Lithos最新文献

A petrographic study of “Verde Prato” (trade name of an ophiolitic serpentinised peridotite quarried in Tuscany) revealed, next to a relict Cr-spinel, another opaque ore mineral apparently containing ca.17 wt% SiO₂. In order to unveil the origin of this anomalous composition, detailed SEM-EDS, μ-Raman spectroscopy and X-ray μ-diffraction investigations were undertaken, which provided useful but somehow contrasting indications. Therefore, in order to definitely unveil the nature of the unusual opaque mineral, a specific TEM study was undertaken. This showed the presence of submicroscopic euhedral Mn-Cr-Fe-spinels in a matrix of a 7 Å layer-silicate (Al-lizardite). Interesting, all the submicroscopic spinel grains were found to share the same crystallographic orientation, suggesting an origin from a single, larger, former crystal. This unusual type of alteration of the mantle spinel probably occurred on the ocean floor during the pervasive polyphase process of hydrothermal peridotite serpentinization. Overall, by mapping the different serpentinite microstructures and compositions, a polyphase serpentinization history consisting of at least eight different cycles has been suggested. Moreover, the spinel composition indicates that the alteration was accompanied by Mn-metasomatism, made possible by the close proximity of Mn-rich hydrothermal vents, such as “black smokers”.

The incorporation of KREEP (potassium, rare-earth element, and phosphorus), mantle-derived mafic melts and trapped liquid into the lunar ferroan anorthosite (FAN) suite plays a pivotal role in generating their geochemical and isotopic variations. Nonetheless, the specific involvement and distinct roles of these different components remain controversial. This study presents in-situ Sr isotopic data for 28 anorthositic clasts found within lunar feldspathic meteorites to trace their sources and post-modification processes. We find that these plagioclases exhibit substantial variations in their measured ⁸⁷Sr/⁸⁶Sr values (0.69978–0.70357), in contrast to the relatively narrow range observed in Apollo 16 FANs, thereby likely reflecting a diverse chemical composition of lunar crustal rocks. In contrast, the analyzed plagioclases have consistently low ⁸⁷Rb/⁸⁶Sr ratios (0.00185–0.03962), similar to those of Apollo samples, reflecting impact-induced loss of Rb. Detailed investigations indicate that certain elevated ⁸⁷Sr/⁸⁶Sr ratios are probably not caused by terrestrial contamination or instrumental analysis, but most likely result from the decay of ⁸⁷Rb from sources with initial ⁸⁷Rb/⁸⁶Sr higher than 0.0119–0.1380. However, such elevated ⁸⁷Rb/⁸⁶Sr values cannot solely result from crystallization of the lunar magma ocean (LMO) and likely involve KREEP components. Combined with trace element data, we estimate the maximum proportion of KREEP melt in the formation of lunar anorthosites. Future analyses of lunar anorthosites collected by China's Chang'e-5 and Chang'e-6 missions will be crucial for validating the observed Sr isotopic heterogeneity.

The origin of coeval magmatic and metamorphic rock associations is of great significance in tectonic interpretations. In this study, spatially associated cordierite granite (S-type), metapelite and diorite from the Qinghai Nanshan (NW China) area were dated to be coeval at ∼247–244 Ma. The cordierite granite and metapelite have almost uniform peak pressure-temperature (P–T) conditions of c. 3.8–5.0 kbar / c. 740–790 °C, indicating a geothermal gradient of higher than ∼40 °C / km. The zircon crystallization temperature of the diorite pluton is estimated to be c. 760 °C by Ti-in-zircon thermometer, putting a lower limit of temperature for dioritic magma. Both cotectic (phenocrysts) and restitic cordierite crystals were identified in the cordierite granite. Similar whole rock compositions of the coarse-grained cordierite granite and the metapelite in the Qinghai Nanshan area as well as the average / median pelite worldwide, imply formation of the granite was from almost complete melting of autochthonous metapelite, followed by in-situ recrystallization with negligible / without melt extraction. These data indicate that the cordierite granite and metapelite are both products of the contact aureole surrounding the diorite pluton. This study presents an example for better understanding the transition from high-grade metapelite to S-type granite.

The formation and evolution of the Neo-Tethys Ocean profoundly influenced the pre-collisional configuration of the Tibetan Plateau before the India-Asia collision. However, the timing of the Neo-Tethys Ocean's opening and the resulting magmatism remain subjects of ongoing debate. Here we present an integrated investigation of a suite of basaltic andesites exposed in the Gyaca area, eastern Tethyan Himalaya, southern Tibetan Plateau. Using zircon U-Pb-Hf isotopes, bulk rock geochemical data, and whole-rock Sr-Nd-Hf isotopic data, we attempt to temporally and petrogenetically constrain the early stages of magmatism associated with the opening of the Neo-Tethys Ocean. The Gyaca basaltic andesites were formed in the Late Triassic (ca. 217 Ma). They exhibit geochemical features resembling those of arc magmatic rocks, characterized by moderate light/heavy rare earth element fractionation ((La/Yb)_N = 5.16–6.57), enrichment in large ion lithophile elements, and depletion in high field strength elements. They also show variable whole-rock SrNd (⁸⁷Sr/⁸⁶Sr_i = 0.709848–0.712233; ε_Nd(t) = −1.12 to +0.19) and zircon Hf (ε_Hf(t) = −6.2 to +3.2) isotope compositions, alongside depleted whole-rock Hf isotopes (ε_Hf(t) = +2.83 – +7.42). Compared to coeval arc magmatism in the southern Lhasa Terrane, southern Tibetan Plateau, the Gyaca basaltic andesites show higher incompatible element contents and more enriched NdHf isotope compositions, ruling out their origin as products of northward subduction of the Neo-Tethys oceanic plate. The negative correlation between the Mg# of these basaltic andesites and ε_Nd(t) suggests that more primitive magmas have more enriched Nd isotopes, likely due to assimilation with sediments during turbulent magma ascent under high thermal conditions. Combining existing petrological and sedimentological evidence, we propose that the Gyaca basaltic andesites likely document the early interaction between the upwelling asthenosphere and the overlying sediments during the initial spreading of the Neo-Tethys seafloor. Consequently, the opening of the Neo-Tethys Ocean in the eastern Himalaya would not postdate the Late Triassic.

