Bulk-rock Geochemistry Research Articles

Using coupled bulk-rock geochemistry and short-wave infrared (SWIR) spectral reflectance data as rapid exploration tools in metamorphosed VHMS deposits: insights from the King Zn deposit, Yilgarn Craton, Western Australia

AbstractBulk rock geochemistry and SWIR reflectance spectroscopy are widely used by companies for rapid and cost-effective exploration of volcanic-hosted massive sulfide (VHMS) deposits. However, few studies have integrated bulk-rock geochemistry with hyperspectral reflectance spectroscopy in greenstone belts that have undergone high-grade metamorphism. Here we present an extensive dataset combining bulk-rock geochemistry with chlorite and white mica SWIR spectral reflectance from the amphibolite-grade King VHMS deposit of the Yilgarn Craton, Western Australia. At King, the footwall stratigraphy is dominated by tholeiitic mafic rocks overlain by a sequence of calc-alkaline intermediate-felsic metavolcanic rocks. The hanging-wall stratigraphy is characterized by a thin metaexhalite layer, overlain by thick succession of interbedded metasedimentary and metavolcanic rocks. Chlorite spectral signatures are more Fe-rich in mafic lithologies and Mg-rich in felsic rocks, particularly where intense Mg-metasomatism occurred before metamorphism. In all units, Fe/Mg ratios of chlorite are strongly tied to bulk rock Fe/Mg ratios. White mica in the footwall is primarily muscovitic, with minor amounts of phengite in deep Fe-rich mafic rocks. By contrast, the hanging-wall sequence is dominated by phengitic signatures in both the Fe-rich metaexhalite, and weakly Ca-Mg altered volcanic rocks. This study concludes that chlorite SWIR reflectance is largely influenced by the bulk Fe/Mg composition of the host rock, whereas white mica reflectance correlates with the type and intensity of hydrothermal alteration prior to metamorphism. These findings underscore the potential of using chlorite and white mica spectral signatures to understand hydrothermal alteration patterns and detect new orebodies in metamorphosed VHMS systems.

Discovery and Geological Significance of Neoproterozoic Bimodal Intrusive Rocks in the Dabie Orogen, China

The Mingshan reservoir of the Dabie Orogen has a number of Neoproterozoic bimodal intrusive rocks. We focused on the zircon U-Pb chronology, Hf isotopes, and bulk-rock geochemistry of these rocks. The results showed the following: (1) The bimodal intrusive rocks mainly consist of monzogranitic gneiss and plagioamphibolite, with zircon U-Pb ages of 785.0 ± 7.1 Ma and 787.3 ± 6.1 Ma, respectively. These ages indicate that they were formed in the late Qingbaikou epoch of the Neoproterozoic era. (2) The monzogranitic gneiss was dominated by peraluminous features and displayed a strong right deviation of REE (rare-earth element) patterns and a negative δEu anomaly. It is enriched in the LILEs (large-ion lithophile elements) Rb, Ba, and K, but slightly depleted in Nb, Sr, P, and Ti, with low 10,000* Ga/Al values, indicating that it is similar to Al-type granite. The plagioamphibolite belongs to the metaluminous, peraluminous series. It has a Mg# (molar ratio of Mg to Mg + Fe) of 36.1~55.9 and is enriched in the LILEs Rb, Ba, and K, with a slight positive anomaly of Ba, and is depleted in Nb and Sr. (3) The monzogranite shows negative zircon εHf(t) values ranging from −13.4 to −7.2 and a Paleoproterozoic TDM2(Hf) (two-stage depleted mantle model age) of 1969–2298 Ma. The zircon εHf(t) values and TDM2(Hf) of the plagioamphibolite were concentrated around 2.9–5.7 and 1257–1410 Ma, respectively. The geochemistry and Hf isotopes show that the monzogranitic gneiss and plagioamphibolite have distinct magmatic sources. The plagioamphibolite formed from mantle and partial continental crustal materials. The monzogranitic gneiss, on the other hand, was formed as a result of the partial melting of the shallow ancient felsic crust caused by mafic rock heating or upwelling. Taking into account regional correlation, the middle Neoproterozoic bimodal intrusive rocks originated in the structural framework of an extensional setting.

First identification of Late Permian mafic dykes in the nucleus of Yangtze Craton: Emeishan large igneous province doubled?

