20.8 to +1.0) much lower than the rhyolites/granites, and were interpreted to be derived from partial melting of an ancient continental crust. In contrast, rhyolites and granites have much higher εNd(t) (−0.4 to +1.8) and εHf(t) (−3.2 to +11.2) values, indicative of their derivation from primary magmas generated by partial melting of newly formed underplating crust, with possibly minor contributions from mantle-derived materials. These A-type felsic rocks have high Y (20–100ppm) and Nb (10–40ppm) contents similar to “within-plate” magmatic rocks in the frequently-used Y vs. Nb and Rb vs. Y+Nb plots for tectonic discrimination. Such a feature, in combination with rift-related, coeval mafic rocks in the region, indicates that they have formed in an intra-plate rifting setting. Our work thus argues against the existence of the so-called Grenvillian Orogen in the southwestern Yangtze Block. In view of the subsequent Neoproterozoic arc magmatism (950–730Ma) in the region, we propose that the ∼1050Ma A-type granitoids and coeval mafic rocks were produced under a continental rifting basin that was transformed to an compression setting during early Neoproterozoic.

Evidence for Ordovician subduction-related magmatism in the Truong Son terrane, SE Laos: Implications for Gondwana evolution and porphyry Cu exploration potential in SE Asia

The Truong Son Fold Belt (TSFB) is characterised by Late Carboniferous-Late Triassic metamorphic, volcanic and plutonic rocks, the product of accretion of the Indochina Terrane onto the South China Terrane and a range of composite subduction, collision and extensional events. This study discusses geochronological and geochemical data obtained from a dioritic intrusion and rhyolitic tuff mapped in the Donken area of SE Laos, and previously assigned to the Permian Antoum Granodiorite rock suite within the TSFB. Magmatic zircon U-Pb Q-ICP-MS dating undertaken in this study suggests ages of ca 470±2Ma for the diorite and ca 476±1.5Ma for a proximal rhyolitic tuff.Whole-rock geochemistry of both units suggests a subduction-related island arc environment, with calc-alkaline and tholeiitic affinities for the diorite and tuff respectively. The intrusion also exhibits an adakitic signature (high Sr, low Y and HREE contents) suggesting that Ordovician magmatism also occurred within the Indochina Terrane, associated with an enigmatic, early Gondwana subduction event. This intrusion appears part of a broader, bilateral Early Ordovician magmatism, newly linked to the south-east subduction of the Tamky-Phuoc Son Ocean underneath the Kontum terrane, and a north-west subduction beneath the Truong Son terrane. Significantly, sub-economic hydrothermal Cu mineralisation observed within the dioritic intrusion, hints at the presence of local Ordovician, porphyry-style base metal enrichment.

Geochronology and geochemistry of the TTG and potassic granite of the Taihua complex, Mts. Huashan: Implications for crustal evolution of the southern North China Craton

Tonalite-trondhjemite-granodiorite (TTG) suite and potassic granite volumetrically expose in the Neoarchean to Paleoproterozoic Taihua complex, Mts. Huashan region, southernmost segment of the Trans-North China Orogen (TNCO). Zircon U–Pb dating on the trondhjemite, granitic gneiss, K-feldspar granite and coarse-grained granite samples show three episodes of magmatism (2.55–2.49Ga, 2.33–2.25Ga and 1.87–1.80Ga) with distinct geochemical features in this area during the Paleoproterozoic. The trondhjemite samples have high SiO2 (67.46–71.73wt%), Na2O (4.83–5.95wt%), low Mg# (31–40) and HREE contents, with moderate (La/Yb)N values (16.8 to 35.3) and Sr/Y ratios (15.8 to 34). The potassic granite samples show high SiO2 (65.2–75.43wt%), K2O (3.9–8.6), low Na2O (1.74–4.17wt%), Mg# (2–46) and HREE contents, with moderate to high (La/Yb)N values (10.3 to 226) and Sr/Y ratios (12.8 to 208). All these granitoid samples are characterized by high SiO2 and low Mg#, Cr, Ni with moderate to high (La/Yb)N values and Sr/Y ratios. The absence of evidence of any pre-existing high Sr/Y and La/Yb sources in this region led to that the granitoids probably generated from partial melting of hydrous mafic rocks with garnet and amphibole in the residue. Therefore, partial melting of thickened lower curst is the most likely origin. Combined with previous studies, the first episodic magmatism (2.55–2.49Ga) represented by TTG and granitic gneisses with positive whole rock ƐNd(t) and zircon ƐHf(t) values is interpreted as melts from partial melting of juvenile thickened lower crust. The second episodic magmatism (2.33–2.25Ga) represented by TTG, granitic gneisses and K-feldspar granite with positive to negative whole rock ƐNd(t) and zircon ƐHf(t) values is suggested as the product of partial melting of both juvenile and pre-existing crustal material. The third episodic magmatism (1.87–1.80Ga) is represented by these potassic granites, which is synchronism with the metamorphism recorded in this region and probably resulted from partial melting of pre-existing crustal materials in a syn-orogenic or post-orogenic setting during the collision between the Eastern and Western Blocks of the North China Craton. Multistage continental accretion (at 2.84–2.72Ga, 2.57–2.43Ga and 2.36–2.25Ga) and reworking (at 2.36–2.25Ga, 2.19–2.07Ga and 1.87–1.80Ga) occurred in the southern segment of the TNCO from Neoarchean to Paleoproterozoic. Transformation from an Andean-type continental margin arc setting on the western margin of the Eastern Block to continent-arc-continent collision in the southern TNCO during Late Neoarchean to Late Paleoproterozoic is suggested in this paper.

