Geochemical Constraints Research Articles

Apatite U-Pb geochronological and geochemical constraints on the Mazhala Au-Sb deposit in South Tibet, China

Due to lack of suitable datable minerals, timing of the Mazhala Au-Sb deposit in South Tibet remains unresolved. Also, previous geochemical studies suggest an ambiguous conclusion on the source of ore-forming fluid, with diverse viewpoints of magmatic water, mixed magmatic–meteoric water or mixed metamorphic-meteoric water, respectively. In this study, hydrothermal apatite U-Pb dating constrains the Mazhala Au-Sb deposits to have formed at ca. 35 Ma, consistent with the timing of Eocene leucogranite intrusions in South Tibet. The Mazhala Au-Sb mineralization occurred during the main collision stage to late collision stage in Eocene to Oligocene, and predates the Cuonadong Sn mineralization in South Tibet which occurred during the crustal extension stage in Miocene. An EPMA analysis shows the apatite belongs to fluorapatite that have F contents in the dominant range of 2–4%. The Mazhala apatite displays an overall MREE enrichment and LREE- and HREE depletion distribution pattern with positive Eu anomalies, and have δ18O values dominantly in the range of 16-19‰ with a mean δ18O‰ value of 17.74 ± 0.15‰ (2σ), which suggest a relatively homogenous chemical composition for the ore forming fluids with a relatively stable temperature regime. In combination with previous temperature estimates, the ore forming fluids are inferred to have a dominant δ18Ofluid value of 13-16‰, which overlap with metamorphic water.The combined age and geochemical data suggest Eocene leucogranite intrusion induced metamorphic fluid circulation within Paleozoic-Mesozoic metamorphic sedimentary rocks around the study area, which prompted precipitation of fluorapatite together with Au-Sb bearing pyrite and arsenopyrite in the Mazhala Au-Sb deposit. This study puts an emphasis on the contribution of metamorphic water as the dominant ore-forming fluid origin in the Mazhala Au-Sb deposit. In contrast, the close spatial and temporal association of the Cuonadong Sn polymetallic mineralization with the Miocene leucogranite intrusion-induced hydrothermal activity in the Cuonadong dome demonstrates that their genetic links, as was also evidenced by C-H-O-S-Pb isotope data that point to a dominant contritution of Miocene leucogranite to ore-forming fluid and materials for the Cuonadong Sn polymetallic ore deposits. As such, from a perspective of ore-forming model in South Tibet it be seen that during the main collision stage to late collision stage ore-forming fluid and materials were mainly derived from metamorphic water, whereas during the crustal extension stage, ore-forming fluid and materials were dominantly sourced from magmatic water.

Recycling of continental crust materials: Geochemical constraints from post-collisional alkaline intrusive rocks in the Dabie orogen

Post-collisional alkaline intrusive rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr-Nd-Hf-Pb isotopes, as well as zircon U-Pb ages and Hf-O isotopes. The results provide geochemical constraints on the nature of their mantle sources and thus insight into crust-mantle interaction in the continental collision zone. The alkaline intrusive rocks are composed of syenite and nepheline syenite. Syn-magmatic zircon U-Pb dating by LA-ICP-MS for them yielded Early Cretaceous ages of 131.3 ± 1.4 Ma to 122.6 ± 0.6 Ma, coeval with the post-collisional magmatism in the Dabie orogen. One relict zircon with U-Pb age of 211 Ma is consistent with the timing of metamorphism for the ultrahigh-pressure (UHP) metamorphic rocks in this orogen. They have arc-like trace element distribution patterns, such as enrichment in LILE (large ion lithophile element) and LREE (light rare earth element) but depletion in HFSE (high field strength element), and enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7077–0.7131 but negative εNd(t) values of −16.0 to −9.4 and εHf(t) values of −17.5 to −12.7. Their syn-magmatic zircons have three groups of Hf-O isotope compositions comparable to those of UHP metamorphic rocks in Central-South Dabie and North Dabie, which represent the upper and middle continental crust of the subducted South China Block, respectively. In this regard, slab–mantle interaction is evident during the Triassic continental collision. We suggest that the melts derived from the subducted South China Block reacted with the lithospheric mantle wedge of the North China Block, resulting in phlogopite-bearing metasomatites, whose partial melting would generate the post-collisional alkaline intrusive rocks during the Early Cretaceous.

