Petrogenetic Relationships Research Articles

Petrogenetic relationship of the variably Ni−Cu−platinum-group element (PGE)-mineralized intrusions of the Thunder Bay North Intrusive Complex, Midcontinent Rift, Canada: Implications for mineral exploration in mafic−ultramafic complexes

The Thunder Bay North Intrusive Complex (TBNIC) of Canada comprises six variably Ni−Cu−platinum-group element (PGE)-mineralized mafic−ultramafic intrusions that intruded the Archean Quetico Subprovince. These are the Current, Escape, Lone Island, Greenwich, and 025 intrusions, with the former three aligned along an east−west-trending intrusion termed the East−West Corridor. These intrusions have a consistent igneous stratigraphy comprising, from bottom to top, peridotite, gabbros, and quartz gabbro. Only the Current and Escape intrusions are known to host sulfide mineralization in the peridotite. Despite the economic significance of the TBNIC, little work has been done to characterize its petrogenetic evolution and the causes of the apparent variable fertility. Our new ages for Escape (1107.6 ± 0.9 Ma) and Greenwich (1105.7 ± 0.9 Ma), combined with previously determined ages for Current (1106.6 ± 1.6 Ma) and Lone Island (1106.3 ± 2.1 Ma), demonstrate that the TBNIC was emplaced as part of the ca. 1.1 Ga Midcontinent Rift event. The similarities in their rare earth element geochemistry and consistently low Th/La−Sri values indicate that the intrusions crystallized from genetically related magmas that did not assimilate significant amounts of crustal material. Accordingly, their systematically negative εNdt values and Nb anomalies likely reflect that their parent magmas were largely sourced from the subdcontinental lithospheric mantle and not the Keweenaw Plume. Given their genetic relationship, the variable fertility of the intrusions cannot be related to processes that occurred in their mantle sources. Since the gabbros have systematically depleted Cu/Zr−Cu/Pd ratios and the peridotites have enriched ratios, the apparent variable fertility is likely related to the timing and location of sulfide segregation and accumulation, with the barren gabbros preserving magmas that lost sulfide liquid during transport to shallow crustal levels, and the mineralized peridotites recording accumulation of sulfide liquid. This interpretation has important implications for exploration as it implies that sulfide mineralization in the apparently barren intrusions may be located at depth in undiscovered ultramafic cumulates.

Petrogenetic relationships between Paleoproterozoic granitoids and rare-element pegmatites from Dibilo (Liptako, West Niger, West African Craton)

The rare-element pegmatites of Dibilo form intrusions in a tonalitic gneiss of the Paleoproterozoic Téra-Ayorou pluton about 4 km from the contact zone with the Diagorou-Darbani greenstone belt, in the Nigerien Liptako (NE portion of the Man Ridge of the West African Craton). The general objective of the present study is to determine the petrogenetic relationships between the granitoids and the Dibilo pegmatites through a multidisciplinary approach combining petrology, geochemistry, structural geology and metallogeny. Two generations of granitoids have been distinguished at Dibilo. A first generation of metaluminous to weakly peraluminous granitoids (tonalitic gneiss, metadiorite and two-mica granitic gneiss) constitute a calc-alkaline series. Trace element geochemical signatures suggest that these granitoids were emplaced in an island arc and subduction zone context. A second generation of granitoids (leucogranites), encompassing two-mica, muscovite and two-mica-garnet granites, are peraluminous and calc-alkaline. These leucogranites are fertile, i.e., rich in incompatible elements, with Li concentrations attaining 250 ppm in the richest. A model of partial melting shows that they could derive from the partial melting of Li-rich metadiorite, which is itself highly enriched in lithium (150 ppm). One sample of migmatite has Li contents of 300 ppm, which evidences the fertile nature of the tonalitic-dioritic-granitic gneiss complex.Three types of pegmatite have been identified at Dibilo: sterile pegmatites (type I), peraluminous and poorly evolved; Nb-Ta pegmatites (type II-a) and Nb-Ta, Mo pegmatites (type II-b), moderately evolved, peraluminous, respectively mineralized with columbite-group minerals (type II-a) and columbite-group minerals and Mo-bearing sulfide (type II-b); and highly evolved, peraluminous Li and Nb-Ta pegmatites (type III) mineralized with spodumene and columbite-group minerals. The type I barren pegmatites share similarities with the leucogranites and could also originate from the partial melting of the dioritic gneiss. The highly evolved type II-a, type II-b and type III pegmatites correspond to the residual liquids resulting from the fractional crystallization of leucogranites. Therefore, we propose that the Dibilo rare-element pegmatites derive from both fractional crystallization and partial melting processes within the tonalitic-dioritic-granitic complex.