Age and geochemistry of zircon from the oldest metamorphic rocks of the Omolon Massif (Northeast Russia)

This study provides SHRIMP-RG data on zircons from garnet gedritites, the products of retrograde metamorphism of eclogite-like rocks constituting belonging to the basement of the Omolon Massif. The earliest episode recorded by oscillatory-zoned cores having high HREE and Ti contents occurred at 3.25–3.22 Ga (Paleoarchean) and is interpreted to represent an upper age limit of a metamorphic or magmatic protolith. One zircon core with a pronounced negative Eu anomaly yielded a concordant age of 2.6 Ga, which is interpreted to mark a Neoarchean episode of granite formation. The studied population of zircons provides the most distinct record of a Paleoproterozoic (1.9 Ga) event, which is marked by formation of garnet gedritites under amphibolite-facies conditions. This event is recorded by transparent recrystallization rims of preexisting large zircon grains and small newly-formed grains, which are characterized, compared with their cores, by lower crystallization temperatures and one order of magnitude lower concentrations of U, Th, and HREE, and the presence of garnet micro-inclusions.

Geology, geochronology and geochemistry of granitic intrusions and the related ores at the Hongshan Cu-polymetallic deposit: Insights into the Late Cretaceous post-collisional porphyry-related mineralization systems in the southern Yidun arc, SW China

The Hongshan Cu-polymetallic deposit is located in the southern Yidun arc in southwestern China, where both subduction-related (Late Triassic) and post-collisional (Late Cretaceous) porphyry–skarn–epithermal mineralization systems have been previously recognized. In this study, two distinct magmatic events, represented by diorite porphyry and quartz monzonite porphyry, have been revealed in the Hongshan deposit, with zircon SHRIMP U–Pb ages of 214±2Ma and 73.4±0.7Ma, respectively. The 73Ma age is comparable to the Re–Os ages of 77 to 80Ma of ore minerals from the Hongshan deposit, indicating that the mineralization is related to the Late Cretaceous quartz monzonite porphyries rather than Late Triassic diorite porphyries. The Late Triassic diorite porphyries belong to the high-K calc-alkaline series and show arc magmatic geochemical characteristics such as enrichment in Rb, Ba, Th and U and depletion in HFSEs, indicating that they were formed during the westward subduction of the Garzê–Litang Ocean. In contrast, the Late Cretaceous quartz monzonite porphyries show shoshonitic I-type geochemical characteristics, with high SiO2, K2O, LILE, low HREE, Y and Yb contents, and high LREE/HREE and La/Yb ratios. These geochemical characteristics, together with the Sr–Nd–Pb isotopic compositions (average (87Sr/86Sr)i=0.7085; εNd(t)=−6.0; 206Pb/204Pb=19.064, 207Pb/204Pb=15.738, 208Pb/204Pb=39.733) suggest that the quartz monzonite porphyries originated from the partial melting of the ancient lower crust in response to underplating of mafic magma from subduction metasomatized mantle lithosphere, possibly triggered by regional extension in the post-collisional tectonic stage. The S isotopic compositions (δ34SV-CDT=3.81‰ to 5.80‰) and Pb isotopic compositions (206Pb/204Pb=18.014 to 18.809, 207Pb/204Pb=15.550 to 15.785, and 208Pb/204Pb=38.057 to 39.468) of ore sulfides indicate that the sulfur and metals were derived from mixed mantle and crustal sources. It is proposed that although the Late Triassic magmatic event is not directly related to mineralization, it contributed to the Late Cretaceous mineralization system through the storage of large amounts of sulfur and metals as well as water in the cumulate zone in the mantle lithosphere through subduction metasomatism. Re-melting of the mantle lithosphere including the hydrous cumulate zone and ancient lower crust during the post-collisional stage produced fertile magmas, which ascended to shallow depths to form quartz monzonite porphyries. Hydrothermal fluids released from the intrusions resulted in porphyry-type Mo–Cu ores in and near the intrusions, skarn-type Cu–Mo ores in the country rocks above the intrusions, and hydrothermal Pb–Zn ores in the periphery.