ABSTRACT The Emeishan large igneous province (ELIP) is known to have a relatively small-exposed area with a radius no longer than 700 km. New zircon U-Pb and plagioclase and pyroxene 40Ar/39Ar age determinations from mafic dykes in the nucleus of Yangtze Craton, located more than 1000 km away from the inner zone of ELIP, reveal that they were emplaced at ~260 Ma. This is synchronous with the ELIP. The zircon εHf(t) values of the dolerite dyke vary between +5.4 and +8.9. These mafic dykes have low Ti/Y ratios (299–413) and are enriched in light rare-earth elements (LREE) and depleted in heavy rare-earth elements (HREE) and high field strength elements (HFSE). These dykes bear similarities to the Emeishan high-Ti basalts despite their relatively low Ti/Y ratios. Whole-rock Sr-Nd isotopes gave highly radiogenic initial 87Sr/86Sr (0.710033–0.710371) and extremely low initial 143Nd/144Nd (0.511515–0.511641) and εNd(t) of −15.4 to −12.9. Electron microprobe analysis of concentric-zoned pyroxenes yielded increasing Mg# and Cr2O3 from core to rim. Bulk rock geochemistry, Sr-Nd isotopes, and mineral chemistry indicate that recharge of a new batch of ascending plume-derived magma into an enriched lithosphere generated the mafic dykes. The synchronous emplacement and geochemical and isotopic similarities between the examined Huangling mafic dykes and Emeishan flood basalts, as well as the presence of titanaugite, all point to the genetic link between the Huangling mafic dykes to the ELIP. The Emeishan-age of the Huangling dykes permits a substantial increase of at least 3 × 105 km2 in the spatial extent of the ELIP.

The key controlling factors on Sn–Cu mineralization: A case study from the world-class Gejiu Sn–Cu-polymetallic deposit

Gejiu, a globally prominent Sn–Cu-polymetallic district, contains an extensive suite of magmatic rocks. Magmatic rocks in the southeastern portion of the Gejiu district comprise the Masong and Laoka equigranular and porphyritic granites, and basalts. Previous studies have suggested that Sn mineralization in this region is primarily associated with the equigranular granite, whereas Cu mineralization is predominantly associated with the basalts. Despite this, the major factors controlling the formation and distribution of Sn–Cu mineralization in this region remain poorly constrained. This contribution characterizes the mechanisms of formation of the Gaofengshan Sn–Cu and Zhuyeshan Cu–Sn deposits, focusing on the petrogenesis of the host Masong and Laoka equigranular granites, and the evolution of the ore-forming magmatic–hydrothermal fluids. This is accomplished by combining bulk-rock geochemistry of the Masong and Laoka equigranular granites, and basalt, with biotite geochemistry from both granites and pyrite geochemistry from associated skarns. The Masong and Laoka equigranular granites crystallized from hybrid crust–mantle-derived magmas. Reduced granites like the Masong that underwent high degrees of fractional crystallization, and have elevated halogen are prospective for Sn mineralization. The abnormally high concentrations of Cu in pyrite in the Zhuyeshan deposit suggest that the Cu was primarily derived from sources external to the granite. Garnets from skarn-type ores and stratiform ores at Gaofengshan and Zhuyeshan yielded U–Pb ages of 88.2 ± 1.4 Ma and 81.8 ± 3.7 Ma, respectively, while vesuvianite from the skarn-type ores at Zhuyeshan yield a U–Pb age of 84.1 ± 0.5 Ma. These ages confirm that both the Sn and Cu mineralizing events occurred during the Late Cretaceous, coeval with emplacement of the granitic intrusions. Notably, there is no direct geochronological link between Cu mineralization and the basalt (ca. 244.4 Ma), which has a close spatial relationship with the Laoka equigranular granite. Combined with previous S–Pb isotope data, we propose that large-scale Cu mineralization in Gejiu resulted from the extraction of Cu from basalt by fluids exsolved from the Late Cretaceous equigranular granitic magmas.

The mid-Cretaceous Hohonu Batholith (South Island, New Zealand): Identifying magmatic sources and processes during onset of crustal extension

The Hohonu Batholith is an aggregation of mostly mid-Cretaceous granitoid plutons on the West Coast of the South Island of New Zealand emplaced during a transitional period between subduction-related compression and continental lithospheric extension. This study reports an integrated dataset, comprising in-situ UPb, O and Hf isotope compositions and REE, Ti and other trace and major elements (ZrHf, ThU) for zircons extracted from four representative plutons within the batholith. Our results provide detailed insight into the protracted thermal, chronological and geochemical histories. LA-ICPMS UPb zircon ages indicate a primary episode of magma genesis and emplacement from 107 to 113 Ma, confirming published SIMS dating. However, a younger previously unrecognized age population of ~91–96 Ma is identified, primarily (although not exclusively) in zircon rims. This younger age event coincides with the timing of protracted lithospheric extension and crustal thinning of the Zealandia continent. The cryptic younger zircon ages suggest that Hohonu granitoids experienced a partial thermal overprint (accompanied by Pb loss) mostly recorded in rims. Differences in bulk rock geochemistry between plutons are inferred to reflect variable conditions of partial melting controlled by source mineralogy and H2O content. Isotope and trace element compositions, along with Ti-thermometry, measured on the same micro-volume of CL-imaged zircons, are used to test if source characteristics were imparted from melt to minerals in zircon-saturated silicic systems. Similarities are revealed in the zircon record of the selected plutonic rocks, confirming their broadly consanguineous relationship and the fundamental role of open-system behaviour, involving hybridization or assimilation between mantle-derived (or juvenile mafic) and a crustal-derived components, as previously inferred from whole rock NdSr isotope systematics. However, intra-sample decoupling of zircon OHf isotope systematics may also be linked to residual source component unmixing. This possibility, in addition to mafic recharge, may have obscured melt source compositional characteristics, and hence zircon REE appear as unsuitable fingerprints of source(s) and conditions of partial melting in this granitoid system. Simple compositional and thermal magma evolution trends appear punctuated by episodes of mafic recharge, presumably during lithospheric thinning.