Petrology and chronology of mare components in lunar basaltic breccia meteorite Northwest Africa 12384

AbstractNorthwest Africa (NWA) 12384 is a lunar polymict breccia composed almost entirely of basaltic components. The clast content includes low‐ to very‐low‐Ti volcanic picritic glass, basaltic vitrophyre, and crystalline pigeonite basalt—an assemblage of volcanic materials that can be tested for petrogenetic relationships. We present the inferred history of select mare components of NWA 12384 as suggested by texture, mineralogy, and petrography, and compare them to Apollo samples and other lunar meteorites. In addition, we used the volcanic glasses in the breccia as a primary composition for crystallization modeling and comparison to the lithic clast compositions. We find that the mafic clasts in NWA 12384 cannot be derived from the picritic glass through a common liquid line of descent because of higher Ti content, though they may have crystallized from a separate, common liquid line of descent. These clasts could represent local source‐region heterogeneity or differential assimilation of more Ti‐rich material. Pb‐Pb SIMS analyses of a large basalt clast in NWA 12384 reveal an age of 3044 ± 41 Ma (2σ), which is used together with the chemical data and 4π cosmic ray exposure age of less than 20 kyr and terrestrial age of between 3.1 and 17.3 kyr to constrain the possible locations of provenance for this meteorite.

Petrographic and geochemical analysis of lunar meteorite NWA 11788: Parallels with Luna 20 and the Apollo magnesian granulites

AbstractThe first in‐depth geochemical and petrological analyses of new lunar meteorite Northwest Africa (NWA) 11788 were conducted with the aim of better understanding the diversity of lunar rock types. Petrography, microcomputed tomography, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry were employed to analyze mineralogic/elemental makeup, petrologic profile, melt history, and inferred composition of the lunar mantle from which the crystals in this sample originated from. Geochemical maps of the lunar surface were generated to constrain potential lunar launch locations for NWA 11788. Potential launch locations are concentrated in the outer rims of impact basins on the lunar Eastern nearside limb (e.g., Crisium, Fecunditatis, Marginis, Smythii) and around the South Pole–Aitken Basin. Similarities in the major, minor, and trace element chemistry of NWA 11788 along with its potential launch locations suggest a petrogenetic relationship with regolith samples returned from the Luna 20 mission and the Apollo 16 and 17 magnesian granulites. Additionally, the results of this study add to the growing body of evidence that KREEP (potassium, rare earth elements, phosphorous)‐poor, Mg‐suite‐“like” lithologies are common in non‐Apollo‐type locales, that KREEP may not be required to generate lithologies like the Mg‐suite, and that KREEP is not globally distributed at present.

Multi-stage evolution of the monzonitic Larvik Plutonic Complex (Oslo Rift, Norway) and its implications for the formation of the Kodal Fe-Ti-P(−REE) deposit

