Origin Of Granites Research Articles

The origin of alkali granites and Th-U ± REE enrichments in Kestanbol Magmatic complex (NW Anatolia) revisited: Evidences from bulk-rock geochemistry and isotopic data, zircon U[sbnd]Pb, biotite Ar/Ar and apatite (U[sbnd]Th)/He geochronology

This paper presents new field, petrographic, geochemical, SrNd isotopic and geochronological data from Kestanbol Magmatic Complex (KMC) in the western Anatolia. Zircon UPb ages from the KMC were in the range 21.91 Ma and 21.52 Ma, indicating Miocene emplacement. 40Ar/39Ar dating results of biotites from the same samples show a narrow range of ages between 20.0 and 22.7 and a weighted mean of 21.41 ± 0.40 Ma, and those of hornblende analysis yield ages between 21.52 and 31.19 Ma with a weighted mean of 22.70 ± 0.99 Ma, are interpreted as the cooling age of the KMC. The average (U-Th/He) ages from the KMC yielded an average of 21.5 Ma and 19.8 Ma. These new age data indicate rapid cooling following the emplacement of the KMC at ∼21 Ma. We suggest that the cooling was due to rapid uplift in the western Anatolia. The studied monzonitic, syenitic and alkaline subvolcanic rocks of the northern KMC are characterized by high K2O (4.34–10.7 wt%), low to moderate SiO2 (50.0–69.9 wt%), and P2O5 (0.03–1.07 wt%). They have moderate initial 87Sr/86Sr (0.707245–0.707875) and high initial 143Nd/144Nd (0.512441–0.512508) ratios, consistent with some crustal contamination. The studied rocks are enriched in Th (up to 204 ppm), U (up to 54.9 ppm), REE (up to 565.9 ppm) and, some LILE's including K (up to 8.85%), Rb (up to 447.1 ppm), Sr (up to 2053 ppm) and Ba (up to 2578 ppm). The geochemical and isotopic data suggest that the magmatic evolution of KMC is dominated by events including post-collisional tectonics, flux induced partial melting, fractional crystallization. The enrichments of incompatible elements are mostly caused by the fractional crystallization and K-metasomatism that affected the earlier magmatic phases during the cooling of the complex.

The late Archaean granite paradox: A case study from the Zimbabwe Craton

Late- to post-tectonic high-K granites are found in many Archaean cratons and are thought to be the product of a major, crustal-scale melting event in the underlying TTG crust leading to the stabilisation of the craton. However, despite the TTG melting model being an obvious explanation for the origin of late-Archaean high-K post-tectonic granites, experimental studies show that TTGs are insufficiently fertile to produce large volumes of potassic granites. This is the late Archaean granite paradox. Here we argue that the paradox can be resolved if the TTG protolith is more potassic than might be expected from a straightforward partial melt of an Archaean basalt. We propose that a likely fertile protolith for late Archaean granites is TTG crust which has incorporated a partial melt of older felsic crust during its emplacement. This hypothesis is validated with a case study from the Neoarchaean rocks of the Zimbabwe Craton.This paradox reflects a more fundamental problem when considering the origin of Archaean TTGs, for the ‘enriched’ basaltic protolith invoked in many models of TTG genesis is not abundant in Archaean terrains, nor should it be if the basaltic protolith is a melt of primitive or depleted mantle. This means that fertile, K-rich, TTGs are not simply the product of the melting of a basaltic protolith, but involve an additional process. Three models of TTG petrogenesis are discussed which might lead to K-enrichment in the melt – the hydrothermal potassic enrichment of the basaltic protolith and the influences of fractional crystallisation and/or crustal contamination on the TTG magmas. We conclude that to produce a sufficiently fertile, K-rich TTG source in the Zimbabwe Craton the contribution of a melt phase from older TTG crust is most consistent with the major and trace element and isotopic geochemistry.