Tonian Carbonates Record Phosphate‐Rich Shallow Seas

AbstractThe early‐middle Neoproterozoic is thought to have witnessed significant perturbations to marine P cycling, in turn facilitating the rise of eukaryote‐dominated primary production. However, with few robust constraints on aqueous P concentrations, current understanding of Neoproterozoic P cycling is generally model‐dependent. To provide new geochemical constraints, we combined microanalytical data sets with solid‐state Nuclear Magnetic Resonance, synchrotron‐based X‐ray Absorption Near Edge Structure spectroscopy, and micro‐X‐ray Fluorescence imaging to characterize the speciation and distribution of P in Tonian shallow‐water carbonate rocks. These data reflect shallow water phosphate concentrations 10–100× higher than modern systems, supporting the hypothesis that tectonically‐driven influxes in P periodically initiated kinetically‐controlled CaCO3 deposition, in turn destabilizing marine carbonate chemistry, climate, and nutrient inventories. Alongside these observations, a new compilation and statistical analysis of mudstone geochemistry data indicates that, in parallel, Corg and P burial increased across later Tonian continental margins until becoming decoupled at the close of the Tonian, implicating widespread N‐limitation triggered by increasing atmospheric O2.

Geochemical constraints on the petrogenesis of mafic rocks (metadolerites) from the Proterozoic Shillong Basin, Northeast India: implications for growth of the Greater Indian Landmass

AbstractThe Paleo-Mesoproterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex is exposed in parts of Northeast India. The studied metadolerites are from the volcano-sedimentary sequence of Shillong Basin from the Borjuri area in the Mikir Massif. This episode of mafic magmatism can be correlated with the Columbia supercontinent formation and bears significance to its reconstruction. The present work discusses the field, petrography and geochemical characteristics of the metadolerites, which occur in close association with the quartzites of the Shillong Group of rocks (metasedimentary rocks of the Shillong Basin). Our data show distinctive characteristics of subduction-related magmatism exhibiting high LREE/HREE, large ion lithophile element/high field strength element ratios and pronounced negative Nb anomaly. Elemental ratios such as Zr/Ba (0.21–0.46), La/Nb (1.23–2.32) and Ba/Nb (30.08–56.90) point to a fluid-enriched lithospheric mantle source in a subduction regime. Metadolerites plot in the field of ‘back-arc basin basalts’ in tectonic discrimination diagrams reinforcing a subduction zone tectonic setting. The mafic rocks correspond to a 6–10 % partial melting of a mantle source incorporating spinel+garnet lherzolite. The metamorphic P-T of the metadolerites estimated from plagioclase-hornblende geothermobarometer (7–8 kbar, 664 °C) is indicative of amphibolite facies metamorphism in a medium P-T zone. Based on the comparative analysis of field observation, petrography, geochemistry and geological ages given by previous workers, we infer that the Shillong Basin represents a back-arc rift region and is the eastern continuation of the Bathani volcano-sedimentary sequence of the Chotanagpur Granite Gneiss Complex marking continuation of the Central Indian Tectonic Zone to the Mikir Massif.

Geochronological and geochemical constraints on petrogenesis of the Hongyuan intrusion in central West Junggar, Xinjiang, NW China

Geochronological and geochemical constraints on petrogenesis of the Hongyuan intrusion in central West Junggar, Xinjiang, NW China

Petrogenesis and Tectonic Evolution of Kab Amiri Ophiolites and Island-Arc Assemblages, Central Eastern Desert, Egypt: Petrological and Geochemical Constraints