Rubidium (Rb) enrichment processes in a highly evolved granitic system: Insights from the petrology and geochemistry of the Guobaoshan Rb deposit in eastern Tianshan, NW China

Highly fractionated granitoids are closely associated with Rb mineralization, yet the mechanisms that control Rb enrichment and details about the enrichment process have not been well constrained. The Guobaoshan Rb deposit in eastern Tianshan, NW China, is one of the largest granite-related Rb deposits; it preserves a series of gradual lithofacies ranging from biotite granite and muscovite granite to Rb-rich amazonite granite and pegmatite, and provides an excellent opportunity to constrain the Rb enrichment processes in granitic systems. The large Guobaoshan Rb deposit hosts 281,000 tons of Rb2O at a grade of 0.12% mainly in its amazonite-bearing granite and amazonite granite lithofacies. In this contribution, we integrate geochronology, bulk-rock geochemistry, and analysis of Nd and zircon Hf isotopes to characterize the evolution of the Guobaoshan granitic pluton and the mechanisms that generated Rb enrichment. The zircon U-Pb age of the biotite granite and columbite U-Pb age of the amazonite granite are 244.8 ± 1.8 Ma (1σ, mean square of weighted deviates [MSWD] = 1.1) and 241.9 ± 2.3 Ma (1σ, MSWD = 1.0), respectively. In situ Rb-Sr analysis of orthoclase, mica, and albite from the amazonite-bearing granite, amazonite granite, and pegmatite yields ages of 249.1 ± 7.3 Ma (2σ, MSWD = 1.6), 245.5 ± 16.8 Ma (2σ, MSWD = 0.63), and 245.8 ± 9.8 Ma (2σ, MSWD = 3.3), respectively. Together, these ages demonstrate that the Guobaoshan granitic pluton formed during the Triassic and underwent a protracted magmatic evolution. The Guobaoshan granitic pluton is characterized by high concentrations of SiO2, alkalis, and Al2O3, and low concentrations of MgO, Fe2O3, CaO, P2O5, and TiO2, with an Al saturation index (A/CNK) of 1.01−1.14. It is enriched in Rb, but depleted in Ba, Sr, P, and Eu, has low total rare earth element (REE) contents (83.1−221 ppm), and is characterized by significantly negative anomalies (Eu/Eu* = 0.01−0.44). The biotite granite is enriched in light rare earth elements (LREEs), whereas the muscovite granite, amazonite-bearing granite, amazonite granite, and pegmatite are depleted in LREEs, and exhibit a marked tetrad effect (TE1.3 = 1.05−1.50). These geochemical characteristics, combined with the non-CHARAC (charge-radius−controlled) ratios of Zr/Hf, Nb/Ta, Y/Ho, and K/Rb, are indicative of melt-fluid interaction during the evolution of the Guobaoshan magma. Given that the biotite granite is the least-evolved rock, its petrological and geochemical characteristics suggest that it is an I-type granite. The slightly variable εHf(t) and consistently negative εNd(t) values indicate that the magma from which the Guobaoshan granitic pluton crystallized was likely sourced from the Mesoproterozoic crust, with limited mantle contributions. The coeval ages and coherent geochemical variations and mineral compositions of these rocks suggest that they formed via fractional crystallization of plagioclase, mica, quartz, and K-feldspar in the same magmatic system. This is supported by fractional crystallization simulations using the rhyolite-MELTS software package. Based on the petrological, mineralogical, and geochemical characteristics of the muscovite granite, amazonite-bearing granite, amazonite granite, and pegmatite, as well as simulations using rhyolite-MELTS, high degrees of fractional crystallization of granitic magma and interaction of this evolving magma with F-rich fluids are suggested to have been the key mechanisms that caused enrichment of Rb, while the melt-fluid interaction was more critical for Rb mineralization.

Melt generation and magma storage conditions of primitive arc lavas in the Macolod Corridor, southwestern Luzon arc, Philippines

