Wall-rock Assimilation Research Articles

Unraveling the Magmatic-to-Carbothermal Processes in the Ba-Sr-REE Mineralization of the Sevattur Carbonatites, India

Abstract The Neoproterozoic Sevattur carbonatite complex (India) consists of calcite carbonatite, silicate-poor- and silicate-rich (banded and blue) dolomite carbonatite. The trace element composition of the calcite and ferroan dolomite shows that most of them have magmatic characteristics. The predominance of dolomite carbonatite over calcite carbonatite and the absence of a genetically related silicate suite at Sevattur suggest a dolomitic parental magma derived by the direct melting of the mantle. Regardless of the prevalent magmatic character of the Sevattur carbonatites they host a plethora of Ba-Sr-REE minerals formed throughout the orthomagmatic-to-carbothermal stages under varying activities of ${\mathrm{F}}^{-}$,${\mathrm{CO}}_3^{2-}$, ${\mathrm{PO}}_4^{3-}$and ${\mathrm{SO}}_4^{2-}$. The orthomagmatic stage begins with the crystallization of calcite carbonatite containing xenocrystic diopside, antecrystic Y-bearing calcite (Cal-I), and primary benstonite, apatite, and richterite in a Sr-Mg-rich calcite (Cal-II) matrix. Fractionation of apatite, carbocernaite, and cordylite-(Ce) together with the enrichment of Mg, Fe, Na, and F in late-magmatic stages, forms the dolomite carbonatite. The decomposition of benstonite to norsethite, Ba-Ca carbonate, and strontianite occurs within dolomite carbonatite at the same stage with minimal wall-rock assimilation. Interaction between the xenocrystal clinopyroxenite and the primary magma during late-magmatic stages has resulted in the formation of aegirine and phlogopite in calcite carbonatite. Similarly, the prolonged assimilation of clinopyroxenite by the evolving magma leads to the formation of magnesio-riebeckite and magnesio-arfvedsonite within the banded- and blue carbonatites. The increasing activities of ${\mathrm{PO}}_4^{3-},{\mathrm{SO}}_4^{2-}$and H2O during the carbothermal stage, destabilizes precursor carbocernaite and cordylite-(Ce), resulting in an assemblage consisting of synchysite-(Ce), ancylite-(Ce), daqingshanite, hydroxylbastnäsite-(Ce), ferriallanite-(Ce), monazite-(Ce), and baryte in dolomite carbonatite. In the banded carbonatite, the carbothermal stage is characterized by ferri-winchite, tremolite-actinolite, and phlogopite crystallization. Further contamination and differentiation of the carbothermal fluid have resulted in the crystallization of quartz and albite in blue carbonatite. Phlogopitization within the blue carbonatite and clinopyroxenite results from late K enrichment in the carbothermal fluids.

Intrasample osmium isotope disequilibrium in young volcanic rocks: Insights from a new progressive digestion procedure

Some young volcanic rocks have been found to possess intrasample Os-isotope heterogeneity. This has important implications for source tracing and dating. Here, we use a new procedure through which it is possible to sequentially extract, from a single aliquot, the 187Os/188Os ratio of: i) the surface contaminant(s) (hydrobromic acid leachate); ii) the residual bulk rock powder (Step I) and iii) the Os-bearing phases entrapped within silicates (Step II). We applied this technique to two young basaltic reference materials (RMs), USGS BHVO-2 and EN026 10D-3. The former samples the 1919 lava from the Kīlauea summit crater (Hawaii) and the latter is a basalt sample dredged from Mohns Ridge (Greenland Sea). Both rock standards have previously been shown to display large intrasample Os isotope heterogeneity. In addition to analyzing these RMs, we also analyzed the RM USGS BHVO-1 for comparison, as well as another sample from the Kīlauea 1919 summit eruption (Kil 1919-6) along with a picritic tholeiite from the Kīlauea Iki 1959 eruption.Our leaching results show that BHVO-2 is contaminated with a highly radiogenic and labile contaminant with 187Os/188Os ratio of 0.525 ± 0.021. Without leaching, the Step I 187Os/188Os ratios varied from 0.1399 ± 0.0013 to 0.1568 ± 0.0014 while those for Step II were uniform (average = 0.1286 ± 0.0006, N = 5). A comparison of Re, Mo concentrations and 187Os/188Os ratio of the leachates of BHVO-2 and its predecessor (BHVO-1) indicates that BHVO-2 was contaminated during preparation. After leaching, the Step I and Step II 187Os/188Os ratios of BHVO-1 and another sample (Kīlauea 1919-6) from the same lava flow are identical within error to that of the Step II ratio of BHVO-2. A picrite from the 1959 Kīlauea Iki eruption also analyzed using the new technique gives homogenous results for both the steps with an average 187Os/188Os ratio of 0.1302 ± 0.0005. The 187Os/188Os ratio of recent Kīlauea lavas is identical within error to that of the primitive upper mantle (=0.1296). The Kīlauea Os-isotope composition could represent a “common component” that occurs globally in many plumes. The Step I 187Os/188Os ratio of EN026 10D-3 is 0.136 with Step II ratio being 0.131 (N = 2). The Step II 187Os/188Os ratios are more radiogenic than the nearby Jan Mayen lavas (187Os/188Os = 0.124–0.129) that have been argued to influence the chemistry of the Western section of the Mohns Ridge basalt but fall within the range of the Primitive Upper Mantle. We also discovered that BHVO-1, KIL 1919-6, 1959 Kīlauea Iki picrite, and EN026 10D-3 show Os-isotope disequilibrium between the leachate and Step I, suggesting wall rock assimilation during magma ascent. The data set presented here indicates a need for re-evaluating the extent to which mantle heterogeneities influence the Os isotope composition of ocean basalts.

