Contributions to Mineralogy and Petrology最新文献

The Himalayan orogeny caused partial melting of rocks within the Greater Himalayan Sequence (GHS), forming migmatites. The extensive occurrence of such migmatites in the lower structural level of the GHS (GHS_L) is a distinctive feature of the Western Arunachal Himalaya (WAH), situated in eastern part of the orogen; meanwhile leucogranite is predominantly found in the highest reaches of the GHS_L. A comprehensive multi-method study incorporating field observations, petrography, phase equilibrium modelling, geochemical analysis, and zircon U–Pb and monazite U–Th–Pb geochronology was conducted on migmatitic paragneiss and leucogranites from the GHS_L along the Bomdila-Tawang section of the WAH. P–T pseudosection modelling reveals a clockwise P–T path characterized by prograde burial and heating, significant melt production, and nearly isothermal decompression during melt solidification. Structural observations, including concordant and discordant relationships between leucosomes and gneissic bands, suggest that deformation established pathways for melt migration. Zircon U–Pb dates reveal bimodal protolith ages of ~ 1350 Ma (Ectasian) and ~ 900 Ma (Tonian). Insufficient zircon overgrowth (< 20 μm), likely due to extensive melt extraction during suprasolidus metamorphism, precludes younger age determination. Monazite U-Th-Pb age indicates peak metamorphism of the GHS_L at ca. 25–26 Ma, synchronous with MCT initiation in the WAH. Melt generation at peak metamorphic conditions in the GHS_L reached ~ 16 vol% in stromatic metatexites and ~ 26 vol% in layered diatexites and of these generated melts, > 50% escaped at depths of ~ 30–34 km. This extensive migration formed complex leucosome networks, contributing to regional leucogranite distribution and rheological weakening, enabling ductile flow within the GHS.

The tectonic re-equilibration after the Variscan orogeny coincided with widespread early Permian post-collisional magmatism in southern Europe. A full understanding of the origin of this magmatism in the South Variscan realm and its relationship to major tectonic events such as subduction, continental collision, rifting or lithospheric foundering hinges on high-precision geochronological data of the magmatic products. Here, we present new high-precision zircon U–Pb geochronological data obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) for the early Permian Athesian Magmatic District (AMD) in NE Italy. Our analysed zircons from felsic intrusive and volcanic rocks give ages spanning from ca. 281.8 to 277.2 Ma, suggesting that the lifetime of the AMD was significantly shorter than previously reported. Our data, when combined with recent high-precision ages from other South Variscan magmatic systems suggest that the Cisuralian (early Permian) post-collisional magmatism in the Southalpine domain occurred over more than 8 m.y. with the magmatic centres migrating from the western to the eastern Southern Alps. Geochemical and radiogenic isotope modelling of published data for magmatic rocks in the Southern Alps and the Corsica-Sardinia batholith suggest a subduction-enriched mantle source for the South Variscan post-collisional magmatism, with melting occurring under a relatively thin lithosphere at depths of ca. 60 km. Our results point to a significant post-orogenic delamination of the thick lithospheric mantle formed during the Variscan orogeny. In this scenario, the migration of the post-collisional magmatism within the Cisuralian district may be due to the lateral migration of the lithospheric foundering.

Water as structural hydroxyl in olivine plays an important role in determining the water budget of the upper mantle and its numerous physicochemical properties. However, the solubility of water in olivine in the deep upper mantle (i.e., 300–410 km depth), which defines the maximum water content under given conditions, still needs to be known with high precision. We examined the water solubility by annealing experiments under conditions controlled by Fe-FeO buffer and peridotite assemblages at 10–13 GPa and 1100–1450 ºC, using a starting olivine of representative chemistry and different fluid materials. The experimental conditions were broadly consistent with those prevailing in the deep upper mantle. The attainment of equilibrium water incorporation in the H-annealed olivine samples was ensured by H diffusion kinetics, water profile analyses and time-series studies. The annealed samples demonstrate infrared hydroxyl bands at 3650–3000 cm⁻¹, but the relative band patterns are different from those observed in the available H-annealing experiments at 1–7 GPa under otherwise comparable conditions (including starting materials). The obtained solubility of water increases with increasing both temperature and pressure over the run conditions, and differs apparently between the runs equilibrated by different fluids that are relevant to the deep upper mantle and water solubility studies. In general, the water solubility of olivine increases nonlinearly with increasing depth in the upper mantle, and can be described as: C_w = (290 ± 78) × exp ((0.0043 ± 0.0006) × depth (km))– (268 ± 89) (H₂O as coexisting fluid) and C_w = (149 ± 72) × exp ((0.0046 ± 0.0011) × depth (km))–(132 ± 85) (CH₄-H₂O as coexisting fluid), where C_w is water solubility (ppm wt. H₂O). The water solubility of olivine in the realistic upper mantle should be defined from the runs coexisting with CH₄-H₂O, and the highest value is only ~ 800 ± 80 ppm wt. H₂O, implying that the actual water contents of olivine in the upper mantle must be mostly (if not exclusively) lower. The inferred storage capacity of water in peridotite in the upper mantle reaches its maximum of 600 ± 100 ppm wt. H₂O (95% confidence level) at the bottom boundary of ~ 410 km depth, and a minimum of 350 ± 50 ppm wt. H₂O (95% confidence level) is expected at mid-depths of 190–230 km. During the upwelling of relatively water-rich materials from the source regions of enriched mid-ocean ridge basalts or ocean island basalts, hydrous melting would be much easier to trigger at the mid-depths of the upper mantle. The data further suggest that, to produce a pervasive hydrous melting at the ~ 410 km depth, the prevailing water content of the mantle transition zone should be greater than ~ 600 ppm wt. H₂O.