Provenance, diagenesis, and depositional environment of Miocene Kamlial Formation, Azad Jammu and Kashmir, Sub Himalayas, Pakistan: Evidences from field observations and petrography

Petrographical characterization and field observations were caried out to evaluate Kamlial Formation in Bagh district, Azad Jammu and Kashmir. Based on detailed petrography, the lithic arenite consisted of quartz (20–25%), feldspar (7–11%), rock fragments (20–37%), cementing materials (11–21%), and accessory minerals. Grains are mostly angular to subrounded and poorly to moderately sorted. The analysis revealed that the lithic arenite is mineralogically immature; also, the current activity during the time of deposition was low. Polycrystalline quartz indicates that the sandstone was derived from metamorphic source, while monocrystalline quartz indicates a granitic origin. Quartz having an angular shape suggests the source rock was near the depositional site, while quartz having a rounded shape represents long transportation. The presence of feldspar in the lithic arenite suggests the rocks were deposited at high relief or cold temperatures. Primary porosity in sandstone was reduced by calcite cements around the grain, while secondary porosity was developed by fracturing of quartz and feldspar. Tectonic uplift in the study area was demonstrated by fractured quartz and mica in thin sections. Field observations of various sedimentary structures were observed such as load casts, ripple marks, and mud cracks, etc. The presence of conglomerates and load casts in the study area indicates that the Kamlial sandstone was deposited by fluvial and shallow marine environment. Furthermore, the ripple marks indicate that the tidal flat environment controlled the deposition of the sediments.

Origin and Tectonic Implications of I-type Granites, North Delhi Fold Belt, NW, India: Insights from Whole Rock Geochemistry and Mineral Compositions

Abstract A comprehensive whole-rock geochemical and mineral chemistry study was carried out of granites from the Narnaul and surrounding isolated hills of North Delhi Mobile Belt, NW India to constrain their genesis, and tectonic environment. The distinctive features of granites are meta to peraluminous nature and decreasing trend of P2O5 with high SiO2 (>71%) ascribed to its metaigneous origin. In detail the chemical diversity of the studied samples is reflected by their affinity to Calc-alkaline to shoshonite series with high K2O/Na2O (mostly>1), low MgO (< 0.11%) and variation in Mg# (upto 0.5) and maficity with increasing SiO2. The positive Eu anomalies, high LILE, highly enriched LREE (La/Sm = 4.13–19.33) pattern and comparatively elevated concentrations of Cs, La, Ta, Dy, Zr, Pb and Yb suggest different episodes of magmatic differentiation. Our data indicate that melting of older subduction-related igneous rocks and the underwent fractional crystallization in hydrous conditions is the possible mechanism for the generation of I-type granites in collisional tectonic setting.

Origin of Silurian granite in the Baiganhu field, Eastern Kulun Terrane, NW China: Implications for the tectonic setting of Sn-W mineralization

The Baiganhu area in the Eastern Kunlun Terrane on the northern margin of the Tibetan Plateau contains world-class Sn-W deposits in close association with monzogranite. However, the origin of granite and the tectonic setting for the Sn-W mineralization have not been well constrained. Here we present the petrological and geochemical data of the granite (431 Ma) associated with the Sn-W mineralization to evaluate the geological-tectonic setting for the mineralization. The monzogranite has high SiO2 contents, ranging from 65.8 to 73.5 wt%, and shows a geochemical affinity with A2-type granites. Relatively high zircon saturation temperatures, ranging between 750 °C and 921 °C, suggest that the parental magmas were hot. The whole-rock εNd (t = 431 Ma) values range from –2.7 to –3.1 with TDM ages between 1810 and 1040 Ma, and zircon εHf (t = 431 Ma) values range from + 0.7 to + 4.1 with TDM ages between 1040 and 900 Ma. The data suggest that the magmas had significant contributions of old rocks. The granitic rocks intruded into quartz-muscovite schist of the Proterozoic Xiaomiao Complex, which underlies the vast area. Zircon grains in the schist are magmatic in origin based on sharp oscillatory zoning and high Th/U ratios (>0.2). The 207Pb-206Pb zircon ages (n = 25) of the schist range from 1898 ± 2 to 1700 ± 6 Ma, suggesting that the zircon grains originate from a Paleoproterozoic magmatic belt. Such ancient rocks, either as lower crustal component or subcontinental lithospheric mantle, contributed ∼ 30 % to the parental magmas. We conclude that post-collisional extension within the Eastern Kunlun Terrane was accompanied by upwelling and adiabatic melting of the asthenospheric mantle which induced partial melting of subcontinental lithospheric mantle and lower crustal rocks and formed granitic magmas. Aqueous fluids separated from the granitic magma which preferentially incorporate W and Sn contributed to the mineralization in and around the granites.