The Larvik Plutonic Complex is a monzonitic complex that was emplaced early during the formation of the Permian Oslo Rift (southern Norway). It extends across its entire width (around 50 km), and is made up of a majority of larvikite (augite monzonite) and lardalite (nepheline monzogabbro to nepheline syenite) distributed in ten successive intrusive units that partially intersect each other. The Larvik Plutonic Complex also hosts occurrences of singular Fe-Ti-P-rich rocks of magmatic origin, including the Kodal deposit. These are mainly composed of titanomagnetite, ilmenite, titanaugite and apatite, the latter also featuring rare earth elements (REE) enrichment. Here, we aim to unravel the conditions that allowed the Kodal deposit to form, and determine why other mineralized bodies are not seen elsewhere in the Larvik Plutonic Complex. We compared the petrography and elemental and isotope (Sr, Nd and Hf) geochemistry of both the Kodal mineralization and the neighboring larvikite, in order to provide evidence of their petrogenetic relationship. Both lithologies share the same isotopic ratios (87Sr/86Sr(i): 0.7035–0.7043; εNd(i): +2.37 − +3.48; εHf(i): +3.39 − +9.16), which would suggest a single homogeneous source. The very low dispersion of our isotopic data also suggests that crustal contamination levels in the area were low to negligible. Normalized trace diagrams of larvikite in several localities of the Larvik Plutonic Complex also show enrichments in most incompatible elements, which becomes more prevalent towards the Kodal deposit. Elements like Sr and Eu(2+) follow an opposite trend, because of their compatibility in plagioclase. We therefore infer that the region around the Kodal deposit hosts more fractionated larvikite due to the previous crystallization of successive plagioclase cumulates. We deduce that the Kodal lobe corresponds to a more evolved intrusion, which is no part of Pluton V per se, as considered in the literature until now, but instead derives at least from a monzonitic magma at the origin of the plutons V to VIII). This also implies that the formation of Fe-Ti-P mineralization at Kodal was most likely a consequence of enrichment of the residual melt in alkaline elements and incompatible elements. These conditions support an early hypothesis of formation by silicate-liquid immiscibility, along with petrographic evidence of disequilibrium at the mineralogical scale. However, further analyses would be required to test the hypotheses of silicate-liquid immiscibility against an accumulation of the Fe-Ti-P mineralization from an evolved intermediate magma.

Melt inclusion heterogeneity in nakhlite and chassignite meteorites and evidence for complicated, multigenerational magmas

AbstractNakhlite and chassignite meteorites are cumulate rocks thought to originate from the same location on Mars. Petrogenetic relationships between nakhlites and chassignites are not fully constrained, and the two cumulus phases in nakhlites—olivine and clinopyroxene—possibly formed either together from one magma or separately from different magmas. Primary magma compositions can potentially be determined from studies of melt inclusions (MIs) trapped within early‐formed mineral phases. MIs frequently undergo post‐entrapment effects, and when such processes occur, there can be significant changes to their compositions. Here, we report major, minor, and trace element abundances for MIs in cumulus phases in nakhlites and chassignites. The melt compositions that they record are variable (MgO = 2.50–13.5 wt%, K2O = 0.03–3.03 wt%, La/Yb = 2.46%–16.4%) and are likely affected by diffusive reequilibration with changing magma composition outside of their host phases. Evidence for diffusive reequilibration suggests that nakhlite and chassignite magmas were generated in an open system, and cumulus phases may have undergone magma storage and mixing. Such processes may be akin to those that occur in terrestrial intrusive magmatic systems by open‐system magma recharge. MIs within the nakhlite and chassignite suite therefore provide insights into magmatic processes during magma storage and transit on Mars.

Petrogenetic relationship between plutonic and subvolcanic rocks in the Jurassic Shuikoushan complex, South China

Petrogenetic relationship between plutonic and subvolcanic rocks in the Jurassic Shuikoushan complex, South China

On the association of Magmatic Microgranular Enclaves (MMEs) and cordierite-bearing monzogranites at the Sierra Bermeja Pluton (SW Spain)

Magmatic Microgranular Enclaves (MMEs) found within the cordierite-bearing monzogranites of the Sierra Bermeja Pluton (southern Iberian Massif) were investigated to understand their potential petrogenetic relationship. Variations in mineralogy, such as the absence of cordierite (Crd) and potassium feldspar (Kfs) in the MMEs, as well as differences in the compositions of biotite (Bt) and plagioclase (Pl), along with the lack of correlations of certain elements in whole-rock geochemistry, suggest a distinct origin for the MMEs compared to the host monzogranites. The MMEs may represent minor globules of a more mafic magma injected into and mixed with the host monzogranitic magma, with no significant alterations in the geochemical characteristics of the latter.