Origins of Early Mesozoic granitoids and their enclaves from West Kunlun, NW China: implications for evolving magmatism related to closure of the Paleo-Tethys ocean

Early Mesozoic granitoids and microgranular enclaves (MEs) are widespread in the West Kunkun, northwestern Tibetan plateau, and record the tectonic evolution of Eurasia–Tethys in this area. This study reports geochemistry, zircon U–Pb and Hf isotopic data for a suite of granitoids and their MEs from the Middle Triassic Bulunkou pluton (BP) and the Late Triassic Akazishan pluton (AP) from the West Kunlun. LA-ICP-MS U–Pb zircon dating of a sample from the BP host monzogranite and an enclave, as well as a AP monzogranite, yielded ages of 236 ± 2, 230 ± 7 and 208 ± 1 Ma, respectively. The BP monzogranite and its enclaves from the northwestern part of the West Kunlun are mainly weakly peraluminous granites characterised by relatively high Rb, Th, Rb/Sr (2.64–9.03) and HREE contents, and low Mg#, Sr/Y and negative Eu anomalies. Zircons from the BP monzogranite have eHf(t) values from −5.7 to −1.6. Zircons from enclaves of the BP show more variable eHf(t) values from −4.1 to 3.8. We consider that the BP granites are likely to have been formed by partial melting of metasedimentary rocks at shallow crustal depth, and their enclaves, composed of quartz + biotite + plagioclase + garnet + K-feldspar, are relics from the melting of a source at middle crustal depths. The AP host and its enclaves from the southeastern part of the West Kunlun have low Rb, Rb/Sr (0.15–1.90) and weakly negative Eu anomalies but high HREE contents indicating limited fractionation of plagioclase without residual garnet in their source. The inferred protolith is an intermediate igneous rock in the middle or lower crust. MEs hosted in the AP have high Mg# (39.7–45.0) and Nb and weakly negative Eu anomalies, as well as high Sr, P and Ti, corresponding to a medium-K basaltic rock, which may have originated from mixing of partial melting of metasomatised mantle wedge that has been modified by upwelling asthenospheric mantle and crustal melting in the deep source. Post-collisional southeasternward younging of Mesozoic granitoids in the West Kunlun records a transition from shallow level, mid-crustal melting to deeper level, lower-crustal melting. The post-collisional granitoids show significant mantle input increasing from northwest to southeast, which was closely associated with mafic magma-driven partial melting of mantle and crustal sources as a consequence of slab break-off. Syn- to post-collisional Early Mesozoic granites emplaced along the Mazar–Kangxiwar suture zone suggest that these tectonomagmatic processes followed the closure of the Paleo-Tethys ocean during the Tarim and Karakorum–Qiangtang continental collision.