Diachronous slab-driven Early Cretaceous crustal extension and pulsed magmatic response in southeastern China: Insight from A-type granite and related rocks

The Late Mesozoic magmatic belt in southeastern (SE) China, stretching over 1300 km, resulted from southwestward subduction of the paleo-Pacific plate. Our study focuses on the origin of Early Cretaceous syenite and A-type granite in SE China, utilizing zircon U-Pb and Hf isotopic analyses, bulk-rock compositions, and Nd-Hf isotopic compositions. Findings indicate that ∼ 142 Ma silica-unsaturated syenites in the southwest form through small degrees of partial melting of the metasomatized lithospheric mantle, while ∼ 142 Ma A-type granites originate from partial melting of juvenile felsic rocks. The ∼ 130 Ma A-type granites in the northeast result from mixing of ancient crustal-derived magma and mantle-derived magma. The southwest and northeast regions display distinct magmatic histories, with flare-ups and lulls occurring at different times. These variations suggest diverse extensional and compressional histories. A spatially migrating extensional regime is observed from the southwest (∼147 Ma) to the northeast (∼120 Ma) of SE China. Magmatic flare-ups in the southwest involve reworking juvenile crust, while in the northeast, they modify ancient crust, likely related to paleo-Pacific plate subduction and northeastward slab rollback. Slab subduction rate correlates strongly with observed geological events, indicating diachronous crustal extension, magmatic flare-ups, and lulls during ca. 147–110 Ma due to northeastward slab rollback and changing subduction rate.

The Sea Point contact, Cape Town, South Africa: a geological site made famous by Charles Darwin's visit

Abstract The Sea Point intrusive contact on the Atlantic coast, South Africa, is a site of historical significance that continues to be useful for studying the details of granite–wall rock relationships, some of which have not been properly evaluated yet. At a time when the origin of granites was still controversial, and the concept of contact metamorphism not yet properly developed, Charles Darwin visited the Sea Point contact to make some geologically important observations. He concluded that the granite originated as a magma deep in the Earth, intruded a country rock of sedimentary origin and changed this country rock's appearance owing to the thermal action of the hot granite magma. He also contemplated the long periods of time required to expose the rocks by erosion. Besides its historic value, the Sea Point heritage site exposes unique features giving rare insights into the physical and chemical interaction between intruding granitic magma and a regionally low-grade metamorphosed sedimentary sequence. Questions regarding mechanisms of magmatic emplacement may be answered from this outcrop, including emplacement dynamics and interrelationships of various granitic phases, pre- versus syn-intrusive deformation and metamorphism, assimilation of country rock by a granitic magma and the origin of feldspar phenocrysts seemingly ‘stranded’ in the country rock.

The origin of high-silica granites and rare metal mineralization: Insights from geochemistry and U-Pb-Sr-Nd-Hf-O isotopes of Early Cretaceous granitoids in the southern Great Xing’an Range, NE China

The origin of high-silica granites and rare metal mineralization: Insights from geochemistry and U-Pb-Sr-Nd-Hf-O isotopes of Early Cretaceous granitoids in the southern Great Xing’an Range, NE China

Paleocene A-type igneous suites in the Sikhote-Alin (the East Asian continental margin): Petrological, geochronological, isotopic, and geodynamic constraints

Global geodynamic reconstruction models of the Pacific margin of Asia refer to materials collected throughout Japan, Korea and northeastern China, but they lack data on southern Sikhote-Alin. Therewith, the Sikhote-Alin orogenic belt (NE Asia) constituted a single eastern margin of the paleo-Asian continent with the abovementioned territories in the Paleocene. New isotopic, geochemical, and geochronological data show that Paleocene igneous activity (∼61–55 Ma) is widely developed in southern Sikhote-Alin. Bulk rock compositions of the igneous rocks of this period yield ferroan, peraluminous, calc-alkaline to alkaline affinities, highly abundant LILE and HFSE (with pronounced Ba, Sr, Eu, and Ti negative anomalies) and depleted HREE. The initial melts, displaying Zr + Y + Ce + Nb > 350 ppm and 10,000 × Ga/Al > 2.6, derived from an OIB-like mantle source crystallised under fairly reducing conditions (below FMQ buffer), and yield high temperature of zircon saturation (>850 °C), indicating typical A-type granite and related volcanic rock features. It is logical to associate variations in A-type rock geochemical composition with an enrichment of the upper part of the magma chamber with fluid-mobile components involving a redistribution of major and trace elements through fluid-magmatic differentiation. Strong depletion in Ca, Mg, Ba, Sr, Eu in the A-type rocks is caused by an outflow of these elements by an oxidizing, initially reduced, acidic fluid beyond the zone of magma generation. Whole-rock Sr-Nd isotope data argue for the generation of the A-type rocks by melting of dominantly a juvenile mantle component with a subordinate component of the ancient crust. Isotope variations of igneous rocks of the reference area: 87Sr/86Sr(t) (0.7024–0.7118), εNd(t) (−0.9 to −5.1) and TDM2 (934–1277 Ma), result from the mixing of the OIB-like mantle source with selective melts or from the metapelite contamination of the Samarka terrane accretionary prism and of the Zhuravlevka-Amur turbidite basin, later followed by fluid-magmatic differentiation that led to the formation of anatectic or hybrid melts. We further suggest that the origin of the A-type granites and related volcanic rocks is the result of the oblique interaction of oceanic and continental plates. This interaction accounts for the simultaneous formation of tears in the slab, enabling sub-slab asthenospheric upwelling, and strike-slip fault-related extensional structures in the overriding continental plate.