Origin of the Permian high silica A-type granites in Halajun, South Tienshan, NW China: Implications for crystal-liquid segregation and rare metal fertilization

High silica A-type granites (HSAGs) are important carriers of rare metal elements. Formation of HSAGs by crystal-liquid segregation would facilitate fertilization of rare metals and rare earth elements (REE). In this work, we present detailed petrology, whole-rock geochemistry, and in-situ zircon UPb ages and HfO isotopic compositions of the Halajun HSAGs in the South Tienshan Orogenic Belt (STOB). In combination with the previous study of the Halajun syenite and monzonite, we unravel the genetic connection of these granitoids and further explore the metallogenetic potentiality of HSAGs along the STOB in China. The Halajun granites have geochemical affinities of HSAGs with high SiO2 contents (> 70 wt%), whole-rock zirconium saturation thermometry (680–860 °C), high Ga/Al ratios and high field strength elements (HFSE) contents. The HSAGs (273–279 Ma), monzonite (280 Ma) and syenite (283 Ma) have similar formation ages, identical mineral assemblages, and consistent variations in major elements, implying a common magma reservoir of their parental magma. The Tamu and Huoshibulake HSAGs have high SiO2 (74.5–78.1 wt%) contents, low Sr (5–28 ppm) and Ba (7–60 ppm) contents, high Rb/Sr ratios (11.6–87.5), and strong negative Eu anomalies (0.01–0.08). Combined with the petrographic features, these geochemical variations suggest that they are the extracted melt. Conversely, the geochemical (high Sr and Ba concentrations, and Eu/Eu* near unity) and petrographic features of monzonite and syenite represent the residual silicic cumulate (crystals ± trapped melt). The Kezi'ertuo and Halajun II HSAGs have SiO2 (71.9–76.5 wt%) contents, Sr (35–101 ppm) and Ba (60–541 ppm) contents, Rb/Sr ratios (1.9–8.9), and negative Eu anomalies (0.04–0.43), indicating the intermediate component (extracted melt + fractional crystals) in the consecutive system of crystal-liquid segregation. In-situ zircon HfO isotopes for the Halajun granitoids indicate a complex trans-crustal magmatic process involving crystal-liquid segregation and wall-rock assimilation. The long-time span of Tarim Large Igneous Province (TLIP) sustained injection of mantle-derived magma and/or advective heat to the magma chamber (> 1 Ma). Owing to the prolonged magma evolution, the highly fractionated silicic magma with high incompatible elements and F contents was extracted from the magma mush, and formed the HSAGs.

Comparison of the major- and trace-element compositions of the Kang-dian and Rössing-Husaib uraniferous leucocratic rocks

The Rössing-Husaib area is well-known for its low-grade and large-reserve U deposits hosted by leucogranites. The leucocratic dykes in the Kang-dian area of China resemble the Rössing-Husaib uraniferous leucogranites in occurrences and mineral assemblages, however, it remains disputable if the Rössing-Husaib and Kang-dian leucocratic rocks have consistent U-mineralizing processes. A comparison of the major and trace element data of the leucocratic rocks from both areas may help to better understand the ore-forming mechanisms and provide implications for exploration in the Kang-dian area. The positive correlation between Rb and P2O5 contents for the Rössing leucogranite samples is typical of S-type granitoids in spite of the low A/CNK ratios (∼1.0). The Rössing-Husaib uraniferous leucogranites are generally rich in K2O and Rb, and have high Rb/Sr and Rb/Ba ratios. In contrast, the Kang-dian leucocratic dykes have lower K2O and Rb contents, and lower Rb/Sr and Rb/Ba ratios, suggesting a clay-poor source. The U contents of the Rössing-Husaib and Kang-dian dyke samples are positively correlated with TiO2 contents, but show no correlation with Rb/Sr ratios, which is inconsistent with fractional crystallization. Low-degree partial melting of a U-rich protolith may account for primary U enrichment in the leucocratic rocks. The variable major- and trace-element compositions of the Kang-dian dyke samples may reflect wall-rock assimilation and/or alteration to varying degrees, which further enhanced U enrichment. Relatively low Th/U ratios (∼0.1) favored the final crystallization of uraninite. The uraniferous leucogranites in the Kang-dian and Rössing-Husaib areas are commonly undeformed and emplaced along regional large faults and/or domes at the late stage of orogeny. We propose that decompression and uplift of basement rocks facilitated the generation of uraniferous melts, and regional large faults provided a pathway for the ascent of leucocratic magma from depth.

Why are S-type plutonic rocks more chemically variable than S-type volcanic rocks and S-types more variable than I-types?

S-type granitic magmas form essentially through partial melting of crustal metasedimentary rocks, so this eliminates many possibilities for the origins of chemical variations in their parental magmas. Global, whole-rock, major-oxide data show that S-type volcanic series have far less chemical variability (analytical scatter) than S-type plutonic rocks. On the other hand, published data show far smaller contrasts between metaluminous ‘I-type’ silicic plutonic and volcanic rocks. There are several possible explanations for these features, but sampling bias and analytical errors can be discounted. Likewise, the differences cannot be explained by plutonic S-types being crystal cumulates or products of wall-rock assimilation, nor can they be due to volcanic suites being products of magma mixing or having compositions controlled by a different differentiation mechanism to that which might control S-type plutonic compositions. Variations in major-oxide concentrations suggest that the most probable explanation is that S-type plutonic magmas are derived from somewhat more aluminous metasedimentary source rocks, with higher ratios of metapelites to metagreywackes. If so, there are complementary implications that plutonic S-type magmas may be formed at lower temperatures, that they may be more hydrous than the S-type volcanic magmas, and that the differences in emplacement mechanism reflect this. The weaker contrasts between ‘I-type’ volcanic and plutonic rocks may be due to their crustal magma sources being more homogeneous and the melting reactions stoichiometrically similar, regardless of source-rock type.