Decoding the origin of primitive arc magmas from petrological, geochemical, and thermobarometric constraints is crucial for understanding their melt generation process and crustal storage conditions. In the Philippine arc setting, primitive basalts have been recognized in the Macolod Corridor, southwestern Luzon arc. The Macolod Corridor is a 30 by 60-km northeast-southwest striking, young rift system that hosts several Quaternary stratovolcanoes including the active Taal Volcano and Mt. Banahaw, lava domes, and ∼ 200 monogenetic centers classified as scoria cones, tuff cones, tuff rings, and maars. This study reports textural, petrological, and geochemical analyses of the Macolod primitive basalts to decipher their petrogenesis and elucidate their pre-eruptive magma storage conditions. We identified at least five distinct primitive lava compositions based on their modal mineralogy: clinopyroxene-olivine basalts, plagioclase-olivine-clinopyroxene basalts, olivine-plagioclase-clinopyroxene basalts, clinopyroxene-plagioclase-olivine basalts, and clinopyroxene-olivine-plagioclase basalts. Clinopyroxene-olivine basalts and clinopyroxene-plagioclase-olivine basalts occur as lapilli and bomb deposits. In contrast, plagioclase-olivine-clinopyroxene-basalts, olivine-plagioclase-clinopyroxene basalts, and clinopyroxene-olivine-plagioclase basalts occur as lapilli and volcanic bombs in monogenetic volcanoes and as basaltic blocky lava flows in small polygenetic volcanoes. Phenocrysts, glomerocrysts, and microphenocrysts assemblages include olivine, clinopyroxene, plagioclase ± spinel in a glassy matrix. The basalts are identified as subalkaline, medium-K, and medium-Fe tholeiitic basalts, based on their bulk-rock geochemistry. Adding 3–4 % equilibrium olivine to the Macolod primitive basalts generates magmas in equilibrium with mantle olivines with Fo90.68–90.82 and 0.392–0.395 wt% NiO compositions. Disequilibrium textures exhibited by olivines, clinopyroxenes, and plagioclases suggest that these are products of magma decompression and dissolution processes. Calculated melt based on olivines reveals that these primitive magmas last equilibrated at depths ranging from ∼36–42 km (1.03–1.23 GPa) at 1286°–1318 °C. Application of clinopyroxene-only thermobarometer results indicate clinopyroxene crystallization depths of around 7–16 and 10–19 km for hydrous and anhydrous estimates, respectively. The segregation depths estimated in this study translate to the uppermost mantle until near the Moho boundary whereas the storage depths correspond to prolonged magma storage regions in the upper crust as modeled by existing geophysical data (i.e., seismic travel-time tomography). Combining textural and geochemical results from this study with existing geophysical data provides new insights into the magma plumbing systems in the region and how these might operate through time.

Geochemistry of shales of middle Permian Barren Measures Formation, West Bokaro Basin, India: implications on provenance, paleodepositional and paleoclimatic conditions

Geochemical analyses of fine-grained clastic sedimentary rocks, such as shales, are significant since they portray the nature of the control factors from source to sink in a sedimentary basin, and help characterizing the heterogeneity in potential source and/or reservoir rocks. Bulk rock geochemistry (including major element oxides, trace elements and rare earth elements) of 20 shale samples from the Permian Barren Measures Formation, West Bokaro Basin, Peninsular India, is presented to decipher the provenance, paleo-weathering, paleotectonic, paleo-depositional and paleoredox conditions of the basin within the Lower Gondwana paleogeographic framework. X-ray diffraction (XRD) peaks and scanning electron microscopy (SEM) reveals an abundance of quartz, muscovite and clay minerals, viz., illite, kaolinite, etc., with less abundant feldspar and glauconite. The concentrations of the major element oxides, the trace elements and rare earth elements (REEs) of the shale samples are compared with predefined standards such as PAAS and UCC. The CIA (chemical index of alteration) and the A–CN–K values, based on the ratios of the major element oxides, signify intense chemical weathering. The ratios of Sr/Cu versus Rb/Sr and K2O/Al2O3 versus Ga/Rb, and high ΣREE values indicate a temperate paleoclimate. Ratios of the major element oxides (e.g., K2O/Na2O versus SiO2) suggest a passive tectonic setting. The Eu/Eu∗ and (Gd/Yb)n values depict predominantly post-Archean granitic and gneissic source rocks. The ratios of trace elements, viz., Th/Sc, Th/Co, Th/Cr, La/Sc and Eu/Eu∗, and the distribution pattern of the REEs (higher LREE in comparison to HREE) attest to a felsic-intermediate source from a nearby provenance. The redox sensitive trace elements, such as V and Ni, indicate a dysoxic condition that prevailed during deposition. The ratios of Fe2O3 versus MgO and Log (MgO/Al2O3) versus Log (K2O/Al2O3), and the ternary plots of Fe2O3–MgO–SiO2/Al2O3 suggest a non-marine to deltaic transitional environment. Such evidence of marine influences during the sedimentation of the Permian Barren Measures Formation confirms the fluvio-marine transgressive paleogeography inferred from sedimentological and paleontological inputs from the Gondwana Basins in Peninsular India, and opens up a scope for regional correlations of paleogeographic changes during the Middle Permian (Guadalupian) across the Gondwanaland.

Onset and tempo of ignimbrite flare-up volcanism in the eastern and central Mogollon-Datil volcanic field, southern New Mexico, USA