Geochemical transition shown by Cretaceous granitoids in southeastern China: Implications for continental crustal reworking and growth

Zircon U–Pb ages and in-situ trace elements and Hf–O isotope compositions, together with whole rock geochemical and Sr–Nd–Hf isotopic data, are presented for Cretaceous granitoids in southeastern (SE) China in order to establish their origin and the evolution of the underlying lithosphere during the Late Mesozoic. Two stages of Cretaceous magmatism, with contrasting geochemical features, have been identified: an earlier adakite-like biotite granite as represented by the Shangying pluton and a later enclave-bearing monzogranite as represented by the Zaoshan pluton. The Shangying biotite granites have a zircon U–Pb age of 99±1Ma. They have relatively low Y and Yb contents, with high La/Yb and Sr/Y ratios, showing geochemical features of adakite. Their Sr–Nd–Hf isotope compositions are similar to those of Early Cretaceous mafic rocks in the same area, indicating that they were generated by partial melting of juvenile granulitic crust at a depth of about 40km. The source was formed by underplating of enriched lithosphere mantle-derived magmas. In contrast, the Zaoshan calc-alkaline monzogranites, their enclaves and associated dolerite dykes from the Zaoshan pluton have an emplacement age of ~88±1Ma. The dolerites have high MgO contents, relatively low SiO2 concentrations and low La/Yb ratios, and depleted Hf isotope compositions. All these geochemical features suggest that they were derived from a depleted spinel Iherzolite mantle source. The enclaves have high SiO2 contents, indicating that they were derived from a crustal source. They have variable zircon Hf and O isotope compositions, suggesting that two components, i.e., a high εHf(t) and a low δ18O component and a low εHf(t) and high δ18O component, were involved in their origin. The high zircon εHf(t) values and low δ18O values are similar to those of the dolerites, indicating a common source. Thus, we suggest that the enclaves were generated by partial melting of newly underplated depleted mantle-derived materials. The monzogranites have distinctly different zircon Hf and O isotope compositions from the enclaves, indicating that the parental magmas were mainly derived from ancient crust that interacted with underplated depleted mantle-derived magmas. The monzogranites have relatively high HREE contents, suggesting a garnet-free source (<32km), distinct from the Early Cretaceous adakite-like granites that were generated from a garnet-bearing source. Combined with previously published data, it is evident that Early Cretaceous adakite-like magmatic rocks (107–99Ma) and associated mafic rocks (100–107Ma) were widespread in SE China, indicating a crustal thickening event that was possibly induced by underplating of mantle-derived magma. Subsequently, between 99 and 87Ma, crustal extension and lithospheric thinning induced the later widespread I-type granites with similar geochemical features to the Zaoshan pluton. A-type granites and syenites of this age are also present in SE China. The transition in geochemical and isotopic data, from enriched to depleted Hf isotope compositions as seen in the monzogranites, their enclaves and intrusive dolerites, suggest that depleted mantle-derived materials were involved in the generation of the monzogranites, further indicating continental crustal reworking and later crustal growth in SE China during the early Late Cretaceous.

FLUORINE-RICH XENOTIME FROM THE WORLD-CLASS MADEIRA Nb-Ta-Sn DEPOSIT ASSOCIATED WITH THE ALBITE-ENRICHED GRANITE AT PITINGA, AMAZONIA, BRAZIL

The Madeira deposit (Sn, Nb, Ta) at Pitinga, Amazonia, in Brazil, is associated with the albite-enriched facies of the A-type Madeira granite (~1,820 Ma). Fluorine (cryolite), Y, REE, Li, Zr, and Th are potential by-products of the disseminated ore. We studied the xenotime from the core albite-enriched (CAG), transitional albite-enriched granite (TAG) and pegmatitic albite-enriched granite (PAG). In all these rocks, xenotime is magmatic, has high HREE contents (0.37 to 0.54 atoms per formula unit; LREE are not detected), and low U, Th and Ca content. The CAG xenotime is the richest in REE, followed by that in the PAG and TAG units. Fluorine was detected in all xenotime crystals from CAG (up to 5.10%) and from PAG (up to 1.40%) and in most crystals from TAG (up to 0.68%). Where the xenotime is richer in F, the Na and Si increase, the P, Ca, and the effectiveness of a thorite-type substitution decrease. Fluorine controls the ratio REE/Y; the richer the xenotime in F, the richer it is in REE, in particular Er and Yb. The cell parameters a and c are significantly shortened in F-rich xenotime. There is no evidence of OH in the structure. We contend that fluorine substitutes for O to form PO3F tetrahedra, as in bobdownsite. The formation of the PO3F tetrahedra in the xenotime accounts well for the compatibility between the crystallographic characteristics and the chemical composition.