Discovery of carbonatite-syenite hosted REE-Nb mineralization in the Amaragh area, Gleibat Lafhouda (South Morocco): Hints toward an extended mineralized alkaline province

We report the results of a reconnaissance study of several small carbonatite and syenite bodies with very low topography (up to 1.5 m), that were recently discovered in the Archean basement in the Amaragh area near the Gleibat Lafhouda (GL) carbonatite, previously interpreted as a primary carbonatite. Two varieties of carbonatite have been identified: 1) brownish to greyish banded dolomite carbonatite, composed of dolomite as the main carbonate phase, and calcite, magnetite, apatite, monazite-(Ce), magnesite and minor bastnaesite-(Ce), 2) calciocarbonatite, mainly composed of calcite and several feldspar aggregates (the latter are interpreted as xenocrysts). The Amaragh syenites consist of small outcrops and show white to dark grey colors. The main outcrop is a nepheline syenite that contains coarse-grained rounded magnetites, rimmed by biotite and epidote within a feldspar and nepheline groundmass, suggesting fluid-mineral reactions. The other syenites consist mainly of alkali feldspar, albite, mica, amphibole and accessory apatite, epidote, and titanite. The host minerals of Nb and REE are pyrochlore and allanite. All syenite samples display a peraluminous composition and strong LREE enrichment with negative EuCN anomalies, except for two samples with a slightly positive EuCN anomaly, suggesting at least two types of differentiated syenites. All samples show strong postmagmatic and hydrothermal overprint, which is revealed by, for example, the occurrence of negative YCN and positive CeCN anomalies and mica-epidote mineral reaction rims between magnetite and feldspar. The close spatial association of the Amaragh carbonatite and syenite bodies with the GL carbonatites along with geochemical and mineralogical similarities and REE-Nb enrichment suggest a close petrogenetic relationship, pointing toward a single carbonatite-syenite alkaline complex that was probably formed in an extensional tectonic setting.

Geochemical variations of anatectic melts in response to changes of P–T–H2O conditions: Implication for the relationship between dehydration and hydration melting in the Himalayan orogen

The anatectic mechanism is the most fundamental criterion in determining the petrogenetic relationship between migmatite, granulite and granite at convergent plate margins. Although two main anatectic mechanisms, namely water-absent (dehydration) and water-fluxed (hydration) melting, have been identified in many collisional orogens, their spatiotemporal relationship and genetic connection remain obscure. To address this issue, we conduct an integrated study of forward phase equilibrium and trace element modelling for partial melting of metapelite in the Himalayan orogen. The results are used to decipher changes of phase relation and melt composition in response to changes of temperature, pressure and water content. Pressure and water content mainly affect the solidus and evolving residual mineral assemblages, whereas temperature controls the anatectic reaction. Hydration melting at lower pressure and temperature would produce anatectic melts with relatively high SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios, but low Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios. In contrast, dehydration melting at higher pressure and temperature would generate anatectic melts with relatively high Al2O3 and Na2O contents and Na2O/K2O and Rb/Sr ratios, but low SiO2, FeOT + MgO, CaO, K2O, and Ba contents and SiO2/Al2O3 ratios. A comparison of geochemical compositions between the modelled melts and the two groups of well-characterized leucogranites in the Himalayan orogen indicates that the coeval dehydration and hydration melting would likely occur at the deeper and shallower crustal levels, respectively. In combination with the secular evolution of metamorphic facies series in the Himalayan orogen, we propose a spatiotemporally coupled dehydration-hydration melting mechanism. The early metamorphism at lower geothermal gradients would result in metamorphic dehydration in the lower crust and hydration in the upper crust during the syn-collisional period in the Early Cenozoic. As a result, the later metamorphism at higher geothermal gradients would lead to dehydration melting in the lower crust and hydration melting in the upper crust during the post-collisional period in the Late Cenozoic. This study highlights the applicability of phase equilibrium and trace element modelling to reveal the geochemical variations of anatectic melts in response to changes of P–T–H2O conditions. This provides new insights into the relationship between dehydration and hydration melting in collisional orogens.

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.

Petrogenesis of the alkali basalt and trachy-andesite suite in the northern Tarim Basin, NW China: Implications for crust-mantle interactions controlled by the Permian mantle plume