Apatite Geochemical and Nd Isotopic Insights into Trachyte Petrogenesis in the Tianchi Volcanic Area of Changbai Mountain, NE China

AbstractWe report the oxide, element geochemistry and Nd isotopic geochemical data of apatite in the middle Pleistocene medium‐ and fine‐grained trachyte in the Tianchi volcanic area (TVA) of Changbai Mountain, discussing the relationship between apatite and the composition of the whole rock. The purpose is to use the apatite geochemical data to constrain the evolutionary process of trachytic magma and the petrogenesis of trachyte in the cone‐forming period of the Tianchi volcano. Apatite (Ca5(PO4)3(OH, F, Cl)) is a common accessory mineral that occurs widely in volcanic rocks in the TVA. The apatites in the trachyte are mainly subhedral–anhedral, having the characteristics of magmatic apatite. In terms of oxide and element geochemistry, they have homogeneous Al2O3, SiO2, MgO, P2O5, K2O, CaO and heterogeneous TiO2, with high F content. They are generally enriched in Th, U and LREEs, depleted in Nb, Ta, Zr, Hf and HFSEs, showing negative Ba, Sr and Ti anomalies, similar to those of the whole‐rock host trachytes. The ratios of high (La/Yb)N, low δEu (Eu/Eu*), Sr/Y value and ΣREE content in apatite, and the F, Sr, Y, Th/U, La/Sm, and Nd/Tb with ΣREE and δEu anomalies showed a linear correlation, all of those indicating that the host magma has the characteristic of high differentiation. The apatite grains generally having 147Sm/144Nd, 143Nd/144Nd ratios and εNd(t) values of 0.1072–0.1195, 0.5123–0.5126 and –3.49 to –0.10, respectively, are similiar to those of the host rock. The Nd model ages TDM1 are 949–803 Ma in apatite. Combined with the εNd(t) value of the apatite core (–7.06 to –3.49), we conclude that the initial magma of the host trachyte was derived from the partial melting of Proterozoic crustal material and there was an assimilation of wall rocks during its evolution.

Extreme Sn isotope fractionation in highly evolved granites

Stable Sn isotopes have been widely used to trace the planetary differentiation processes, the provenance and trade paths of ancient bronze artifacts, the mineralization history of tin ore deposits, and the terrestrial magmatic processes. However, systematic Sn isotope studies of felsic igneous rocks especially the Sn mineralization-related highly evolved granites are still lacking. In this study, we present the first high-precision Sn isotope data of two Sn-rich highly evolved S-type granites (Laochang and Kafang) from the world-class Gejiu Sn ore district. These granite samples show remarkably variable δ122/118Sn values (relative to the Sn standard solution NIST 3161a) ranging from 0.11 ± 0.02‰ to 0.93 ± 0.02‰ (2σ), which is the largest variation range reported in granites so far. The absence of correlations between Sn isotopes and LOI (loss-on-ignition) and εNd(t) values, along with the low magmatic oxygen fugacity (△FMQ < 0) for these granites, rule out hydrothermal alteration, wall-rock assimilation, source composition heterogeneity, and anatexis processes as causes of the Sn isotope fractionation. The Laochang granites show negative correlations between δ122/118Sn values and magmatic differentiation indicators (e.g., CaO, Fe2O3, Ti, Eu/Eu*, F/Cl), suggesting fractional crystallization increased the δ122/118Sn isotopes in the residual melts. The fractional crystallized minerals are mainly composed of biotite, plagioclase, and minor ilmenite. Compared with the Laochang granites, the Kafang granites show higher SiO2 contents, F/Cl ratios, and δ122/118Sn values, suggesting the magma at this stage was fluid-saturated and fluid exsolution coupled with fractional crystallization would further enrich the heavy Sn isotopes in the residual melts. We further quantified the contributions of crystal fractionation and fluid exsolution on Sn isotopic variability using a Rayleigh fractionation model. The modeling indicates that △122/118Sncrystal-melt = −0.6 to −0.5‰ and △122/118Snfluid-melt = −1.9 to −0.7‰ with an initial H2O content of 2 wt.% can explain the observed δ122/118Sn values of Gejiu granites. This study implies that both crystal fractionation and fluid exsolution processes cause increase of Sn isotopic values in the highly evolved granites and Sn isotopes are useful for tracing the evolution of highly differentiated igneous rocks.

Sulfate recycling at subduction zones indicated by sulfur isotope systematics of Mesozoic ultramafic island arc cumulates in the North American Cordillera

The isotopic composition of sulfur in convergent margin-related orthomagmatic sulfides is critical to the debate on: 1) petrological mechanisms of sulfide saturation in relatively oxidized magmatic systems, and 2) the relative contribution of subducted sulfate to the overall sulfur budget of the sub-arc mantle. Magmatic sulfides that crystallize in deep-seated plutonic rocks are less susceptible to isotopic fractionation arising from magmatic degassing and are thus better suited for studies of primary S-isotope compositions of arc magmas than their extensively degassed volcanic counterparts. Isotopic compositions of chalcopyrite and pyrrhotite, which crystallized from trapped immiscible sulfide melt, and secondary fibrous pyrite after pyrrhotite in the Polaris Alaskan-type ultramafic-mafic intrusion (Early Jurassic) of the North American Cordillera were determined by secondary ion mass spectrometry. Chalcopyrite grains from five olivine clinopyroxenites are fresh and homogeneous and they have uniform near-chondritic δ34S values (-0.19 +0.48/-0.32‰; n = 97). In contrast, pyrrhotite (δ34S = -1.6 +2.8/-0.6; n = 34) and secondary pyrite (δ34S = -2.7 +1.8/-4.4‰; n = 21) grains display subchondritic but highly variable δ34S. The marked difference in measured δ34S of different sulfide phases attests to the critical advantage of using in-situ microanalytical techniques over whole-rock isotopic analysis. Pyrite grains from hornfelsed volcanogenic country rocks have elevated S-isotope values (δ34S = +7.4 +1.3/-1.7‰; n = 11). The near- to subchondritic isotopic compositions of sulfides from the Polaris intrusion demonstrate that wall-rock assimilation played a minor role and that sulfur in the Polaris magmas is largely magmatic. Fibrous textures and variable, but negative, δ34S of pyrrhotite and pyrite indicate post-magmatic mobility of sulfur due to low-temperature alteration by hydrothermal fluids. The narrow range of chalcopyrite δ34S, however, indicates that it remained unaffected by hydrothermal processes and equilibrated under uniform magmatic physicochemical conditions. Assuming sulfide liquid immiscibility at T ≥ 800 °C, geochemical modeling of S-isotope fractionation indicates that the Polaris parental magmas had suprachondritic δ34S between +1‰ (at log fO2 = FMQ+1) and +5‰ (at log fO2 =FMQ+2), consistent with the contribution of subducted sulfate to the sub-arc mantle.