The Cenozoic ignimbrite flare-up (40–18 Ma) generated multiple volcanic fields in the southwestern United States and northern Mexico resulting from asthenospheric mantle upwelling after removal of the Farallon slab. The correlation of tuffs to one another and to source calderas within these volcanic fields is essential for determining spatiotemporal patterns in volcanism and magma geochemistry, which have been used to deduce migration of the Farallon slab at depth and associated mantle melting. However, the correlation of Eocene–Oligocene tuffs in the southwestern U.S. is difficult because of post-emplacement erosion and faulting. This study focuses on spatiotemporal patterns of the initial episode of ignimbrite flare-up activity (ca. 36.5–33.8 Ma) in the Mogollon-Datil volcanic field in south-central New Mexico, USA. We show that alkali feldspar major and trace element geochemistry is an effective tool for correlating tuffs when combined with high-precision, single-crystal 40Ar/39Ar geochronology and bulk-rock geochemistry. Using these data, we correlate several tuff units and differentiate other tuffs that have the same eruption age but very different geochemistry, and we conclude that there was a broadly northwestward migration in volcanism over time. The new tuff correlations are used to investigate spatiotemporal variations in magma geochemistry, erupted volumes, and recurrence intervals during the initial episode of Mogollon-Datil volcanic field volcanism. Early-erupted tuffs restricted to the eastern Mogollon-Datil volcanic field share similarities with western U.S. topaz rhyolites, which suggests that the silicic magmas were generated by partial melting of mafic lower crustal rocks. We also find differences in the compositions, crystallinities, and mineral assemblages between the early- and late-erupted tuffs. The early-erupted tuffs tend to have single-feldspar mineralogies, lower feldspar Or contents, large negative Eu anomalies, and low-whole–rock Ba concentrations. Conversely, late-erupted tuffs have two feldspar plus quartz assemblages, lesser Eu anomalies, higher whole-rock Ba concentrations, and feldspars have higher Or contents. Thus, we suggest that for some of the early eruptions, after magmas underwent crystal fractionation in the crust, the silicic melt largely separated from the crystalline mush prior to eruption, whereas late-erupted tuff magmas underwent crystal fractionation at near the eutectic minimum and were remobilized and erupted with a larger proportion of their crystalline mush. Using our new ages, correlations, and previously published data, we find that the initial phase of Mogollon-Datil volcanic field volcanism produced at least 15 eruptions between 36.5 Ma and 33.8 Ma, with a minimum total erupted volume of ~1350 km3 and an average recurrence interval of 170 k.y. However, eruptions were generally smaller in volume (most <15 km3) than in other coeval fields, and most eruptions (n = 11) occurred in the first 1.2 m.y. (ca. 36.5–35.3 Ma) of activity. Altogether, our work sheds new light on variations in the composition, timing, and migration of volcanism during the initial phase of Mogollon-Datil volcanic field activity and highlights the utility of feldspar geochemistry in both “fingerprinting” tuffs and elucidating magma evolution.

Recycling of terrigenous sediments recorded in late Permian-early Triassic Fe-rich intrusions from Yunkai Massif, South China

The origin of Fe-rich intrusion remains debatable regarding the respective role of magmatic evolution and Fe enrichment in the source. Here, we report detailed mineral and bulk-rock geochemistry of Permian-Triassic (251–250 Ma) mafic intrusions from the Tengxian area in Yunkai Massif, South China. These rocks consist of hornblende norites, show Fe-rich affinities with high FeO*(FeO* = FeOt / (FeOt + MgO) in weight ratio, >0.8, based on total iron oxide in the rock), and exhibit significant enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but depletion in Sr and high field strength elements (HFSEs), resembling subduction-related magmas. In addition, they show remarkable features that include very high δ18O values in zircon (δ18O = 8.8–10.6 ‰) and apatite (δ18O = 8.7–10.8 ‰), and extremely enriched SrPb [e.g. 87Sr/86Sr(i) = 0.7181–0.7196, 207Pb/204Pb(i) = 15.77–15.78, 208Pb/204Pb(i) = 38.95–39.05] and nonradiogenic NdHf isotopic compositions [e.g. εNd(t) = −11.1 ~ − 10.1, εHf(t) = −8.8 ~ −7.3] in bulk rocks. These characteristics distinguish the Tengxian norites from modern arc basalts and subduction-related magmas in the Paleo-Pacific Tectonic Domain. Instead, they are isotopically similar to Cenozoic Tibetan and Mediterranean potassic to ultrapotassic rocks in the Tethys Tectonic Domain. Regardless of variable influence by orthopyroxene and plagioclase accumulation, the intrinsically low SiO2 and high FeOt and Fe/Mn ratios in these rocks were likely attributed to significant contribution of a Fe-rich mantle component such as Si-poor pyroxenite, which might have formed through crystal accumulation of mafic magmas at mantle conditions. The highly evolved Sr-Nd-Pb-Hf isotopic signatures and very high δ18O values in zircon and apatite required substantial (10–20 %) involvement of recycled crust in the mantle source. The combined mineral and bulk-rock geochemical data suggest that the parental magmas for the Tengxian norites originated from a metasomatized mantle wedge through addition of terrigenous sediment-derived melt following the subduction of Paleo-Tethys Ocean beneath the Yunkai Massif.