Boron and Molybdenum Isotope Evidence for Source‐Controlled Compositional Diversity of Cenozoic Granites in the Eastern Tethyan Himalaya

AbstractThe origins of Cenozoic granites in the Himalaya are key to understanding the evolution of the Himalayan orogen. However, it is unclear whether these granites represent primary melts, and the nature of their magma source is controversial. Here, we present a systematic element and Sr–Nd–B–Mo isotope study of Cenozoic granites from the Yardoi area in the eastern Tethyan Himalaya, China. These granites can be divided into two groups: mid‐Eocene porphyritic two‐mica granites with low SiO2 contents (65.9−69.6 wt.%) and adakitic geochemical signatures, and mid‐Eocene to Miocene equigranular granites with high SiO2 contents (71.6−75.5 wt.%). The high‐SiO2 granites (HSG) have similar Sr−Nd isotope compositions to the low‐SiO2 granites (LSG), but they have distinct δ11B values of −19.4‰ to −11.4‰ and −10.6‰ to −6.89‰. This indicates that the two groups have different sources, with the LSG derived by partial melting dominantly of metamafic rocks at thickened lower crustal conditions, and the HSG generated by partial melting of the mid‐crust metasedimentary rocks with less enriched Nd isotope compositions. The δ98/95Mo of the LSG and HSG are highly variable with values of −0.68‰ to 0.12‰ and −1.13‰ to 0.46‰, respectively. δ11B values of the HSG correlate positively with δ98/95Mo and Sr/Y values and correlate negatively with K2O, Rb, Zr, and Rb/Sr, reflecting the addition of external metamorphic fluids during anatexis of the metapelites. The B–Mo isotope data robustly suggest source‐controlled compositional diversity of the Himalayan granites, which could provide clues to the physical and geochemical responses during the evolution of a large orogen.

Understanding the provenance and depositional conditions of Triassic sedimentary rocks from the Spiti region, Tethys Himalaya, India

The Spiti region, renowned as the Museum of Indian Geology, is a world-famous sedimentary succession containing well-exposed sequences from Neoproterozoic to Cretaceous age. In this study, Triassic siliciclastic sedimentary rocks of the Lilang Supergroup were chosen to understand weathering history, provenance, paleoclimate, and depositional conditions using a geochemical and isotopic approach. Triassic shales show more or less similar compositions with substantial enrichment in CaO compared to PAAS (Post Archean shales from Australia), which may be attributed to the association with limestones in the region. However, the sandstones display significant depletion in the trace element concentrations signifying the effect of quartz dilution. The relative depletion of mobile elements (Rb, Ba) as against immobile elements (Zr, Nb, Hf) can be noticed in the trace element spider diagram of the shales. The Triassic sedimentary rocks are characterized by enriched LREE and depleted HREE patterns with pronounced negative Eu anomalies. The Chemical Index of Alteration (CIA; 56–86) indicates low to intense chemical weathering in the source area. The unusual decrease in CIA and other weathering indices in the stratigraphically up section is attributed to changes in climate and environmental conditions during the deposition of sediments in the Triassic period. Detangling the signatures is crucial to understanding the mass extinction crisis, particularly the role of anoxia in these events. Triassic black shales represent suboxic to anoxic depositional conditions in the redox-sensitive elemental binary diagrams. The carbon isotope data of the present study is very well supported by the Total Organic Carbon (TOC), which infers that the oceanic biological system tried to recover from the depletion of biological life. The εNd and 87Sr/86Sr systematics record a shift in source terrains from the Early to Late Triassic period. The Early Triassic samples show much older depleted mantle model ages (TDM = 1.94–1.98 Ga) compared to Late Triassic sediments (TDM = 1.76–1.91 Ga). Similar interpretations can be drawn from Th/Sc ratios (from ∼ 6 to ∼ 0.05) and (La/Yb) N ratios (from ∼ 32 to ∼ 5), which record an increase in these ratios from Early Triassic to Late Triassic formations of the Spiti sedimentary rocks. Overall, trace elemental ratios and radiogenic isotopic signatures of the Triassic rocks of the Spiti region point towards Pan African granitic origin with minor impressions from the juvenile mafic-rich sources, such as Panjal Traps, the African craton, and Arabian-Nubian shield.