Formation of Fe-Ti oxide and Ni-Co sulfide ores by concentric tube flow and hydrous metal-rich melt recharge into the cooling crystal mush: Example from the Hongge intrusion in Panxi region, SW China

The Panxi (ultra-)mafic intrusions are relatively small compared with world-class ore-related (ultra-)mafic intrusions, but they host several super-large Fe-Ti oxide deposits and few small Cu-Ni (PGE) sulfide deposits. The mechanisms that concentrate huge amounts of oxide minerals into these small intrusions and form the distinct oxide and sulfide mineralization remain poorly understood. Here, we investigate the magma dynamics that produced extensive Fe-Ti oxide and coexisting Ni-Co sulfide mineralization in the Hongge intrusion of Panxi region, based on detailed field cross-cutting relations and petrographic observations, bulk-ore/rock geochemistry, and in-situ LA-ICP-MS magnetite trace element analyses. The Fe-Ti oxide ores include magnetite-poor (<60 vol%) disseminated and magnetite-rich (>60 vol%) massive types, with the former further divided into the gabbro-hosted (GH) and (olivine-)pyroxenite-hosted (OPH) subtypes. The GH oxide ores have Cr-poor (median 86.9 ppm) magnetite and abundant apatite interstitial to silicate minerals. In contrast, the OPH and massive oxide ores have Cr-rich (median 13,956 ppm) magnetite that envelops resorbed silicate minerals. Principal component analyses of bulk-ore and magnetite elemental contents reveal genetic links between the OPH and massive oxide ores, but remarkable genetic distinction from the GH oxide ores. Considering the overall lopolith-shape orebody geometry of several branch intrusions, the GH oxide ore formation may have involved spontaneous development of concentric tube flow zones within the cooling crystal mush. However, the lower P but much higher Cr-Cu-Co-Ni contents than the GH oxide ores and the sharp intrusion of massive oxide ores into the OPH oxide ores suggests that the OPH and massive oxide ores were probably formed by upward percolation and later intrusion of hydrous Fe-rich melts from a deep-seated magma reservoir into semi-solidified and fossilized mush, respectively. In addition, sulfide ores occur mainly as irregular/lenticular aggregates in massive oxide ores and are depleted in Cu and platinum group elements (PGEs), indicating early, deep-level sulfide segregation that scavenged the Cu and PGEs from the magmas. Hence, we envisage that the assimilation of S-deficient wall-rocks into basaltic magmas may have induced the early sulfide saturation and retain the majority of Fe for later precipitation as principal oxide and minor PGE-Cu-depleted sulfide ores.

Mineralogy and Geochemistry of Nepheline Syenite From the Bang Phuc Massif of the Alkaline Cho Don Complex in North-Eastern Vietnam—Implications for Magma Evolution and Fluid–Rock Interactions

Abstract The alkaline Cho Don complex in NE Vietnam comprises several mafic-felsic suites related to the widespread magmatism developed during the early Permian–late Triassic. The contribution explores the petrogenesis of nepheline syenite from the Bang Phuc massif and its petrogenetic relationship with cogenetic scapolite-rich gabbro. The nepheline syenite formed through fractional crystallization of pristine mantle-derived basaltic melt modified by subduction-related components (chiefly sediment-derived melts), as shown by, e.g. low Ba/Th and high Th/Nb ratios of the rocks. The transition from gabbro to syenite follows a within-plate enrichment trend (e.g. increasing Ta/Yb, Nb/Yb, and Th/Yb ratios) that might reflect switch from post-orogenic to intra-plate regimes, accompanied by subduction–collision–extension events related to the Indosinian Orogeny. Furthermore, magma evolution involved the progressive contribution of asthenospheric-derived melts that resulted in the appearance of OIB-like signatures (e.g. high Nb/La ratios) in the nepheline syenite. Fractional crystallization of fluorapatite and mafic phases, as well as assimilation of carbonate wall rocks ultimately led to the decrease of LREE contents and/or modification of Zr/Hf ratios. Magmatic phases of the nepheline syenite include nepheline, sodalite, oligoclase, orthoclase, and annite, as well as accessory fluorapatite, fluorite, and minor amounts of zircon and metamict allanite-Ce. The nepheline equilibrated at temperatures ranging between 850°C and 700°C, which reflects protracted residence at a higher temperature. Later, it has been locally altered to cancrinite, dawsonite, and natrolite via CO2- and alkali-rich fluid influx. The fluid–rock interactions were also manifested by the presence of chessboard-twinned albite and coarsening of braid-perthite into patch-perthite, as well as recrystallization of primary orthoclase into microcline. The orthoclase→microcline conversion, albeit fairly indiscrete under a polarizing microscope and confirmed by Raman micro-spectroscopy, is followed by the change of cathodoluminescence colours, i.e. from light-blue (activated by Ti4+ and/or Al-O—Al centres) in orthoclase towards brownish and/or greenish (activated by Mn2+ and structural defects) in microcline.