The Kab Amiri area in the Central Eastern Desert (CED) of Egypt comprises ophiolitic rocks, island arc metasediments, and granitic rocks. This study deals with the petrography and geochemistry of the ophiolitic and island arc rocks to understand their petrogenesis and geodynamic evolution of the CED ophiolitic belt. The Kab Amiri ophiolite is dismembered, comprising serpentinites and metabasalt (spilite). Serpentinites have low contents of Al2O3 (1.14 wt%) and CaO (0.65 wt%), suggesting they are depleted peridotite protoliths (e.g., depleted harzburgites to dunites). The high L.O.I. value (13.7 wt%) of serpentinite rocks indicates intense hydration and serpentinization during alteration processes. Petrographic and geochemical studies suggest that serpentinites were likely formed after depleted peridotites in a supra-subduction zone (SSZ) setting (e.g., a fore-arc setting). Spilitic basalt shows a tholeiitic affinity of the depleted mantle source. The arc-related metasediments are represented mainly by schists and slate. Many samples of metasediments are relatively low in alumina (Al2O3 < 15%), suggesting a low clay content and formation in an island arc setting. In contrast, protoliths of island-arc metabasalts and meta-andesites crystallized from calc-alkaline magmas in the immature oceanic arcs.

The South Armenian Block: Gondwanan origin and Tethyan evolution in space and time

The geodynamic evolution of the South Armenian Block (SAB) within the Tethyan realm during the Palaeozoic to present-day is poorly constrained. Much of the SAB is covered by Cenozoic sediments so that the relationships between the SAB and the neighbouring terranes of Central Iran, the Pontides and Taurides are unclear. Here we present new geochronological, palaeomagnetic, and geochemical constraints to shed light on the Gondwanan and Cimmerian provenance of the SAB, timing of its rifting, and geodynamic evolution since the Permian. We report new 40Ar/39Ar and zircon U-Pb ages and compositional data on magmatic sills and dykes in the Late Devonian sedimentary cover, as well as metamorphic rocks that constitute part of the SAB basement. Zircon age distributions, ranging from ∼3.6 Ga to 100 Ma, firmly establish a Gondwanan origin for the SAB. Trondhjemite intrusions into the basement at ∼263 Ma are consistent with a SW-dipping active continental margin. Mafic intraplate intrusions at ∼246 Ma (OIB) and ∼234 Ma (P-MORB) in the sedimentary cover likely represent the incipient stages of breakup of the NE Gondwanan margin and opening of the Neotethys. Andesitic dykes at ∼117 Ma testify to the melting of subduction-modified lithosphere. In contrast to current interpretations, we show that the SAB should be considered separate from the Taurides, and that the Armenian ophiolite complexes formed chiefly in the Eurasian forearc. Based on the new constraints, we provide a geodynamic reconstruction of the SAB since the Permian, in which it started rifting from Gondwana alongside the Pontides, likely reached the Iranian margin in Early Jurassic times, and was subject to episodes of intraplate (∼189 Ma) and NE-dipping subduction-related (∼117 Ma) magmatism.

Hydrogen and Silicon Effects on Hexagonal Close Packed Fe Alloys at High Pressures: Implications for the Composition of Earth's Inner Core

AbstractHexagonal close‐packed (hcp) structured Fe‐Ni alloy is believed to be the dominant phase in the Earth's inner core. This phase is expected to contain 4%–5% light elements, such as Si and H. While the effects of individual light element candidates on the equation of state (EoS) of the hcp Fe metal have been studied, their combined effects remain largely unexplored. In this study, we report the equations of state for two hcp‐structured Fe‐Si‐H alloys, namely Fe0.83Si0.17H0.07 and Fe0.83Si0.17H0.46, using synchrotron X‐ray diffraction measurements up to 125 GPa at 300 K. These alloys were synthesized by cold compression of Fe‐9wt%Si in either pure H2 or Ar‐H2 mixture medium in diamond‐anvil cells. The volume increase caused by a H atom in hcp Fe‐Si‐H alloys is approximately eight times greater than that by a Si atom. We used the improved data set to develop a composition‐dependent EoS that covers a wide range of compositions. Our calculated density and bulk sound velocity of hcp Fe‐Si‐H alloys suggest a large trade‐off between Si and H contents in fitting the seismic properties of the inner core. Combining our new EoS with geophysical and geochemical constraints, we propose 1.6–3 wt% Si and 0.15–0.6 wt% H in the Earth's inner core.