Zircon reveals crystal-melt segregation processes in the genesis of Cretaceous A-type granites and related cumulated rocks in southeastern China

A-type granites are a distinctive group of igneous rocks classified based on chemical and mineralogical criteria. Although numerous theories for the origin of A-type granite have previously been proposed, their genesis remains highly controversial. In the current study, we present bulk-rock geochemistry and zircon UPb isotopes, Hf isotopes, and trace elemental data for a suite of quartz monzonite, porphyritic granite, and miarolitic alkali feldspar granite from the coastal area of Fujian, southeastern China, to evaluate the genetic link between these rocks. Zircon UPb dating indicates that these rocks were crystallized at ∼94–102 Ma. Our data strongly suggests that lithological differences in these rocks are the result of crystal-melt segregation in the shallow crust. The quartz monzonites are enriched in Sr and Ba, with high Zr/Hf and low silica content and a weak negative Eu anomaly, representing the cumulate residue from the crystal-melt segregation of a magma reservoir. A limited fraction of zircon from the quartz monzonite has evolved trace element signatures (high Hf, Nb, Y, U; low Eu/Eu⁎), indicating that the quartz monzonite represents a mixture of accumulated crystals and high silicic melt trapped in cumulate mush. The miarolitic alkali feldspar granites and porphyritic granites are enriched in silica and Rb, and depleted in Sr and Eu. They also display low whole-rock Zr/Hf and Eu/Eu⁎ ratios, and high Rb/Sr ratios, representing highly evolved silicic melts that were segregated from a magma reservoir. The majority of the zircon grains from the miarolitic alkali feldspar granites exhibit the most evolved trace element signatures (high Hf, Nb, Y, U; low Zr/Hf and Eu/Eu⁎) of the entire suite. This reveals that these zircons crystallized from the high silicic melts extracted from a magma reservoir. Magma recharge and the exsolution of volatiles from the interstitial melt have promoted the segregation and upward extraction of high silica magmas from a compacting magma reservoir in the upper crust. Our work demonstrates that the miarolitic alkali feldspar granites with A-type granite features from the coastal area of southeastern China were generated by the crystal-melt segregation process in a shallow crustal magma reservoir.

Experimental Crystallization of the Beauvoir Granite as a Model for the Evolution of Variscan Rare Metal Magmas

Abstract New experiments on the strongly peraluminous, Li-, P- and F-rich Beauvoir granite (Massif Central, France) provide a basis for understanding crystallization and chemical fractionation in Variscan rare metal magmas. Crystallization experiments were performed on two natural granite compositions under H2O-saturated conditions at 100, 200, and 300 MPa, from 540°C to 700°C and between ~NNO + 3 and NNO-1.4. Experimental charges were examined by SEM and their products (glasses and crystals) analyzed for major elements by EMPA. Trace element concentrations in selected glasses were determined by LA ICP-MS. Despite experimental durations commonly exceeding 1000 h and some up to 4000 h, kinetic problems were encountered in particular in the 100 MPa charges whereas, at 200 and 300 MPa, results consistent with previous melting experiments were obtained. Beauvoir melts crystallize quartz, plagioclase, K-feldspar and mica as major phases. At NNO-1.4, mica is a biotite, whereas it is a Li-mica between ~NNO+3 and NNO-1. Apatite, Fe-Ti oxides, either hematite or magnetite, topaz, amblygonite, cassiterite and columbite-tantalite appear as accessory phases between ~NNO + 3 and NNO + 1. Experimental plagioclases are albitic (An <4.5 mol%) and more Ca and K-rich than natural albites in the granite whereas experimental K-feldspars are more sodic (Ab <45 mol%) than the natural crystals. The less evolved starting melt crystallized Li phengites whereas the most evolved yielded Li-, F-rich micas near the polylithionite-zinnwaldite series, similar to natural micas in the granite. Equilibrium crystallization increases A/CNK, F and P and concentrates Li, Be, B, Rb, Cs, W, U in the melt. Nb and Ta are also enriched, their behavior being controlled by the solubility of columbite-tantalite in the melt. Other elements are either unchanged (Mn, Zn, Ti) or depleted (Sr, Pb) during magmatic fractionation. Sn is concentrated in Li-mica and hematite, and it behaves compatibly at high fO2. Beauvoir melts crystallize at very low temperatures, below 670°C for the two compositions studied and solidus temperatures, determined from previous melting experiments and confirmed by the new crystallization experiments, are near 550°C. The experiments demonstrate that most of the mineralogical and geochemical characteristics that make the Beauvoir granite distinctive result from magmatic rather than hydrothermal post-magmatic processes. Albitic plagioclase, Li-mica, topaz, and amblygonite are of magmatic origin. Glass major element compositions suggest that the two granite samples represent crystallized liquids. Trace element fractionations for most elements at Beauvoir can be accounted for by magmatic crystallization–differentiation processes. Implications for the mineralogy, fO2, volatile concentrations, crystallization and conditions of emplacement, fractionation mechanisms and origin of the Beauvoir granite are discussed.