Coupled U–Pb and Lu–Hf zircon data of Late Devonian magmatism from Bajo de La Leona (Deseado Massif, Argentina): Tracking the southernmost exposure of the Devonian magmatic arc of Patagonia

In Bajo de La Leona, located in the eastern Deseado Massif, pre-Permian basement rocks of the Río Deseado Complex crop out. They consist of high- to medium-grade metamorphic rocks intruded by two Paleozoic granitoids, namely the Mina de La Leona Granite and the Laguna Larga Granite. The latter is the main goal of this work. It is a calc-alkaline to high-K calc-alkaline, ferroan, calcic to alkali-calcic, and mainly peraluminous monzoleucogranite which plots within the I&S-type and volcanic-arc fields of the tectonic discrimination diagrams. Its trace elements composition (e.g., Sr/Y, LaN/YbN, Sm/Yb, Eu/Eu*, Dy/Dy*, Dy/Yb) suggests plagioclase ± clinopyroxene fractionation in a shallow source at low pressures. Assimilation could have played a role in magma differentiation, thus the magmatic development of the Laguna Larga Granite is likely a consequence of assimilation-fractional crystallization (AFC) processes. Coupled LA-ICP-MS U–Pb and Lu–Hf zircon data for the studied granite, as well as Sm–Nd whole-rock data, are here presented for the first time. A Famennian U–Pb zircon crystallization age of 361.9 ± 1.7 Ma was obtained, together with εHf(t) zircon values from -15 to +6 with TDM-Hf model ages between 2.0 and 0.9 Ga. A thin crust, as suggested by low values of thickness proxies such as Sr/Y (1.06–1.83) and LaN/YbN (3.97–4.96), would allow emplacement of juvenile mantle-derived material, giving rise to the suprachondritic εHf(t) values. A mantle-derived source is also suggested by a suprachondritic εNd(t) whole-rock value of +2.34 (TDM-Nd = 0.93 Ga). On the other hand, the subchondritic εHf(t) values could be related to wall rock assimilation. In a regional context, the obtained results for the Laguna Larga Granite allow to correlate it with coeval rocks from northern Patagonia (e.g., San Martín de los Andes batholith, Paleozoic Central Patagonian igneous-metamorphic belt). Therefore, the Devonian magmatic arc might have extended from the North Patagonian Andes and the western North Patagonian Massif to the eastern Deseado Massif along a NNW-SSE-striking belt, confirming previous proposals.

Open-system differentiation of mafic magmas before the formation of layered Fe-Ti(V) deposits in Southeast China

Mafic magmatic differentiation is an important mechanism to drive Fe-Ti enrichments in melt and final formation of layered Fe-Ti-(V) oxide deposits, but how the differentiation operates and how the state of magmatic reservoirs controls differential metallogenic potential are unclear. The Xialan magmatic complex in northern Guangdong Province is the only complex hosting layered Fe-Ti (V) deposits in Southeast China and the formation of such oxide deposits is not addressed in detail. Here, we present an integrated study of geochronology, mineral geochemistry, whole-rock major and trace elements, as well as Sr-Nd-Pb isotopes, on the mafic rocks associated with the Xialan layered Fe-Ti (V) oxide deposits. The investigated mafic rocks are cogenetic and can be classified into three types according to field and textural features: diabase, Fe-Ti rich gabbro, and Fe-Ti barren gabbro. Based on petrographic observation and major and trace element geochemistry, the gabbros and diabases represent cumulates and residual melts, respectively. However, the Fe-Ti rich gabbros show different Nd isotopes from the other rocks, which indicates an open-system differentiation for the Xialan parental magma. A two-stage evolution process for the Xialan parental magma before the final crystallization fractionation to form the oxide deposits is further proposed based on MELTs modeling results and the characters of mineral assemblages. At the early stage, the magmas started differentiation with 0.5 wt% initial H2O content and moderate oxygen fugacity (△FMQ = ca. + 0.5 – +1.5) at 5 kbar and generated Fe-Ti enriched melts. At the later stage, the Fe-Ti rich melts gradually evolved to a much higher H2O content (>3 wt%), which stabilized the Fe-Ti oxides and hornblendes. Wall-rock assimilation during the late-stage magma evolution is suggested to be the key factor to elevate the H2O contents and oxygen fugacity of the magma which finally produced the layered Fe-Ti oxides. As such, this study demonstrates that open-system differentiation of mafic magmas can play a key role in the final generation of some hornblende-bearing layered Fe-Ti oxide deposits.

Transcrustal and source processes affecting the chemical characteristics of magmas in a hyperactive volcanic zone

Arc magma generation and evolution are affected by transcrustal processes, such as assimilation and fractional crystallisation, and source processes like slab influx into the mantle wedge. However, the relative contribution of these mechanisms remains contentious. The Pleistocene to Holocene Taupo Volcanic Zone (TVZ) is unusually dominated by felsic volcanism with more than 95% of the total eruptive volume corresponding to rhyolitic magmas. Despite the relatively thin crustal basement (∼16–30 km), combined wall rock assimilation and fractional crystallization (AFC) of primary basalt has previously been presented as a necessary means to generate the chemical characteristics of these magmas. This petrogenetic model is here assessed using a new, internally consistent set of major and trace element concentrations, and Sr-Pb isotope data from a suite of basaltic to rhyolitic samples that are representative of the entire TVZ with respect to geographical distribution and age. The dataset was modelled for AFC using the energy- and mass-constrained Magma Chamber Simulator (MCS) and the local Permian to Early Jurassic Torlesse composite terrane basement used as the principal assimilant. Regardless of the intensive parameters employed, the energy- and mass-constrained models presented here fail to realistically reproduce the combined major oxide, trace element, and isotope systematics of the sample set. The Pb isotope ratios of TVZ basalts to rhyolites reveal a broadly linear trend ranging from an unradiogenic (206Pb/204Pb: 18.77) to a more radiogenic crustal endmember (206Pb/204Pb: 18.84). This trend can be reproduced by a composite mélange dominated by the Torlesse terrane (representative of tectonically eroded forearc crust), with some contribution of subducting sediments (GLOSS-II), interacting with the sub-arc mantle to generate the source of the magmas observed in the TVZ. This model is also consistent with Sr-Pb isotope systematics. At New Zealand’s active margin, tectonic erosion has been reported based on geophysical data. Therefore, it is proposed that slab input can have a stronger relevance in the magma generation of the TVZ than previously thought. The broad compositional variations in the TVZ may result from source contamination by a subduction mélange, with differentiation processes within the overriding crust being subordinate.