The influence of Cenozoic Eurasia-Arabia convergence on the Southeast Arabian Foreland Basin: new geochronological and geochemical constraints from syn-kinematic carbonate mineralization

The Cenozoic succession of the Jabal Hafeet anticline yields the most complete surface expression of the deformation that affected the Southeast Arabian Foreland Basin (SEAFB). The carbonate rocks of the Eocene Rus Formation comprise the core of the Jabal Hafeet anticline and host a network of fractures and carbonate veins associated with dynamic fracture opening and sealing events. These fracture networks developed during the propagation of compressional stresses from the Makran and Zagros fold-and-thrust belts into their foreland basin system (the SEAFB) and are associated with Arabia-Eurasia convergence. Syn-kinematic calcite veins associated with the Cenozoic folding events in the SEAFB were dated by U–Pb LA-ICP-MS carbonate geochronology and characterized further by Raman fluid-inclusion geochemistry. The U–Pb data show that Cenozoic compression linked to the propagation of the Makran fold-and-thrust belt into the SEAFB took place from c. 20 Ma (early Miocene) to c. 2 Ma (mid Pleistocene). Raman fluid-inclusion data reveal the presence of complex hydrocarbons within the parent carbonate-bearing fluids, reflecting a fluid transport pathway between the upper Cenozoic rocks and deeper hydrocarbon-bearing Mesozoic sequences. Combined isotopic and geochemical datasets show that the deformational history of the SEAFB is likely related to the reactivation of inherited deep-seated structures in the upper Cenozoic stratigraphic sequence due to the far-field stress propagation from the Makran belt into the Arabian peninsula, rather than the propagation of a thin-skinned deformation architecture.

Structural and geochemical constraints on the genesis of the Kuangshanchang carbonate-hosted zinc–lead deposit, Huize district, southwestern Yangtze Craton

Structural and geochemical constraints on the genesis of the Kuangshanchang carbonate-hosted zinc–lead deposit, Huize district, southwestern Yangtze Craton

The petrogenetic relationship between migmatite and granite in the Himalayan orogen: Petrological and geochemical constraints

Partial melting of crustal rocks contributes to the formation of granites in collisional orogens. It is generally recognized that residual granulites are compositionally complementary to crustal-derived granites, yet the petrogenetic relationship between migmatites and granites remains obscure. To shed light on this issue, a comprehensive study of field geology, petrographic observations, whole-rock major and trace elements, mineral major elements, zircon and monazite U–Th–Pb isotopes and trace elements, and monazite SmNd isotopes was carried out for the Higher Himalayan leucosomes, leucocratic dikes, and leucogranites from the Cona area in eastern Himalaya. These rocks were formed by partial melting of the Higher Himalayan metasedimentary rocks and granitic gneisses in the Miocene, and are characterized by distinct petrological and geochemical features. Leucosomes and leucocratic dikes, which were derived from different source rocks (granitic gneisses and metasedimentary rocks, respectively), share many similarities. Both of them are rich in K-feldspar and have high K2O, Ba, and Sr contents and CaO/(CaO + Na2O) ratios, but low Rb/Sr and Sm/Nd ratios; LREE contents correlate positively with Th contents, but negatively with P2O5 contents; zircon and monazite have low MREE/HREE ratios. In contrast, leucogranites, which originated from mixed sources of metasedimentary rocks and granitic gneisses, are rich in plagioclase and have low K2O, Ba, and Sr contents and CaO/(CaO + Na2O) ratios, but high Rb/Sr and Sm/Nd ratios; LREE contents correlate positively with Th and P2O5 contents; zircon and monazite have high MREE/HREE ratios. The K2O-rich leucosomes and leucocratic dikes and the Na2O-rich leucogranites do not show geochemical signatures indicative of a link owing to melt evolution processes, suggesting that the two main groups are the products of distinct and independent anatectic processes in the source. The leucosomes and leucocratic dikes are associated with water-fluxed (hydration) melting at lower temperatures and pressures, whereas the leucogranites are related to water-absent (dehydration) melting at higher temperatures and pressures. In this regard, the leucosomes and leucocratic dikes in migmatites and the leucogranites likely represent two different types of anatectic products, which were respectively derived from partial meltig of the hydrated crustal rocks at the shallower level and the dehydrated crustal rocks at the deeper level. The hydrated-dehydrated crustal architecture is ascribed to the influx of water into the shallower crustal rocks caused by metamorphic dehydration at the deeper crustal level. The spatiotemporally coupled dehydration-hydration anatectic processes may be a common mechanism for the formation of coherent granite, migmatite, and granulite in collisional orogens.