Contrasting origins of A-type granites in the Late Triassic-Early Jurassic Pitou complex, southern Jiangxi province: Implications for Mesozoic tectonic evolution in South China

The origin of A-type granites is purely circumstantial. Granitic complexes are quite common and generally composed of different type granites. I- and A-type granite association in a granitic complex is often described in the literature. Here we present a granitic complex, i.e. the Pitou complex in southern Jiangxi province, South China that is composed of different varieties of A-type granites. Detailed zircon LA-ICP-MS U Pb chronology and in situ Hf isotope, mineral chemistry and whole-rock element and Sr Nd isotope data of this granitic complex are used to explore their origin and to further constrain the geodynamics of Mesozoic magmatism in South China. Our new data indicate that the Pitou complex was emplaced during the Late Triassic (237 Ma) (central to northern part) and Early Jurassic (188–185 Ma) (southern part), respectively and that both the Late Triassic and Early Jurassic granites belong to A-type but show contrasting origins. The Late Triassic granites show an association of alkali-feldspar granite-syenogranite-monzogranite and the rock type of the Early Jurassic granites is alkali-feldspar granite. All the granites are composed mainly of K-feldspar, quartz, plagioclase and biotite (ferri-biotite or annite) with biotite occurring as anhedral crystal, interstitial to quartz and feldspar. Both the Late Triassic and Early Jurassic granites have high SiO 2 (>70 wt%) and total alkalis (K 2 O + Na 2 O > 8 wt%) with high FeO T /(FeO T + MgO) and low Mg#. They are enriched in rare earth elements (except Eu) and depleted in Sr and Ba. They display Zr + Y + Ce + Nb >350 ppm, 10,000 × Ga/Al >2.6 and have high zircon saturation temperatures (>800 °C). Biotites in the granites are Fe-rich with very low Fe 3+ /Fe 2+ , indicating fairly reducing conditions. However, the Late Triassic and Early Jurassic granites also show some geochemical differences. The Late Triassic granites have lower FeO T /(FeO T + MgO) and higher Mg# than the Early Jurassic granites, with the Mg# of the former consistent with pure crustal melts and the Mg# of the latter much lower than pure crustal melts. In addition, the Late Triassic granites have lower zircon saturation temperatures (up to 843 °C, average 823 °C) than the Early Jurassic granites (up to 946 °C, average 875 °C). Furthermore, the Late Triassic granites have high 87 Sr/ 86 Sr (t) (0.7132–0.7226) and negative ε Nd (t) (−10.3 to −10.9) and ε Hf (t) (in situ zircon) (−11.5 to −11.6) that are similar to those of the early Paleozoic granitoids in the region whereas the Early Jurassic granites have low 87 Sr/ 86 Sr (t) (down to 0.7034) and positive ε Nd (t) (up to +3.0) and ε Hf (t) (in situ zircon) (up to +8.3) that are similar to those of the Early Jurassic gabbroic rocks in the region. Geochemical data and major element modeling suggest that the Late Triassic A-type granites were formed by shallow (ca. 15–20 km depth) dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas and the Early Jurassic A-type granites were produced by extensive fractionation (ca. 90% to 95%) from gabbroic magmas by removal of plagioclase, amphibole, K-feldspar, clinopyroxene and magnetite. We further suggest the origin of the Late Triassic A-type granites was related to the transtension of the regional NE–trending strike-slip faults caused by the early Indosinian continental collisions and the Early Jurassic A-type granites were emplaced in a back-arc extensional setting coupled with the slab rollback and subsequent slab break-off of the subducted Palaeo-Pacific plate. Our new data confirm that the early Mesozoic magmatism in South China was controlled by continental collisions during the Late Permian to Triassic amalgamation in Southeast Asia whereas the late Mesozoic magmatism was related to the northwestward subduction of the Palaeo-Pacific plate. A transition in the dominant tectonic regime from the Tethyan to the Pacific tectonic domain most likely took place during the Rhaetian. • The Pitou granitic complex emplaced during the Late Triassic and Early Jurassic. • Both Late Triassic and Early Jurassic granites are A-type but show contrasting origin. • Late Triassic granites formed by shallow dehydration melting of Paleozoic granitoids. • Early Jurassic granites formed by extensive fractionation from gabbroic magmas. • Origin of both A-type granites suggests different geodynamic mechanisms.