The Permian Tarim large igneous province (LIP), which contains both large volumes of basalt and silicic volcanic sequences, has been intensively studied in the past decades, but some issues still remain controversial, including the petrogenesis of each component in the LIP, their interrelationships, and the geodynamic processes that produced them. In the Halahatang area of the northern Tarim Basin, a well-developed eruptive suite composed of basalt and trachy-andesite has been penetrated by numerous boreholes, which provide us an opportunity to better understand these issues. In this paper, we carried out an integrated study on the eruptive suite, including petrography, geochronology and geochemistry. The results indicate that the eruptive suite formed at around ∼290 Ma, generally synchronous with the main episode of magmatism within the Permian Tarim LIP. The basalts have similar compositions to typical oceanic island basalts (OIBs), which have alkaline, high-Ti and sodic compositions and show enrichments in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs). Whole-rock Sr–Nd isotopes are slightly enriched, with εNd(t) and (87Sr/86Sr)i values from −4.5 to −3.4 and 0.7065 to 0.7086, respectively. The trachy-andesites are alkaline, metaluminous and ferroan, and display highly enriched LREEs and LILEs. High Ga/Al ratios and Zr + Nb + Ce + Y values constrain their A-type affinity and A1-type nature. They have εNd(t) values from −8.2 to −4.5 and (87Sr/86Sr)i values from 0.7083 to 0.7189, more enriched than those of the basalts. Based on these results, we proposed that the different components in the igneous suite have close petrogenetic relationships. The alkali basalts may have been formed by mantle plume-derived melts coupled with somewhat assimilation of continental crust, while the trachy-andesites were products of variable fractionation of the basaltic magmas involving more continental crustal components in the magma evolution. In combination with the existing results in Tarim, we further suggested that the alkali basalt and trachy-andesite suite may have been generated via a crust–mantle interaction process related to the Permian mantle plume activity.

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.

The stable chromium isotope composition of different mantle reservoirs

The stable chromium isotope composition of Earth’s mantle has been suggested to be heterogeneous, with mantle-derived chromitites slightly enriched in heavier Cr isotopes than mantle-derived silicate rocks. To better understand such heterogeneity and to search for a potentially complementary light Cr isotope reservoir in the mantle, we analyzed the stable Cr isotope compositions (δ53Cr expressed as deviation of sample 53Cr/52Cr from NIST SRM 979) of whole-rock pyroxenite and peridotite xenoliths from the North China Craton using a double spike technique. The δ53Cr values of the peridotites vary from −0.21‰ to −0.07‰ with an average value of −0.14 ± 0.10‰ (2SD, N = 5), which overlaps with the range of the Bulk Silicate Earth (−0.12 ± 0.10‰). In contrast, pyroxenites in this study yield lower δ53Cr values (−0.30‰ to −0.14‰ with an average value of −0.21 ± 0.08‰, 2SD, N = 20). The variation in the δ53Cr values of the pyroxenites can be explained by a Rayleigh distillation model during partial melting of the primitive mantle. The model provides minimum Cr isotope fractionation factors during partial melting ranging from 0.01‰ to 0.12‰, with light isotopes enriched in the melt. Combined with literature data, our results suggest that both peridotites and basalts are indistinguishable from the Bulk Silicate Earth in terms of δ53Cr. However, pyroxenites and chromitites are isotopically lighter and heavier than the Bulk Silicate Earth, respectively. Future work is needed to investigate the petrogenetic relationship between chromitites and pyroxenites in order to understand why they seem to complement each other in δ53Cr.

Petrogenetic relationships between peraluminous granites and Li- Cs-Ta rich pegmatites in south Issia zone (Central-West of Côte d’Ivoire): Petrography, Mineralogy, Geochemistry and zircon U–Pb Geochronology

Petrogenetic relationships between peraluminous granites and Li- Cs-Ta rich pegmatites in south Issia zone (Central-West of Côte d’Ivoire): Petrography, Mineralogy, Geochemistry and zircon U–Pb Geochronology

Zircon-modeled melts shed light on the formation of Earth's crust from the Hadean to the Archean

Abstract Elucidating the compositions of melts from which Hadean zircons crystallized can provide insight into early crust construction. We calculated model melts using Ti-calibrated zircon/melt partition coefficients and trace element data for zircons from the Hadean, Archean, and possible analogue environments (e.g., rifts, hotspots, arcs) to constrain petrogenetic relationships. Model melts from oceanic settings (mid-ocean ridges, arcs, Iceland) showed higher heavy rare earth element (HREE) contents and shallower middle REE (MREE) to HREE/chondrite (ch) slopes than those from continental arcs and tonalite-trondhjemite-granodiorite suites (TTGs). However, Hadean and Archean model melts were consistently similar to each other and to those from continental arcs, hotspots, and TTGs (and dissimilar to oceanic settings), with depleted HREE contents and slope reversal in heaviest REEch. Despite close similarities that suggest comparable petrogenesis of Hadean and early Archean magmas from which Jack Hills detrital zircons crystallized, subtle variabilities in REEch and Zr/Hf suggest thickening crust and evolving igneous systems through time.