Mineralogical and Geochemical Constraints on the Occurrence Forms of REEs in Carboniferous Karst Bauxite, Central Guizhou Province, Southwest China: A Case Study of Lindai Bauxite

Karst bauxite is a major source of aluminum and contains high concentrations of trace elements such as Li, Ga, Sc, and rare earth elements (REEs). It is regarded as a potential non-conventional REE source due to the increasing demand for REEs. This study provides new mineralogical and geochemical evidence of a Lindai bauxite deposit in central Guizhou Province, aiming to analyze the genesis of REE minerals and reveal the forms of REEs occurring in these deposits. The results indicate that a large number of detrital and authigenic rare-earth phosphate minerals, including monazite and xenotime, were identified. In terms of their genesis, the authigenic monazite was mainly precipitated under alkaline and reducing conditions, whereas the authigenic xenotime was formed in acidic and reducing conditions. The mineralogical evidence detected by scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and a TESCAN Integrated Mineral Analyzer (TIMA) suggests that the REEs in the Lindai bauxite exhibit multiple forms, including rare-earth phosphate minerals (monazite and xenotime) and REE scavenging by hematite and anatase phases. Among them, the anatase controls some LREE (such as La, Ce, and Pr) distributions in addition to monazite, whereas the hematite also controls relatively large amounts of Sm, Eu, and HREE (such as Gd, Tb, Dy, Ho, Er, Tm, and Lu) distributions. According to the calculation of the percentage of REE-bearing mineral phases obtained by a TIMA analysis, REE scavenging by hematite accounted for 93.28% of all forms of REEs, REE scavenging by anatase accounted for 5.88%, monazite accounted for 0.67%, and xenotime accounted for 0.17%. This study provides new evidence of the forms of REEs occurring in Carboniferous karst bauxite in central Guizhou Province.

Origin and Paleoenvironmental Conditions of the Köprüağzı Evaporites (Eastern Anatolia, Turkey): Sedimentological, Mineralogical and Geochemical Constraints

Köprüağzı evaporites mainly consist of primary (selenite and gypsarenite) and secondary (massive, laminated and satin-spar) gypsum lithofacies, as well as minor anhydrite, and are interbedded with clastic and carbonate units. Sedimentological-mineralogical and geochemical findings reveal that the depositional basin extends from a lagoon to the hinterland. These data confirm that too much detrital input was transported into the basin with multiple salinity and pH value fluctuations, organic matter activity and reducing conditions. Geochemical data point out the shallow environment and the mixing of hydrothermal fluids and fresh waters. The investigated evaporites were exposed to diagenesis and alteration under the influence of a hot—dry and minor humid climate, tectonism and pressure. The 87Sr/86Sr, δ34S and δ18O isotope values of Köprüağzı evaporites reveal that they are of Miocene marine sulfate origin.