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 Role of Latent Heat Buffering in the Generation of High-Silica Rhyolites

Abstract The physical process of crystal-melt separation is responsible for the accumulation of small to very large volumes (>100 km3) of eruptible rhyolitic melt in the shallow crust. Granitic intrusions, although providing a terminal, time-integrated image of melt segregation processes, host an unmatched record of the physical properties controlling mechanisms and rates of interstitial melt extraction from a crystal-rich source. We applied mass balance calculations and thermodynamic modeling simulations to an extensive bulk rock geochemistry dataset (>150 samples) collected in a Permian upper-crustal granitoid intrusion of the Italian Southern Alps. Textural and geochemical evidence indicate that this intrusion constituted a single, zoned magma body, with a crystal-rich base and a thick (~2 km), high-silica cap (75–77 wt% SiO₂). The large compositional variability of the crystal-rich materials suggests variable degrees of melt extraction efficiency and corresponding terminal porosities. Specifically, the loosely bimodal distribution of porosity values (φ) indicates that at least two distinct melt segregation mechanisms were operating in this system, which produced both high (0.65–0.45) and low terminal porosities (0.45–0.25) in the crystal-rich, cumulate materials. Modeling of latent heat budget shows that coexistence of cumulate products with differing terminal porosity signature can be explained by melt segregation processes taking place at different depths across a thick, interconnected magmatic reservoir with an initial homogenous water content (~4 wt% H2O). Deep in the mush column, low water activities (aH₂O < 0.5) promoted thermal buffering of cooling magma at high crystallinities, enabling residual melt extraction by percolation through a crystalline framework accompanied by compaction. Instead, at shallower depths, high water activities (aH₂O > 0.5) ensured prolonged magma residence at porosities that promoted crystal melt separation via hindered settling. Distinct melt extraction processes, acting synchronously but at different depths in vertically extensive silicic mush columns, can account for the large volumes of residual, haplogranitic melt mobilized during the relatively short lifespan of upper crustal magma reservoirs (~105 years).

Zircon U[sbnd]Pb geochronology and geochemistry of hot subduction volcanic suite in the deep bore-hole well-core KBH-5: Evidence of Neoarchean Shimoga greenstone belt beneath the Cretaceous Deccan Traps, India

The continents across Earth have grown through accretion and amalgamation of plume-generated oceanic plateaux and subduction-originated magmatic arcs. The evidence of such evolutionary processes is well preserved in the greenstone belts of Canada, South Africa, China, Australia and India. Unlike accretion that leads to lateral growth of the continents, flood basalt volcanism results in the superposition of large volumes of dense mafic material above relatively less dense sialic upper continental crust. The resulting density inversion is liable to generate tectonic stresses resulting in faulting of under burden along weak planes. The nature of the crust obscured beneath flood basalt provinces that have accumulated over the past 500 My in Large Igneous Provinces (LIPs) has remained enigmatic, largely because of the voluminous lava pile that deters access to the concealed basement, unless otherwise retrieved through deep continental drilling. Here, we present in situ zircon UPb geochronology and bulk-rock geochemistry of a volcanic sequence discovered at −400 m m.s.l in the deep bore-hole well-core KBH-5 of the southwestern Deccan Traps, India. The ∼300 m thick metavolcanic succession includes a basalt – Nb-enriched basalt – andesite – Mg-andesite – dacite – adakite suite. The zircons yielded a 2.58 Ga U-Pb age for the suite, which also incorporates entrained zircon xenocrysts of 2.7 Ga age from a preexisting juvenile material during its emplacement. The bore-hole KBH-5 is located on an NNW-SSE trending normal fault, paralleling a series of major faults west of the Western Ghat Escarpment. Although the crustal block plummeted ∼1000 m downward along the fault plane, the geochemical attributes of the metavolcanic rocks are thoroughly comparable to those reported in the Medur Formation of the Shimoga greenstone belt, ∼300 km south of KBH-5 in the western Dharwar craton. Our study provides the first physical evidence of Dharwar greenstone belt(s) extending further beneath the Deccan Traps, which was previously only speculative based on short-wavelength gravity highs. The metavolcanic sequence evolved in a subduction-related back-arc tectonic regime and accreted to the active continental margin of India in the Neoarchean. Unveiling of potentially similar scenarios in cratons elsewhere entrapped beneath the vast canopy of continental flood basalts, would reveal new crustal growth events during the Archean -Proterozoic transition across the Earth.

Origin of Early Triassic Hornblende Gabbro from the Yunkai Massif, South China: Constraints from Mineral and Bulk-Rock Geochemistry

The early Triassic (~250 Ma) hornblende gabbro from the Tengxian area of Yunkai Massif, South China, contains a mineral assemblage of clinopyroxene, hornblende, biotite, plagioclase, K-feldspar and quartz and accessory apatite, and zircon and ilmenite. Based on mineral association and crystallization sequence, two generations of the mineral assemblage have been identified: clinopyroxene + plagioclase + apatite (zircon) in Generation I and ilmenite + hornblende + biotite + K-feldspar + quartz in Generation II. The high crystallization temperature (T = 999–1069 °C) of clinopyroxene and its coexistence with labradorite (An = 52–58) indicate that Generation I crystallized in a basaltic magma, while the hornblende’s relatively low crystallization temperature (T = 780–820 °C) and coexistence with K-feldspar and quartz suggest that Generation II formed in an evolved alkaline melt. The mineralogical records are likely attributed to pulsed intrusion of the late-stage evolved magma into a crystal mush, like in Generation I. The bulk-rock geochemical data include a sub-alkaline affinity, arc-type trace element features, and highly enriched Sr-Nd-Pb-Hf isotopic compositions, consistent with the isotopic records from the accessory minerals, e.g., the very high δ18O values in both zircon and apatite and significantly negative εHf(t) in zircon. The combined mineral and bulk-rock geochemical data suggest that the primary magma for the Tengxian hornblende gabbro was derived from a mantle wedge that had been metasomatized by voluminous subducted terrigenous sediment-derived melts in response to subduction of the Paleo-Tethys Ocean.