Deep crustal cumulates directly represent the geochemical composition of the lower crust and can provide insights into magmatism at the mantle–crust boundary. However, the scarcity of exposed deep crustal cumulates, which is due to their high density causing such rocks to sink into the mantle, limits our access to deep crustal samples. This study investigated hydrous late Mesozoic mafic–ultramafic cumulate rocks from northeastern Pamir. These rocks are the first of their kind identified in this region and exhibit features typical of deep sub-arc hydrous cumulates worldwide. Petrography, zircon U–Pb ages and zircon Lu–Hf isotopes, whole-rock geochemistry and Sr–Nd isotopes, and mineral major and trace element chemistry were used to constrain the magmatic evolution from source to surface and the crystallization conditions of the primary magma at depth. In situ zircon U–Pb dating yielded a concordant age of 199 ± 1.3 Ma. The mafic cumulates are hornblende gabbros, which had a crystallization sequence of amphibole/magnetite → plagioclase → biotite → apatite. Hornblende geobarometry yielded an equilibrium pressure of 0.65–0.80 ± 0.14 GPa, corresponding to depths of 20–26 km. The ultramafic cumulates, are lherzolites and olivine clinopyroxenites that have a crystallization sequence of olivine/spinel → clinopyroxene → ± orthopyroxene. The estimated pressure, based on published experimental constrains, suggests high-pressure crystallization occurred at ~ 1 GPa. The elevated magmatic oxygen fugacity (ƒO₂) is consistent with values expected for sub-arc conditions, where FMQ is 1–4 log units more oxidized than mid-ocean ridge basalts. The trace element composition of melts calculated to be in equilibrium with clinopyroxene is comparable to the global average composition of continental calc-alkaline basalts. Based on the petrography, mineral chemistry, and uniform whole-rock Sr–Nd isotopic data, the mafic–ultramafic cumulate rocks are inferred to have formed by fractional crystallization of a common hydrous (~ 2 wt% H₂O) parental melt derived from a depleted mantle source (⁸⁷Sr/⁸⁶Sr = 0.7046–0.7132 ε_Nd(t) = 1.5–3.3, ε_Hf(t) = 1.1–11). These results support the notion that the polybaric differentiation in the lower crust can significantly influence the diversity of geochemical composition in the upper crust and highlight that the final closure of the Paleo-Tethys in the northeastern Pamir may not have occurred before the early Jurassic.

Pressure–temperature (P–T) modeling and U–Pb monazite petrochronology provide a detailed P–T-t history for the Funeral Mountains metamorphic core complex, revealing different aspects of the geologic history at different structural depths and enabling the dating of tectonic mode switching cycles in the southwestern US Cordillera. Monazite petrochronology and yttrium X-ray element maps reveal several generations of monazite that formed during the Jurassic to Late Cretaceous. In the Monarch Canyon study area, the staurolite-out isograd separates samples with predominantly Jurassic monazite from those with predominantly Cretaceous monazite. Monazite grains yielding Jurassic to Early Cretaceous dates are chemically distinct from those yielding mid- and Late Cretaceous dates. Jurassic monazite dates from the Funeral Mountains record both prograde and retrograde metamorphism, with the latter associated with garnet breakdown during decompression. Heavy rare earth elements and yttrium (HREE + Y) in a mid-Cretaceous 104 to 88 Ma monazite population link recrystallization to prograde garnet growth from staurolite breakdown, and in a Late Cretaceous 88 to 74 Ma population to retrograde garnet breakdown via a reversal of the staurolite breakdown reaction. Modeling and mineral textures indicate peak metamorphic conditions of 6–10 kbar at ca. 650–700 °C in the structurally deepest rocks in Monarch Canyon. In contrast, structurally shallower rocks experienced peak temperatures between 610 and 650 ºC during Jurassic metamorphism. Monazite petrochronology elucidates the progression of monazite dissolution-reprecipitation along this P–T path. Modeling reactions and mineral stability link specific reactions to changes to the HREE + Y concentrations in monazite, particularly related to garnet and staurolite reactions. This dataset, in conjunction with previous studies, enables the timing and duration of tectonic mode switching cycles in the Funeral Mountains to be quantified, improving our understanding of the complex geological evolution of this core complex.