Geological and Geochemical Constraints on the Origin of the Sr Mineralization in Huayingshan Ore District, Chongqing, South China

There are many celestine deposits and mineralization points in the Huayingshan ore district which form the largest strontium resource base in China. Among these celestine deposits, the Yuxia and Xinglong are two of the larger deposits. Previous studies have displayed different views on the genesis of the celestine deposit in the Huayingshan ore district. In this study, we conducted field obversions, geochemistry, and fluid inclusion studies to investigate the sources of ore-forming matters and the metallogenic mechanism of the celestine deposit. Four types of fluid inclusion (FI), namely PL (pure liquid FI), PV (pure vapor FI), L-V (liquid-vapor two-phase FI), and L-V-S (liquid-vapor-solid three-phase FI) have been identified in celestine from different types of ore in the Xishan anticline. The ore-forming fluids belong to the NaCl-H2 O system with moderate to low temperature (190–220 °C) and moderate salinity (5–9 wt%, NaCl equiv.). Different types of ores were formed by the same period of hydrothermal activity, which is supported by the results of the microthermometer study. Geological, thermometric data, and published hydrogen and oxygen isotope results indicate that the hot brines associated with mineralization mainly originated from meteoric water and some of diagenetic fluid. The Sr (87Sr/86Sr = 0.7076–0.7078) and S (δ34S = 36.4–39.0) isotope values of celestine are consistent with those of the Jialingjiang Formation, indicating that ore metals in hot brines were predominantly derived from that formation. In situ analysis of celestine shows that there is a strong negative correlation between Sr and CaO (R2 = 0.95) and combined with mineralogical and isotope geochemical evidence, we concluded that the precipitation mechanism of celestine is the replacement of gypsum with Sr-rich hot brines. Based on the above research and the classification of celestine deposit type, we classified the celestine deposits in Huayingshan as being of hydrothermal type. The formation of celestine deposits can be divided into three periods: (1) evaporation period, forming the source bed; (2) hydrothermal activity period, forming celestine by replacement of gypsum with Sr-rich hot brines; (3) supergene period, where meteoric water dissolves orebodies and strontianization occurs.

Origin of the post-collisional younger gabbroic rocks and the associated Fe–Ti oxide ores, Abu Ghalaga area, Southern Eastern Desert, Egypt: mineralogical and geochemical constraints

The Abu Ghalaga pluton in the southern eastern desert of Egypt (SED) consists of undeformed norite and hornblende gabbro intruding island arc metavolcanics, ophiolitic rocks, and syn-tectonic older granitic rocks. The Fe-Ti ore (mainly ilmenite) occurs in the form of lenses at the base of the norite. Based on microtextures, mineralogy, and chemical composition, three types of ilmenites are recognized: massive high-Mg type, disseminated high-Mg type, and disseminated low-Mg ilmenite. The three ilmenite types are chemically distinguished by their MgO, MnO, TiO2, Fe2O3, FeO, and V2O3 contents. The variations in the concentration of these oxides are interpreted as a function of variations in T, fO2, and the composition of the parental magma. Reconstruction of the orthomagmatic conditions shows that T and fO2 of the magma from which the norite and its Fe-Ti deposit formed decrease during the course of magmatic differentiation, whereas the hornblende gabbro and its Fe-Ti mineralization have formed from a more evolved magma under relatively lower T and fO2. The REE patterns and variation diagrams support a model of crystal fractionation as the primary means of formation of the Fe-Ti deposits. Chemical features also suggest that the parent magma of both types of gabbroic rocks was generated in the mantle, but in two different pulses. Subduction zone, MORB, and WPB chemical signatures for these gabbroic rocks suggest a complex tectonic history consistent with compression followed by within plate extension possibly contributing to the development of a back-arc/intra-arc basin.