The magma plumbing system of the potentially hazardous Laoguipo volcano in the Tengchong Volcanic Field, SW China

The Late Pleistocene-Holocene Laoguipo volcano in the Tengchong Volcanic Field (TVF), southwestern China, displays significant geochemical and geophysical anomalies characteristics. Here we present petrographic observations, mineral chemistry, bulk rock geochemistry, thermobarometry, and thermodynamic simulation to evaluate the crystallization conditions and pre-eruptive magmatic processes occurring within the magma plumbing system. This study reveals the existence of two magma reservoirs beneath the Laoguipo volcano. The deep magma reservoir is composed of basaltic trachyandesite (SiO2 = 54–57 wt%), which is located at 15–19 km depths with 1087–1160 °C, 1.5–2 wt% H2O content, oxygen fugacity of ΔNNO+1 (Ni-NiO buffer), melt viscosity of 101.7–102.6 Pa·s, and density of 2.5–2.6 g/cm3. The formation of the deep magma reservoir is attributed to the 31% mass fractional crystallization of primitive basalt in the TVF. The shallow magma reservoir is composed of trachyte (SiO2 = 63–64 wt%), which is located at 6–11 km depths with 780–825 °C, 5.9–6.5 wt% H2O content, oxygen fugacity of ΔNNO+1 (Ni–NiO buffer), melt viscosity of 103.9–104.8 Pa·s, and density of 2.2–2.3 g/cm3. The shallow magma reservoir formed after the basaltic trachyandesite had assimilated 19% mass of the upper crustal material and fractionated 41% mass of the crystals. This study suggests that the shallow trachyte magma reservoir is being heated by the ascending deep basaltic trachyandesite magma, resulting in crystal dissolution, remobilization of crystal mush, and magma convection, which may be the main factors responsible for the geochemical and geophysical anomalies characteristics. The Laoguipo volcano is forming a mature magma plumbing system, which is of significance for forecasting future volcanic eruptions.

The behavior of Fe isotopes in Fe skarns: A case study from the Yeshan Fe skarn deposit, Eastern China

To characterize the applicability of Fe isotopes to skarn petrogenesis and exploration, a robust understanding of their fractionation behavior during skarn alteration and mineralization is required. Here, we characterize the bulk-rock Fe isotope composition of endo- and exoskarn, magnetite ore, and the intrusive rocks that comprise the Yeshan Fe skarn deposit in Eastern China, aiming at better constraining the behavior of Fe isotopes during skarn formation and mineralization. The δ56Fe values of garnet and garnet–diopside skarn (–0.19 ‰ to 0.15 ‰, n = 15) are negatively correlated with bulk-rock MgO/Al2O3, suggesting that the variation in δ56Fe of these samples is mainly controlled by mineralogy. Garnet skarn is characterized by δ56Fe values (0.07 ‰ to 0.15 ‰, n = 11) that are similar to those of the spatially associated quartz monzonite pluton (0.12 ‰ to 0.16 ‰, n = 2), and it has systematically higher Fe2O3 contents. Based on petrological observations and bulk-rock geochemistry (e.g., REE and P2O5 contents), garnet, the main Fe-bearing mineral in the garnet skarn, is inferred to have achieved equilibrium (or near equilibrium) with the hydrothermal fluids that circulated throughout the mineralized system, implying that the δ56Fe values of garnet skarn can be used to trace the δ56Fe values of the fluids. Epidote skarn has a similar Fe2O3 content as garnet skarn, but lower δ56Fe values (–0.07 ‰ to 0.01 ‰, n = 2). Epidote skarn was genetically associated with low temperature, FeCl2(H2O)4-dominated fluids, which are in contrast with that (high-temperature, [FeCl4]2--dominated fluids) formed the garnet skarn. Iron in diopside skarn is mainly hosted by magnetite; the δ56Fe values of this lithology (0.00 ‰ to 0.12 ‰, n = 2) are, therefore, mainly controlled by magnetite. Taken together, the Fe isotopic signatures of various types of skarn at Yeshan could enhance our understanding of skarn deposits worldwide, especially those sharing geological features similar to the Yeshan Fe skarn deposit.