Diapir Melting of Subducted Mélange Generating Alkaline Arc Magmatism and Its Implications for Material Recycling at Subduction Zone Settings

Alkaline rocks crop out in subduction-related volcanic arcs globally, whose genesis is explained by partial melting of hybrid diapiric mélanges in the mantle wedge. Such a new model has not yet been evidenced by field-based investigations, where mélanges and alkaline arc rocks have spatial-temporal and genetic connections. In this study, we identified these two types of rock units exposed in Western Tianshan, and established their petrogenetic relationship through field, geochronologic, geochemical and thermodynamic investigations. Our results suggest that partial melts produced by melting of the mélange matrix at pressures of 2.8–2.4 GPa and 1.8–1.4 GPa with temperatures of 1000–1200 °C can replicate the major-element contents and trace-element abundances of the regional alkaline arc rocks. This finding not only supports the hypothesis that diapir melting of subducted mélanges has a capacity to generate alkaline arc magma, but also provides insight into material recycling at subduction zone settings.

Pre-Eruptive Magma Configurations and Petrogenetic Relationships of the Rattlesnake Tuff, Oregon–Insights From Spectacularly Banded High-Silica Rhyolite Pumices

The 7.1 Ma Rattlesnake Tuff (RST) of eastern Oregon is a widespread and voluminous (>300 km3) ignimbrite composed of 99% crystal poor (≤1%) high-silica rhyolite (HSR) and <1% dacites. Basaltic andesitic to basaltic inclusions within dacites are samples of underpinned mafic magmas. The RST HSR is comprised of five increasingly evolved compositional Groups (E–A), and HSR pumices range from white to dark grey, often co-mingled in spectacular banded pumices. Previously, Groups were interpreted as rhyolites generated by crystal fractionation within a single reservoir, where more evolved rhyolite melts formed from relatively less evolved rhyolite parents. To reassess compositional HSR Groups and their implications for tapping a single or multiple rhyolite reservoirs as well as reevaluating the petrological relationships among groups, we focus on large banded pumices for geochemical analysis. Statistical analysis of existing and new data verified these five compositional Groups and gaps, best characterized by variations in Ba, Eu/Eu*, Eu, FeO*, Hf, and Zr. Wet-liquidus temperatures, storage temperatures, and storage pressures calculated for all HSR Groups indicate similar pre-eruptive conditions (∼6.1–7.5 km depth; storage temperatures of ∼805–895°C). Differentiation trends, trends in storage pressure and temperature, and lack of crystal-rich tuff or country rock corroborate existing models for HSRs that involve a single, density-stratified magma reservoir prior eruption. Density differences are sufficient to prevent convection between layers of HSRs in a single reservoir when water content increases from 2–4 wt% from Groups E–A. However, if HSRs do not represent a liquid line, it is possible to generate HSRs through batch melting of various regional country rock. Yet, HSRs would still accumulate within the same storage zone, where density variations kept HSRs from mixing until eruption when these banded pumices formed. In either scenario, our study underscores the significance of water content and density variations for accumulating rhyolite magmas in a contiguous magma body without mixing. This has implications for other compositionally heterogenous rhyolitic ignimbrites where natural samples do not provide comparable evidence to argue for pre-eruptive confocal storage of different rhyolite magmas as is the case for the Rattlesnake Tuff.

Magma hybridization and crystallization in coexisting gabbroic and granitic bodies in the mid-crust, Akechi district, central Japan

Petrological and geochemical features of gabbros and fine-grained mafic rocks (mafic microgranular enclaves; MMEs) in the Inagawa Granite of the Ryoke Plutonic Complex were investigated to assess the interactions between coexisting mafic and silicic magmas, and the petrogenetic relationships between the MMEs and surrounding gabbros. The MMEs exhibit mingling textures that imply the coexistence of mafic and silicic magmas that did not undergo complete mixing, but the geochemical compositions of the MMEs require substantial hybridization and homogenization. The gabbroic rocks exhibit disequilibrium textures and mineral compositions, such as quartz–hornblende ocellar textures and patchy plagioclase crystals with bimodal anorthite contents. These textures and compositions record an abrupt decrease in crystallization temperature and mechanical mixing between crystallizing gabbroic mush and silicic (granitic) melt. Geochemical variations of the gabbroic rocks can be explained by hybridization and fractional crystallization (HFC) processes between crystallizing gabbroic mush and granitic melt. Extrapolation of the mixing trend to a basaltic composition suggests that the primitive mafic end-member was a low-K basaltic magma. Given that HFC yields magnesian andesite by the addition of a small amount of silicic melt to a primitive mafic end-member, the compositional modification of mafic magmas by magma mixing might be an essential process in the formation of andesitic magma in arc crust.