Deep melt migration processes occurring beneath spreading ridges largely occur by porous flow and involve reaction with the pre-existing crystal matrix. The control of the melt/rock ratios and melt fluxes involved in these reactive percolation processes on the structural and chemical evolution of oceanic magmatic systems is yet to be fully constrained. We here report a combined petro-geochemical study of variably evolved gabbroic layers in the Oman Moho Transition Zone, atop the Maqsad mantle diapir, ranging from dunites, troctolites and wehrlites to olivine gabbros. The layering characterizing the base of the crustal section formed during a process of reactive porous flow and hybridization of a dunitic precursor. Positive feedback between melt distribution and deformation focusing allowed for the development of two distinct percolation behaviours, between focused melt percolation and diffuse melt impregnation. This geological setting provides an ideal case study to assess the impact of the melt/rock ratios and percolation dynamics on the evolution of textures and chemical compositions during focused and diffuse percolation. Namely, the former leads to a modification of the crystallographic preferred orientation and complete chemical reequilibration of the matrix, while the latter allowed for preservation of the pre-existing structure and buffer of the melt composition by the matrix and reactive processes. We quantify the melt/rock ratios associated with the two magmatic systems using Plate Models to demonstrate that focused percolation easily resets the matrix composition from melt/rock ratios integrated over time ~ 2–3, whereas diffuse, low-flux melt impregnation would require elevated melt/rock ratios (> 20) to allow for chemical reequilibration. Furthermore, we provide a global overview of the evolution of mineral compositions and textures of a percolated olivine-rich protolith as a function of the melt migration style and the involved melt/rock ratios, both instantaneous and integrated over time.

Abundant Archaean komatiite and basalt erupted through evolving cratons, indicating melt transfer through the ancient mantle lithosphere. However, this process has rarely been identified in cratonic peridotite xenoliths, in contrast to exposed Phanerozoic mantle sections where melt-rock reactions are well-documented. We present a combined microstructural and mineral chemical investigation of eight coarse-grained (up to 20 mm), silica-rich, spinel facies peridotites from the Kaapvaal craton. These peridotites exhibit mild to strong silica-excess with 30–55 vol% orthopyroxene. Microstructural evidence of former melt presence is abundant in all samples, including low apparent dihedral angles, irregular grain boundaries, and extremely elongate grains. Despite varying silica-excess, all peridotites are highly refractory, with olivine Fo-content of 92.9 ± 0.3, reconstituted whole rock Mg-number of 92.9 ± 0.4, and negligible TiO₂ concentrations. Thermobarometry and comparisons with experimental compositions and thermodynamic models suggest a continuum of reactions in open systems, where evolving komatiite melt sourced from greater depth interacted with precursor mantle lithosphere at 2–3 GPa. We propose that silica-excess in cratonic spinel peridotites results from high time-integrated (i.e., aggregated) melt flux through melt channels, without requiring a highly silicic melt. Evidence for reactive flow of komatiite melt through cratonic mantle supports an intraplate setting for many Archaean greenstone belts and a co-evolution of Archaean crust and mantle.