Using Zircons to Disentangle Back-Veining and Hybridization of Diorite Dykes: an Example From the Gangdese Arc, Tibet

AbstractThermo-chemical modeling and chemical data suggest that the genesis of arc melts commonly involves re-melting of older intrusions, triggered by the injection of mantle-derived melts, followed by magma mixing. Remelting and mixing may lead to complex zircon populations, which can be used to gain insights into the conditions of mixing. This paper investigates a case where such processes can be studied through the compositional and thermal record provided by zircons preserved in a diorite dyke swarm that remelted host rock tonalites in the Gangdese Batholith in Tibet. Autocrystic zircons from the diorite yield consistent ages of 46 to 47 Ma even though they can be altered, having highly enriched trace elements, reaching ~1 wt % in Fe, Ca, Y, U, Th, and anomalously high values of LREE and Ti. Granitic magmas derived from the tonalite, back-veined the dykes and mixed with the dioritic mush, transferring small quantities of 77–79 Ma zircon xenocrysts. The xenocrysts are euhedral with little evidence for resorption, indicating that they were apparently stable during the process of tonalite anatexis and transfer to the diorite magma. This requires that: (i) tonalite melting occurred at low temperatures with minimal zircon dissolution, and (ii) the diorite either cooled rapidly before significant resorption of the grains or was already saturated in zircon when mixing occurred. Zircon saturation temperatures of the diorite are relatively low, indicating that xenocrystic zircons were transferred to a highly crystalline dioritic mush. This requires either liquefaction by injection of the back-veining felsic magma to allow for mixing, or pervasive throughflow of the diorite mush by the felsic magma leaving behind zircon xenocrysts. The findings suggest that the dykes triggered low-T, water-fluxed melting of the host tonalite and that these anatectic melts invaded the diorite mush initially through the interstices leading to mixing. This may have caused the breakdown of the crystalline framework leading to liquefaction and renewed magma flow. Dyking and assimilation of wall-rock through back-veining as recorded in these rocks could be common in transcrustal arcs. However, this process could be hidden due to a combination of the similarity in the isotopic and chemical nature of arc rocks and resorption of zircon xenocrysts during mixing. This process may explain some complex chemistry of arc magmatic rocks and their minerals that are not easily explained by endmember models, such as pure re-melting of older arc rocks or fractional crystallization of mantle-derived melts.

Potential flare-ups and lulls in the multi-stage magmatism of the Dom Feliciano belt, southern Brazil: Evidence from geochemistry and isotopic data

The genesis of Precambrian granitic batholiths related to supercontinent assembly is controversial, especially regarding their generating processes and tectonic settings. The present study focused on the Pelotas Batholith (Magmatic-arc domain) of the Dom Feliciano Belt, which comprises the southern segment of the Mantiqueira Province in southeastern South America. New U–Pb and Lu–Hf zircon data from plutonic rocks with evidence of magma mixing and mingling, as well as assimilation of wall rocks, suggest multi-stage magmatism related to flare-up episodes of intense magmatism, potentially triggered by changes in the tectonic setting. U–Pb zircon analyses yielded crystallization ages of granodioritic gneiss xenolith at 691.5 ± 7 Ma, microgranodiorite xenolith at 645.9 ± 3.7 Ma, host monzogranite at 634.4 ± 4.4 Ma, and intrusive syenogranite at 620.1 ± 4.7 Ma, all intruded by syenogranitic pegmatite at 606.9 ± 1.7 Ma. Other analyses were performed on calc-alkaline granite (633 ± 4 Ma) and peraluminous granite (609.6 ± 2.2 Ma). The Lu–Hf data highlights a model age interval ranging from 1.7 to 1.3 Ga with εNd values between −4 and 1, except for the pegmatite sample which also includes Paleoproterozoic and Archean model ages (3.3-1.6 Ga) and εNd values between −33 and −1. Furthermore, the acquired data, together with the gathered dataset Dom Feliciano Belt granites, emphasizes a contribution from slightly juvenile to crustal sources, with model ages ranging primarily from Statherian-Orosirian to Mesoproterozoic. Signature patterns are consistent with sources from African basement rocks.

Heavy rare earth element and crustal-derived silicon enrichment in Huayangchuan carbonatites, Qinling orogenic belt

The Huayangchuan carbonatite is found in the North Qinling orogenic belt. It is distinguished from most carbonatites worldwide by its abundance of quartz and high heavy rare earth element (HREE) content. The origin of Si supersaturation and HREE enrichment in carbonatite is still unknown. Here, we conducted a detailed mineralogical investigation and in situ geochemical analyses on a suite of drill-core samples obtained from Huayangchuan carbonatite. In situ titanite and monazite UPb dating yielded ages of 212.3 ± 6.4 Ma and 211.8 ± 5.0 Ma, respectively, corresponding to the post-collision stage of the Qinling orogeny. Clinopyroxene and amphibole in the carbonatite are commonly intergrowth with quartz grains and trapped by quartz as mineral inclusions. The mineralogical textures and compositions indicate that quartz crystallized simultaneously with the mafic minerals from a Si supersaturation magma. The high Si activity during clinopyroxene formation may be related to Si assimilation from the wall-rocks. In situ Nd isotopic compositions of apatite and titanite show consistent εNd(t) ranging from −4.8 to −9.0, similar to the values of the whole-rocks and calcites. This indicates that the REE were derived from carbonatite magma without an external source. Two types of calcites were identified based on their cathodoluminescence characteristics, and they have variable chondrite-normalized REE patterns (La/Yb(CN) ranging from <0.01 to 19) but similar HREE contents. This implies that the carbonatite underwent strong crystal fractionation of early LREE-rich minerals, which may be essential for HREE enrichment. Therefore, we proposed that Si supersaturation in the carbonatite was probably caused by wall-rock assimilation, and fractional crystallization yielded primary HREE enrichment in the residual melts. Moreover, HREE enrichment in highly evolved carbonatites is further aided by high Si and alkaline activities.