Mantle and Crustal Contributions to the Mount Girnar Alkaline Plutonic Complex and the Circum-Girnar Mafic-Silicic Intrusions of Saurashtra, Northwestern Deccan Traps

ABSTRACT Continental flood basalt (CFB) provinces, while dominated by tholeiitic basalts and basaltic andesites, often also contain alkaline mafic to felsic lavas and intrusions. The tholeiitic and alkaline magmas may reflect different degrees of partial melting of the same mantle source, or the alkaline magmas may be derived from metasomatised, incompatible element-enriched mantle sources. The tholeiitic and alkaline suites, even if closely associated spatially or temporally, require independent magmatic plumbing systems. In the Saurashtra region of the northwestern Deccan Traps CFB province, India, tholeiitic lavas have been intruded by the ~66 Ma Mount Girnar plutonic complex, which comprises olivine gabbros (often with cumulate textures), diorites, and monzonites, profusely intruded by dykes and veins of foid-bearing syenites and lamprophyres. In the region surrounding the complex the tholeiitic lavas have been intruded by a large (12 km-diameter) silicic ring dyke, as well as tholeiitic dykes and sills. The region thus provides an excellent opportunity to study potential petrogenetic relationships between tholeiitic, alkaline, and silicic magmatism in a CFB province, evaluated here using field, petrographic, mineral chemical, and whole-rock geochemical (including Sr–Nd–Pb isotopic) data. Initial (at 65 Ma) Sr–Nd–Pb isotopic ratios of an olivine gabbro and diorites of the Girnar plutonic suite are in the ranges (87Sr/86Sr)t = 0.70499 to 0.70584, (143Nd/144Nd)t = 0.512675 to 0.512484 (εNdt = +2.4 to −1.4) and (206Pb/204Pb)t = 18.270–18.679. Foid-bearing syenites and lamprophyres have broadly similar isotopic ratios and marked enrichments in the most incompatible elements. Thermobarometric calculations indicate crystallisation of mineral phases in the Girnar plutonic suite at varied crustal pressures (0.02–0.9 GPa). Small but significant Sr–Nd–Pb isotopic variations within the plutonic suite rule out closed-system fractional crystallisation as a viable process, whereas a lack of correlation between isotopic ratio and degree of magmatic evolution (rock type) also negates any simple scheme of combined assimilation–fractional crystallisation. The circum-Girnar tholeiitic intrusions, hitherto practically unstudied, are low-Ti and moderately to fairly evolved (MgO = 8.0–3.9 wt %); olivine gabbro and picrite dykes with cumulus olivine show higher MgO (10.1–15.7 wt %), Ni (360–700 ppm), and Cr (410–1710 ppm) contents. The circum-Girnar tholeiitic intrusions have a large range of Sr–Nd–Pb isotopic ratios (e.g. εNdt = +4.2 to −18.7) indicating open-system processes. We infer that magmas of the alkaline Girnar plutonic suite were derived from enriched mantle, with only minor crustal residence or material input, possibly reflecting a very thin basement crust under the complex. In contrast, magmas forming the circum-Girnar tholeiitic intrusions were derived from depleted mantle (εNdt > +4.2) by high degrees of melting, and they experienced olivine fractionation or accumulation in crustal chambers and significant contamination by ancient granitic basement crust. These features probably reflect a much thicker crust surrounding the plutonic complex than directly under it. The circum-Girnar silicic ring dyke has Sr–Nd–Pb isotopic ratios suggesting an origin by anatexis of the basement crust. Based on a range of evidence, the tholeiitic and silicic circum-Girnar dykes and sills are petrogenetically and structurally unrelated to the alkaline Girnar plutonic suite.