Identification and quantification of chemical reactions in a coastal aquifer to assess submarine groundwater discharge composition.

In coastal aquifers, two opposite but complementary processes occur: Seawater intrusion (SWI), which may salinize heavily exploited aquifers, and Submarine groundwater discharge (SGD) which transports oligo-elements to the sea. Aquifers are expected to be chemically reactive, both because they provide abundant surfaces to catalyze reactions and the mixing of very different Fresh Water (FW) and Sea Water (SW) promote numerous reactions. Characterizing and quantifying these reactions is essential to assess the quality and composition of both aquifer water, and SGD. Indeed, sampling SGD is difficult, so its composition is usually uncertain. We propose a reactive end-member mixing analysis (rEMMA) methodology based on principal component analysis (PCA) to (i) identify the sources of water and possible reactions occurring in the aquifer and (ii) quantify mixing ratios and the extent of chemical reactions. We applied rEMMA to the Argentona coastal aquifer located North of Barcelona that contains fluvial sediments of granitic origin and overlies weathered granite. The identification of end members (FW and SW) and the spatial distribution of their mixing ratios illustrate the application procedure. The extent of reactions and their spatial distribution allow us to distinguish reactions that occur as a result of mixing from those caused by sediment disequilibrium, which are relevant to recirculated saltwater SGD. The most important reaction is cation exchange, especially between Ca and Na, which promotes other reactions such as Gypsum and Fluorite precipitation. Iron and Manganese are mobilized in the SW portion but oxidized and precipitated in the mixing zone, so that Fe (up to 15 μEq/L) and Mn (up to 10 μEq/L) discharge is restricted to SW SGD. Nitrate is reduced in the mixing zone. The actual reaction amounts are site-specific, but the processes are not, which leads us to conjecture the importance of these reactions to understand the SGD discharge elsewhere.

U-Pb geochronology of the Silurian-Devonian Bega Batholith, south-eastern Australia: Insights into the origin and development of I-type granites

Earth’s continental crust is primarily composed of buoyant, felsic igneous (granitic) material that has been extracted from the mantle along convergent plate margins. Establishing the timescales of subduction-related granite batholith assembly is key to understanding the processes of magma generation, segregation, and transport that define the dynamics and rates of crustal growth. The ‘Cordilleran’ Bega Batholith is the largest I-type granite batholith in the Lachlan Fold Belt (LFB) of south-eastern Australia. The product of accretionary orogenesis along the Terra Australis Orogen, it has been extensively characterised by isotopic and geochemical studies yet lacks a robust geochronological framework. We present new ID-TIMS and SHRIMP U-Pb zircon ages for twenty-two plutonic rocks of the Bega Batholith. This geochronological survey indicates a prolonged duration for I-type magmatism, with crystallisation ages ranging from ca. 420 Ma in the west, to 385 Ma in the eastern Bega Batholith. These I-type granites collectively occupy ∼90,000 km3 and were emplaced over a period of ∼20–35 Ma, corresponding to a magma production rate of ∼8–15 km3/Ma km−1, considerably lower than that of S-type granites of the region and other subduction-related granite terranes globally. Fundamentally, the temporal overlap between the Bega I-type granites and S- and A-type granites also argues against a hiatus between each phase of granite magmatism in the LFB, suggesting a continuum between the granite types and derivation from common source components. This new geochronological data provides the first comprehensive compilation of U-Pb ages from across the Bega Batholith, establishing initial constraints on the development and evolution of I-type magmatism in south-eastern Australia. Our results demonstrate that I-type granite petrogenesis is linked with crustal growth processes, requiring the modification of global crustal growth curves during the Phanerozoic to account for new continental crust generated within I-type granite terranes of the Paleo-Pacific Gondwanan margin.