Baddeleyite U-Pb age and Hf isotopes, and constraints on genesis of the Panzhihua carbonatite in SW China

The Panzhihua district is well-known for its magmatic V-Ti-magnetite deposit, with plenty of carbonatite intrusions and extrusives (total length: over 20 km), making it one of the world’s largest carbonatitic belts. Mantle-derived xenoliths (dunite, lherzolite and olivine lamprophyre) of different sizes are the most direct and important evidence of magmatic carbonatite. The carbonatite consists mainly of calcite, dolomite, olivine, clinopyroxene, orthopyroxene and biotite, almost without felsic minerals. We performed baddeleyite U-Pb dating on the four largest carbonatite intrusions at Panzhihua, which yielded weighted mean 206Pb/238U ages (from NE to SW: Zhujiabaobao, Jianbao, Gongshan and Lanaqing) of 261.8 ± 3.8 Ma (MSWD = 0.084, n = 28), 263.7 ± 2.3 Ma (MSWD = 0.10, n = 27), 260.6 ± 2.3 Ma (MSWD = 0.27, n = 22), and 262.3 ± 1.8 Ma (MSWD = 0.44, n = 34). Samples of three extrusive carbonatites and one carbonatitic breccia yielded weighted average baddeleyite 206Pb/238U ages of 261.7 ± 3.0 Ma (MSWD = 0.095, n = 33), 263.8 ± 2.5 Ma (MSWD = 0.21, n = 30), 263.2 ± 2.9 Ma (MSWD = 0.13, n = 28), and 259.1 ± 3.6 Ma (MSWD = 0.12, n = 28), respectively. The weighted 206Pb/238U ages of baddeleyites for all intrusions and extrusives are 262.1 ± 1.2 Ma (MSWD = 0.26, n = 111) and 262.4 ± 1.5 Ma (MSWD = 0.18, n = 119), respectively. Baddeleyites from different intrusions and extrusives have varying Hf isotope compositions, with the weighted average εHf(t) of −0.8 ± 0.5 (Zhujiabaobao), −3.2 ± 0.8 (Jianbaobao), −5.5 ± 1.5 (Gongshan) and −11.9 ± 0.5 (Lanaqing), which are largely negative and decrease progressively from northeast to southwest. εHf(t) values of the two extrusive carbonatite and one carbonatitic breccia samples from the southwest of Panzhihua are −14.5 ± 0.7, −15.1 ± 0.8 and −7.8 ± 1.3, and one extrusive sample from the Lanjian ore-block has weighted mean εHf(t) of −7.8 ± 1.3. δ13CVPDB and δ18OV-SMOW values of the Panzhihua carbonatite range, respectively, −13.4 to +0.4 ‰ (avg. −3.7 ‰) and +15.4 to +21.8 ‰ (avg. +19.5 ‰), similar to those of most global carbonatites. εHf(t) of baddeleyites in Panzhihua carbonatite are dominantly negative, and analogous to that of the standard Phalaborwa baddeleyite, indicating that carbonatitic magma may have been derived from partial melting of the metasomatized lithospheric mantle or an enriched asthenospheric component during the Permian intracontinental rifting (Emeishan mantle plume-related). The hyperthermal carbonatitic magma has partially or completely assimilated mantle xenoliths in the chamber and during its ascent, implying it was of high temperature greater than the solidus curve of peridotite (≥1100℃). Assimilation of xenoliths and wallrocks may have altered the Hf isotope compositions and yielded the distinctive alternating dark peridotite and white carbonatite layers around each xenolith. As a newly identified rock type, the Panzhihua may be one of the global largest carbonatite belts, which also enrich the rock sequences of Emeishan large igneous province.

Integrated O, Fe, and Ti isotopic analysis elucidates multiple metal and fluid sources for magnetite from the Ernest Henry Iron oxide copper gold (IOCG) Deposit, Queensland, Australia