Compositionally zoned crystals can record changing melt composition and trace element partitioning behavior during magmatic differentiation. Diffusive reequilibration between compositionally distinct zones in crystals can also produce compositional gradients. Here, we compare the length scales of concentration gradients for different elements in clinopyroxene that originate from the Tshirege Tuff and late Valle Toledo Member rhyolites of the Bandelier magmatic system in New Mexico to determine what petrogenetic information is recorded in the zonation. Within these rhyolites there are unzoned ferrohedenbergite crystals, as well as less common normally-zoned clinopyroxene with ferrohedenbergite rims and ferroaugite cores. Compared to the ferroaugite cores, the ferrohedenbergite rims are enriched in Dy and Yb, but depleted in Co, Ti, Sc, Ce, and Nd. The length scales for fast and slow diffusing elements for most gradients measured are indistinguishable, which argues that the gradients emerged predominantly from changing magmatic composition during crystallization, with diffusion having little to no role in establishing the concentration gradients. Fractional crystallization of the phases present in the rhyolites fails to reproduce all trace-element zonation that occur in the clinopyroxene, however, indicating a more complex origin. Based on the compositional similarity of the ferroaugite cores with pyroxene from rhyolites that erupted ≥ 165 kyr earlier, we interpret the ferroaugite cores as antecrysts scavenged from crystal-rich mush during magmatic rejuvenation. The magmatic rejuvenation that remobilized the parent mush of the ferroaugite antecrysts was likely initiated near the end of a > 100 kyr eruption hiatus that preceded the final runup to the catastrophic Tshirege eruption.

The trachydacitic Alpehué tephra from Sollipulli volcano (Andean Southern Volcanic Zone), consists of ignimbrite and fallout from a Plinian eruption about 3000 years ago. It is mainly composed of (1) crystal-rich pumice and ash but also contains (2) chilled knobbly basaltic lava clasts and (3) mostly highly inflated glomerocrystic fragments with high crystal-glass ratios interpreted to represent a crystal mush zoned from basaltic to dacitic bulk compositions. Knobbly lava clasts are of three types: (a) a very phenocryst-poor basalt, (b) a basalt with large, unzoned olivine and plagioclase phenocrysts and glomerocrysts, and (c) mixtures of microcrystalline basalt with various fragments, glomerocrysts and crystals derived from a crystal mush. Clast type (4) in the tephra is banded pumices in which the three magmatic components occur variably mingled. Thermobarometry and petrographic observations, particularly presence or absence of amphibole, constrain an upper-crustal succession of a lower basaltic reservoir, a zoned basaltic to dacitic crystal mush reservoir, and a separate trachydacite magma chamber on top. All Alpehué magmatic components form a coherent liquid line of descent which supports the interpretation that the crystal mush reservoir is a gradually solidifying magma chamber, not the result of large-scale crystal-liquid segregation. The trachydacite magma may originally have formed as melt escaping from the crystal-mush reservoir but subsequently underwent a long and complex evolution recorded in large strongly zoned plagioclase phenocrysts including resorption horizons. The ascending mafic magmas collected samples from the crystal mush body and intruded the trachydacite reservoir. The phenocryst-poor basalt (a) arrived first and entrained and partially resorbed plagioclase from the host magma. The phyric basalt (b) arrived later and did not resorb entrained plagioclase before eruption. Estimated cooling times, plagioclase resorption times and ascent rates avoiding amphibole breakdown limit the duration of these pre-eruptive processes to not more than a few days.

Lower crustal xenoliths from the Missouri Breaks diatremes and Bearpaw Mountains volcanic field in Montana record a multi-billion-year geologic history lasting from the Neoarchean to the Cenozoic. Unusual kyanite-scapolite-bearing mafic granulites equilibrated at approximately 1.8 GPa and 890 °C and 2.3 GPa and 1000 °C (67 and 85 km depth) and have compositions pointing to their origin as arc cumulates, while metapelitic granulites record peak conditions of 1.3 GPa and 775 °C (48 km depth). Rutile from both mafic granulites and metapelites have U-Pb dates that document the eruption of the host rocks at ca. 46 Ma (Big Slide in the Missouri Breaks) and ca. 51 Ma (Robinson Ranch in the Bearpaw Mountains). Detrital igneous zircon in metapelites date back to the Archean, and metamorphic zircon and monazite record a major event beginning at 1800 Ma. Both zircon and monazite from a metapelite from Robinson Ranch also document an earlier metamorphic event at 2200–2000 Ma, likely related to burial/metamorphism in a rift setting. Metapelites from Big Slide show a clear transition from detrital igneous zircon accumulation to metamorphic zircon and monazite growth around 1800 Ma, recording arc magmatism and subsequent continent-continent collision during the Great Falls orogeny, supporting suggestions that the Great Falls tectonic zone is a suture between the Wyoming craton and Medicine Hat block. U-Th-Pb and trace-element depth profiles of zircon and monazite record metasomatism of the lower crust during the Laramide orogeny at ~60 Ma, bolstering recent research pointing to Farallon slab fluid infiltration during the orogeny.