Geochemical and Zircon Hf-O Isotopic Constraints on the Origin of Wulian A-Type Granite in Shandong Peninsula, Eastern China

Geochemical and Zircon Hf-O Isotopic Constraints on the Origin of Wulian A-Type Granite in Shandong Peninsula, Eastern China

Origin of Tibetan post-collisional high-K adakitic granites: Anatexis of intermediate to felsic arc rocks

Abstract Potassium-rich adakitic rocks have been used to infer high-pressure (HP) melting of thickened or foundered mafic lower crust in post-collisional settings. However, their origin remains debated because of their potassic rather than sodic nature. We address this debate by investigating the source of post-collisional Oligocene high-K adakitic granites in southern Tibet, which are widely attributed to melting of overthickened mafic lower crust. Our new data provide evidence for the generation of these high-K adakitic granites by anatexis of exposed migmatitic intermediate to felsic arc orthogneisses. These granites contain high-Th/U Oligocene magmatic zircons (30−22 Ma), and inherited zircons (66−48 Ma) with the same εHf(t) (0 to +12) as the coeval migmatites. The migmatites have in turn low-Th/U metamorphic Oligocene zircon rims around 66−48 Ma magmatic zircon cores recording the anatectic event at 29−25 Ma. Phase equilibrium modeling yields an anatectic temperature of ~740 °C and pressure (P) of ~0.9 GPa and shows that garnet is stable at P > 0.7 GPa in the melt-present field. The results indicate that high-K adakitic magmas are derived from melting of older intermediate to felsic arc rocks at intermediate pressures, in the garnet stability field, without any involvement of HP melting of metabasaltic rocks. We propose that hybridization between such purely crustal magmas and subcontinental lithospheric mantle–derived shoshonitic magmas results in the hybridized post-collisional Oligocene–Miocene mafic microgranular enclave–bearing potassic adakitic granitic rocks in southern Tibet.

New geochronological and geochemical constraints on the Banke, Dutsen Wai, Guraka, and Zuku alkaline granite complexes from the northcentral Nigerian younger granite province

ABSTRACT An integrated study was carried out on four anorogenic syenite-granite complexes (i.e. Banke, Dutsen Wai, Guraka, and Zuku) from the Nigerian Younger Granite (NaYG) province, in a bid to constrain their magma sources, rare metal mineralization potentials, and tectonic implications. These four complexes primarily consist of quartz syenite, granite porphyry, arfvedsonite aegirine granite, and biotite granite. The studied rocks are chemically characterized by high alkalis, HFSEs, Ga/Al ratios, and zircon saturation temperature (776–1127°C, average = 932°C), typically of A1-type granites. The zircon εHf(t) values of the arfvedsonite aegirine granite (−6.0 to −0.79) are higher than those of the granite porphyry (−9.57 to −7.67) and biotite granite (−9.49 to −5.86) suggesting moderate crustal contamination of mantle-derived melts for the origin of the peralkaline granites. Generation of the biotite granite likely involved inputs of both crustal- and mantle-derived melts and is evidenced by the presence of inherited Pan-African (600–610 Ma) zircons and low whole rock Nb/U and Nb/Ta ratios. Furthermore, input of crustal material likely increased the metal budget of the residual melt and enhanced Nb, Sn, and sulphide mineralization in the Banke granite porphyry and biotite granite and Dutsen Wai biotite granite. Zircon U-Pb dating results constrain the emplacement ages to early Mesozoic era, from the Banke granite porphyry of 180 ± 1 Ma, Dutsen Wai biotite granite of 177 ± 3 Ma, Guraka arfvedsonite aegirine granite of 227 ± 2 Ma and Zuku arfvedsonite granite of 186 ± 2 Ma. These age data indicate a local E-W age migration pattern for the alkaline magma emplacement in northcentral NaYG province. Integrated with other geological evidence, we favour a formation model for the anorogenic NaYG province likely driven by the reactivation of ancient shear zones and structures.

Contrasting Origins of A-Type Granites in the Late Triassic-Early Jurassic Pitou Complex, Southern Jiangxi Province: Implications for Mesozoic Tectonic Evolution in South China

Contrasting Origins of A-Type Granites in the Late Triassic-Early Jurassic Pitou Complex, Southern Jiangxi Province: Implications for Mesozoic Tectonic Evolution in South China