Iron oxide copper gold (IOCG) deposits are globally important sources of Cu; however, their origins remain poorly understood with respect to the metal and fluid sources involved in their formation. In this work, we utilize integrated Fe, Ti, and O isotopic data for magnetite to trace major metal and fluid inputs for the Proterozoic-aged Ernest Henry IOCG deposit—the largest known IOCG deposit in the Cloncurry District, Queensland, Australia. Magnetite separates from ore stage and pre-ore biotite-magnetite (bt-mgt) and magnetite-actinolite (mgt-act) alteration assemblages from Ernest Henry were analyzed for their O, Fe, and Ti isotope abundances, reported as δ18O (VSMOW), δ56Fe (IRMM-14), and δ49Ti (OL-Ti). Select magnetite samples were also analyzed for 17O (reported as Δ’17O0.5305) and range from −0.118 to −0.056 ‰—suggesting evaporitic input. The δ18O values from ore stage (+1.57 to +7.36 ‰), bt-mgt (+0.34 to +5.68 ‰), and mgt-act (+1.68 to +2.10 ‰) samples are consistent with a magmatic-hydrothermal origin for ore and pre-ore mineralizing fluids at Ernest Henry; non-magmatic δ18O values > c. 5 ‰ may be explained through localized carbonate wall rock assimilation. Despite this, δ56Fe values for ore stage (−0.50 to +0.33 ‰), bt-mgt (−0.65 to +0.38 ‰), and mgt-act (−0.26 to +0.30 ‰) magnetite are generally isotopically lighter than the accepted range (c. +0.06 to +0.49 ‰) for igneous and magmatic-hydrothermal magnetite and exhibit a relatively large range of c. 1 ‰, suggesting Fe source mixing within the deposit. Ore stage magnetite δ49Ti (−1.64 to + 3.79 ‰; avg: +1.49 ‰; 2σ = 2.63 ‰) compositions are generally higher and more variable than either the bt-mgt (-0.44 to + 1.49 ‰; avg: +0.62 ‰; 2σ = 1.13 ‰) or mgt-act (+0.27 to + 1.89 ‰; avg: +1.05 ‰; 2σ = 1.56 ‰) and suggest that Ti isotope fractionation occurred due to differential mobility in the fluid. The data are best explained by models invoking both magmatic and non-magmatic metal and fluid input. Fluid flow channeled between the footwall and hanging wall shear zones introduced non-magmatic Fe that may have been leached from local mafic units by magmatic-hydrothermal fluids, resulting in neoformed and regeneratively replaced magnetite with magmatic δ18O, but non-magmatic δ56Fe values during pre-ore alteration. These fluids may have mixed with Fe-poor evaporitic fluids prior to magnetite formation. Later magmatic Fe and O contributions during ore stage mineralization resulted in magnetite with variable δ56Fe and irresolvable δ18O overprinting. Ernest Henry is the first known example of an IOCG deposit with a major leached metal component identified through metal stable isotope geochemistry.

Rejuvenation of silicic magmatic systems: Insights from the formation of garnet-bearing, crystal-rich tuff in Southeast China

Crystal-rich volcanic rocks are important for understanding the evolution of large silicic magma systems. However, the magmatic evolution and rejuvenation processes that form the eruptible crystal-rich magma remain controversial. In this study, we conducted petrological, whole-rock major and trace element geochemical and zircon U–Pb and Hf–O isotopic analyses on the Banshan crystal-rich tuff from the large Cretaceous volcanic belt of southeast China. Systematic SIMS and LA-ICP-MS zircon U–Pb geochronology indicate that the Banshan tuff was formed at ∼114–112 Ma. Their zircon Hf isotopic compositions [εHf(t) = −10.9 − 4.2; TDMC = 2.18–1.68 Ga] indicate that the magma of Banshan tuff was derived from a combination of mantle-derived melts and partial melting of the Paleoproterozoic crust of southeast China. In addition, they have low zircon O isotopic compositions (δ18OVSMOW = 4.39–5.36‰) reflecting that the assimilation of wall-rocks which underwent high-temperature hydrothermally exchange with meteoric waters. Extremely Mn-rich garnets in the Banshan tuff (MnO = 30.48–35.82 wt%) suggest that the magma once reached a high degree of crystallization which was nearing consolidation. The subsequent mush rejuvenation caused by magma recharge is evidenced by partly resorbed alkali feldspar, quartz and garnet, as well as the CL-bright rims of zircon. Crystal-rich volcanic rocks are widely distributed in the lower sequences of Cretaceous volcanic strata from southeast China, but rarely developed in the upper sequences. We suggest this reflects that the magmas of lower sequences have higher volatile contents and were formed in a subduction tectonic setting.

The role of crystal mush in porphyry systems: A case study from the Baishiya ore field, East Kunlun orogenic belt, northern Qinghai-Tibet plateau

Since a cold storage crystal mush reservoir has been gradually used to describe magmas in shallow crust, traditional mechanisms, such as wall-rock assimilation, melt-crystal separation, as well as magma mixing and mingling, must be used with caution because recycled antecrysts can play a significant role. As a result, magmatic processes accounted for texture, geochemistry, and mineralization diversity among ore-related porphyry, overlying (sub)volcanic rocks, and underlying pluton in porphyry systems remain ambiguous. The Baishiya igneous suite (East Kunlun orogenic belt, China) with porphyry-related skarn mineralization provides an opportunity to resolve this issue. Within the crystal mush concept, we propose a comprehensive model based on the subtle changes. First and foremost, large K-feldspar grains characterized by high Sr/Y ratios (52–134) in the top area of granodiorite have a slightly enriched lead isotopic signature, indicating a previously existed crystal mush equilibrated with K-feldspar. The locked crystal mush was injected by mantle-derived mafic melts, leading to a transfer in heat and volatiles from hot melts to cold crystal mush. The buoyant volatiles and loaded metals introduced by episodic mafic replenishments concentrated on the roof, providing a fluid-rich and thermal cycling condition for K-feldspar coarse graining. Some enlarged K-feldspar grains sunk into the bottom area due to gravity, and subsequently reacted with upwelling hot melts to form rapakivi textures. As a gradually waning of injection of mafic melts, crystallization drove fluid exsolution, forming magmatic-hydrothermal events. The finally solidified granodiorite acted as a host rock for a newly produced magma reservoir, but the following development was different. Catastrophic eruptions induced by tectonic stress transition led to a strong depletion of volatiles and associated metal endowment in the underlying magma reservoir, and subsequently left a barren magma source, terminating potential mineralization events. The residual felsic melts migrated upwards along previously formed magma conduits, and thus emplaced as barren porphyry dike. Our work reveals that recycling of K-feldspar antecryst induced by mafic injection contributes to high Sr/Y ratio in ore-related porphyritic granodiorite. Within a long-lived replenishment of mantle-derived magma, a crystal mush reservoir at shallow crustal levels can act as a ‘cooling pond’ to quench these hot melts, but accept volatiles and loaded ore-forming elements, concentrating metals by volatile buoyancy with the help of a compressional stress, which is presumably essential for late fluid exsolution from the reservoir’s roof.