Petrography, geochemistry and detrital zircon geochronology of the Srisailam Quartzite Formation, Cuddapah Basin, India: Implications for depositional age, correlation and provenance

The Mesoproterozoic Cuddapah Basin consists of four unconformity-bound sequences, i.e., Papaghni Group, Chitravati Group, Srisailam Formation and Kurnool Group. A fair amount of geochronological and geochemical data is available from the basal Papaghni Group and Chitravati Group as well as the overthrusted Nallamalai Group. Due to limited geochronological and geochemical constraints, the Srisailam Formation, however, remains a subject of speculation regarding the time of sediment deposition, sediment source locations, and tectonic setting. In this work, we present new geochronological and geochemical data (zircon U-Pb) from samples of the Srisailam Formation. The new data suggest that the Srisailam Formation correlates temporally with the basal Chitravati Group and is largely disparate from the allochthonous Nallamalai Group. The sediments of the Srisailam Formation contain a minor yet significant number of Palaeo-Mesoarchaean detrital zircon with traces of Haedean zircon. Based on our detrital zircon data, we argue that the sedimentation in the Srisailam sub-basin initiated after ca. 1840 Ma and continued till ≤ ca. 1770 Ma and the sediments were mainly sourced from Neoarchaean to Paleoproterozoic terrains including the Dharwar Craton, and sedimentation was initiated in a rift basin that subsequently evolved to a passive margin setting.

Mineralization age and genesis of the makeng-style iron deposits in the Paleo-Pacific tectonic domain of South China: In situ LA-ICPMS garnet U-Pb chronological and geochemical constraints

To reveal the genesis of Makeng-style iron polymetallic deposits from SE China in the paleo-Pacific tectonic domain, a new analytical method of LA-ICPMS garnet U-Pb dating and rare Earth element analysis was conducted for the Makeng, Luoyang, Dapai and Pantian deposits. The U-Pb dating results of nine garnet skarn samples from these deposits suggested that the Makeng-style iron polymetallic deposits mainly formed during 137–130 Ma, which is consistent with the zircon U-Pb and molybdenite Re-Os ages. This study provides more direct evidence of the mineralization age and the relationship between mineralization and granite, compared with previous studies on the zircon U-Pb dating for granites in the ore fields. Rare Earth element (REE) analysis results and REE patterns of four representative garnet samples from the Makeng, Luoyang, Dapai and Pantian deposits show that they are similar to typical skarn deposits, but obvious differences in the REE distribution types indicate that the ore-forming process may be distinct due to different mineralizing fluid for these Makeng-style deposits. Our new garnet U-Pb dating and rare Earth element analysis result not only provides new evidence for the mineralization age and genesis of the Makeng-style deposits but is also of great significance to promote the application of U-Pb dating methods to research skarn type deposits.

Cumulate Formation and Melt Extraction from Mush-Dominated Magma Reservoirs: The Melt Flush Process Exemplified at Mid-Ocean Ridges

Abstract Volcanism is the surface expression of extensive magmatic systems, with their intrusive counterpart representing ~80% of the total magma budget. Our knowledge of igneous processes therefore largely relies on our understanding of deep plutonic processes. In continental or oceanic environments, most of the intrusive igneous rocks bear geochemical cumulate signatures (e.g. depletion in incompatible elements and enrichment in compatible ones) that are commonly explained by mineral-melt segregation during differentiation. Deformation-assisted compaction aided by melt buoyancy is usually referred to as the main process involved in melt extraction. However, buoyancy alone is not sufficient, and a number of cumulative rocks are lacking any compaction evidence, opening the potential for the involvement of other processes. In addition, our view of magmatic systems has shifted in the last decades from large melt-rich bodies to crystal-rich magma reservoirs. This paradigm shift challenges some of the long-established first-order igneous concepts like the idea that melt differentiation at depth is mainly governed by (fractional) crystallization; alternatively, the presence of mush potentially favors additional processes such as melt-mush reactions. We propose a novel igneous process for the formation of igneous cumulates, consistent with the mushy nature of oceanic igneous reservoirs, their continuous/cyclic replenishment by primitive melts, and the widespread occurrence of reactive porous flow (RPF) during magma differentiation identified in a growing number of magmatic systems. The melt flush process relies on melt-mush reactions between the primitive recharge melt(s) and crystal mush. Replacement of the more evolved interstitial melt by the primitive recharge melt leading to reactions (dissolution+crystallization) and concomitant extraction of the more evolved melt from the cumulate by buoyancy participate in the acquisition of the final cumulate signature. This process relying on oceanic igneous systems considers for the first time melt inputs and not only melt extraction and matches the petrographic (e.g. mineral dissolution evidence) and geochemical constraints (trace element signatures) brought by natural oceanic samples. We tested various melt-mush reactions likely involved in the early stages of the melt flush process during RPF to investigate their thermodynamic feasibility with the Magma Chamber Simulator. First-order results show that one-step equilibration of primitive melts with primitive to moderately differentiated mush crystals triggers mineral assimilation. Together with the constraints established from the natural rock record, it strengthens the idea that RPF is a potential key process for magma differentiation in magma reservoirs at different evolution stages. The proposed melt flush process eventually adds to other processes involved in cumulate formation like magma compaction or crystal settling and is likely to apply to any other magmatic system from various settings sharing similar reservoir characteristics.