Post-collisional calc alkaline-adakites from a metasomatized subcontinental lithospheric mantle: Sr-Nd isotopes and bulk-rock geochemistry of Miocene volcanic rocks from the Varzaghan area, NW Iran

ABSTRACT The NW Iran is located at the western termination of the Alborz Mountain range and continuation of the Sanandaj-Sirjan magmatic-metamorphic zone at the NW. The volcanic rocks in the Varzaghan area are distributed over different compositional ranges from high-K basalt, andesite, dacite, and trachyandesite, dacite, to rhyolite with adakitic signatures. Chondrite-normalized REE diagrams show flat MREE and HREE patterns for calc-alkaline volcanic rocks and enrichment in LREE, indicating a garnet-free source. Adakitic rock sample chondrite-normalized REE patterns are relatively steep with significant enrichment in LREE and depletion in HREE, indicating a garnet-bearing source or garnet fractionation process. The arc trace element signatures, narrow range of initial87Sr/86Sr(i) = 0.70425–0.70487, different initial143Nd/144Nd(i) = 0.51264–0.51286 and the presence of adakitic rocks reveal that the volcanic rock sources were not the same and can be related to the partial melting of metasomatized mantle and lower continental crust sources. Slab break-off and upwelling of hot asthenosphere in a post-collisional tectonic regime resulted in the melting of the subcontinental lithosphere and lower continental crust which produced calc-alkaline-adakitic volcanic rocks in the Varzaghan area. The main petrological processes involved in the volcanic rocks petrogenesis were fractional crystallization and crustal assimilation.

Distinct mafic magmatism of the northeastern Longgang Block: Evidence for coexisting mantle plume and subduction during the Neoarchean North China Craton

The question of which specific tectonic regimes played an essential role in shaping the Neoarchean evolution of the North China Craton (NCC) has been a contentious and controversial topic. Mafic rocks, containing valuable geochemical information from the mantle and deep crust, serve as an important source of data to provide essential constraints on above issue. This study presents bulk-rock geochemistry, zircon U-Pb geochronology and Hf-Nd isotopes for the Neoarchean mafic rocks in the core area of the Longgang Block of the NCC. Petrographically, these mafic rocks are composed of the amphibolite and metadiabase. Zircon U-Pb dating results revealed that they were synchronously emplaced at ca. 2.5 Ga. Geochemically, the amphibolites belong to subalkaline tholeiite series, exhibiting enrichment in light rare earth elements (LREEs), intensely negative Nb, Ta and Ti anomalies, and slightly enriched Nd and relatively variable zircon Hf isotopic compositions. These geochemical and isotopic features show large affinity to arc-like magmatism, implying that they were derived from subduction-related metasomatized lithospheric mantle. In contrast, the metadiabases exhibit high-Ti alkaline basalt affinities, OIB-like REE, and trace element patterns. They also show clearly positive Nb, Ta and Ti anomalies, indicating that they originated from a mantle plume-related tectonic environment. The new geochemical and isotopic data reveal that these Neoarchean mafic rocks could have resulted from coexisting mantle plume and subduction processes. Combined with available regional structural, geochemical, and metamorphic data, it is likely that during the Neoarchean, both a mantle plume and subduction jointly controlled the crustal growth and tectonic evolution of the Longgang Block in the eastern NCC.

Magma Evolution During Main‐Phase Continental Flood Basalt Volcanism: A Case for Recharge‐Evacuation‐Assimilation‐Fractional Crystallization in the Ethiopian Low‐Ti Province

AbstractLavas erupted in Continental Flood Basalt (CFB) provinces are not primary magmas; they are differentiated products that result from large volumes of melt migrating and stalling in the lithosphere prior to eruption, resulting in complex liquid lines of descent. Geochemical models can be used to constrain the various influencers of magma differentiation (recharge, assimilation, fractional crystallization (FC), eruption, and mixing). Temporal constraints for changes in plumbing system dynamics are recorded in the petrography and stratigraphy of the erupted lava flows. This study focuses on the flow‐stratigraphy preserved within the Oligocene Ethiopian low‐Ti flood basalt province, located on the NW Ethiopian Plateau. We present new bulk rock geochemistry from 107 lavas and interpret these data within a petrostratigraphic framework. Our model results suggest that both a deep (∼0.6 GPa) and shallow (<0.2 GPa) magmatic system are active throughout the main phase of volcanism. Our recharge evacuation assimilation and fractional crystallization models (REAFC) show that during the main phase of magmatism evacuation from both the deep (65%) and shallow (55%) systems reached an apex. During the terminal phases, magma evacuation from the deeper system ceases while evacuation from the shallow system is much reduced (25%). The degree of crustal contamination predicted by REAFC (4%–10%) is lower than previous estimates determined for this region using assimilation with FC only models (12%–25%). Our study highlights the importance of evaluating petrography while interpreting geochemical models in CFB.

Geochemical diversity of continental arc basaltic mushy reservoirs driven by reactive melt infiltration

The reactive melt flow emerges as an important factor for diversification of basaltic magmatic reservoirs, but whether and how it influences continental arc basaltic mushes are enigmatic. Here, we used mineral and whole-rock geochemistry to examine the petrogenesis of a suit of mafic and intermediate plutons in western Yangtze Block, which were emplaced at continental arc crust and primarily had plagioclase and clinopyroxene as early cumulate mineral phases. We found the crystal mushes were infiltrated by externally-derived reactive melt with high δ18O and fertile crustal signatures, resulting in the changes of mineral phases (e.g., clinopyroxene transformed to hornblende) and bulk-rock geochemistry (including isotopes). Then, the reacted granitic melt was prone to either be extracted from or stall in the crystal mushes, generating quartz-poor (mafic) or quartz-rich (intermediate) plutons, respectively. This study supports the reactive melt infiltration may serve as an important engine for compositional diversity of basaltic mush system within continental arc settings.