Geochemical constraints on the multistage evolution of the Yilashan ophiolite, central Tibet

The Yilashan ophiolite in the southern part of the central Bangong-Nujiang suture zone comprises variably deformed harzburgite, dunite, and less abundant gabbro and pyroxenite pods and crosscutting veins. Cr-spinels in podiform chromitite bodies are characterized by high Cr# (64.3–74.3), high Mg# (58.2–67.5), and consistently low contents of TiO2 (≤0.12 wt%). Primary melt calculations designate parental boninitic magmas with Al2O3(liquid) contents of 11.67–13.49 wt%, and (FeO/MgO)liquid values of 0.92–1.12.Whole-rock geochemical data of fresh pyroxenite samples are characterized by negative K, Ta, Nb and Zr anomalies, no Eu anomaly, and chondrite-normalized REE patterns comparable to the N-MORB melts. The gabbro samples, in contrast, are high-K calc-alkaline, with elevated contents of LILE (Rb, K, Th) and depleted HFSE (i.e., Nb, Ta, P, Ti), consistent with subduction-related magmas. The fractionated chondrite-normalized REE patterns, high LREE/HREE values, and negative Eu anomalies of these gabbroic rocks are similar to the island arc basalts. Pyroxenite shows high PPGE/IPGE values, variable Pd/Ir (55.7–316.7), and left-leaning primitive mantle-normalized PGE pattern, whereas massive chromitite is enriched in IPGE relative to PPGE. The PGE patterns of pyroxenite and chromitite, though distinct, depict significant fractionation and partial melting in their formation. The elevated contents of PPGEs in pyroxenite suggest derivation from sulfur-saturated magmas as chromitite crystallization scavenged the compatible IPGEs from the parental melt. This assumption is supported by clinopyroxene in pyroxenite with variable Al2O3 contents and variable Cr2O3 contents (0.32 to 0.73 wt%). For the fractionated gabbro unit, zircons yielding a concordia U-Pb age of 162.8 ± 3.0 Ma and widely variable εHf(t) values (−9.2 to +12.0) suggest formation from heterogenous or reworked source materials during a Middle Jurassic subduction increment in the Neo-Tethyan Ocean.

Geochemical Evolution of Dolomites in Thanagazi Formation of Mesoproterozoic Alwar Basin, Northwest India

The geochemistry of dolomite in part of the Thanagazi Formation in the Mesoproterozoic Alwar Basin has been studied employing major and trace elements, including rare earth elements (REEs) to understand their provenance and paleoweathering conditions. In the current study, two distinct types of dolomites were identified viz. amorphous and crystalline. Various geochemical discriminants such as SiO2 vs Al2O3 and CaO vs MgO indicate that the dolomites were originated from sedimentary sources with substantial metasomatic and biogenic contributions. Geochemical constraints from plots of Fe2O3/Al2O3 vs. Al2O3/(Al2O3+Fe2O3) and rare earth element compositions for provenance signatures suggest likely deposition of dolomites in shallow marine sedimentary environment. Based on geothermometric estimates and base metal concentrations in dolomites, it is inferred that the mixing of sediments derived from two different sources (metasomatic and biogenic) were likely responsible for the dolomitization in the Thanagazi Formation of the Mesoproterozoic Alwar Basin.