Geochemical analyses of individual minerals provide more detailed insights into the petrogenesis of igneous rocks than whole-rock analyses. This study conducted in situ geochemical and Nd isotope analyses of apatites from 10 Jurassic granitic plutons in the Yanbian area, NE China, to establish the petrogenesis and regional tectonic evolution. The results indicate that the apatite geochemistry of Jurassic granitoids in the Yanbian area was controlled primarily by the composition of parental melt. Post-magmatic alteration may lead to geochemical decoupling between apatite and parental melt, while Nd isotopes exhibit some resilience to such alterations. Apatites from Early Jurassic granitoids display characteristics that are consistent with an I-type origin, whereas those from Middle and Late Jurassic granitoids exhibit an adakitic affinity. Variations in apatite compositions indicate the fractional crystallization of other rare earth element (REE)-bearing minerals during magma evolution. The early crystallization of plagioclase and allanite led to decreases in Sr and Th contents in apatite, respectively, resulting in a negative Eu anomaly and light REE depletion. The fractional crystallization of titanite and hornblende resulted in the depletion of middle REE in apatite. Hornblende is regarded as the main residual phase in the magma source of Middle and Late Jurassic adakitic granitoids in the Yanbian area. Apatite Nd isotopic compositions suggest that the Jurassic granitoids in the Yanbian area originated from two crustal sources: the Central Asian Orogenic Belt and the North China Craton. Additionally, increasing trends in apatite Sr/Y and (La/Yb)_N ratios from the Early to Late Jurassic suggest a gradual thickening of the regional crust, which is likely driven by the continentward migration of the subduction zone associated with the Paleo-Pacific Plate.

Chromitite occurrences in ophiolites are commonly classified into high-Cr (Cr# > 60) and high-Al (Cr# < 60) varieties. High-Cr chromitites have been extensively studied, whereas the origin of high-Al chromitites remains enigmatic mainly due to strong alteration of the relevant rocks. This study undertook a comprehensive examination of alteration-free high-Al chromitites and their hosting peridotites from the Kudi ophiolite in the NW Tibetan Plateau, complemented by a synthesis of global data of high-Al and high-Cr chromite deposits. Our results reveal that harzburgites, hosting both types of chromite deposits, exhibit similar major oxide compositions in their constituent minerals. In contrast, the constituent minerals in the dunite envelopes and chromitites display distinct major and trace element compositions, which are probably controlled by the compositions of infiltrating melts. For both chromite deposits, the forsterite (Fo) contents of olivine increase from harzburgite (90.0–91.6) to dunite (89.9–94.6) and chromitite (90.0–97.2). Mineral compositions of olivine show significant variabilities at the contact boundaries between densely disseminated and sparsely disseminated chromitites. These features suggest that fluid immiscibility plays an important role in the formation of high-Al chromitite. A continuous spectrum of major oxide compositions of chromite from high-Al to high-Cr chromitites suggests a progressive shift in the parental magma compositions from mid-ocean ridge basalt-like to boninitic melts, implying a rapid tectonic transition during subduction initiation.

Based on petrological association, cathodoluminescence (CL), trace element signatures and orientation relationships, two generations of kyanite are distinguished in a high temperature, high pressure garnet-biotite-aluminosilicate bearing migmatite of South Harris, NW Scotland. The migmatite shows a garnet and biotite rich domain (Grt-Bt domain) which is cut at a low angle by a dominantly coarse-grained plagioclase-quartz leucosome. In addition, a fine-grained plagioclase-quartz-kyanite domain (Plag-Qtz-Ky domain) is present intercalated with the Grt-Bt domain and subparallel to the plagioclase-quartz domain. Type 1 kyanite is coarse grained and associated with Bt clusters and garnet within the Grt-Bt domain. It grew relatively early, syn- to post-garnet growth, in a suprasolidus environment resulting in crystallographically determined oscillatory CL and trace element zoning. Grains record evidence of progressive deformation in the crystal plastic regime, where deformation is accommodated by dislocation glide, climb and deformation twinning. The dominant activated slip system is (100)<001> with minor component of <100>, while deformation twins show a ∼ 180° rotation around ∼<001> axis and a twin plane near (001). Grains appear to be impervious to deformation induced diffusion with all zoning remaining sharp despite crystal plastic deformation. Grains in direct contact with the Plag-Qtz-Ky domain show late modification of the CL and trace element signature suggesting melt-mediated interface-coupled dissolution-precipitation reaction. This modification resulted in crosscutting lobate high trace element regions and irregular rims with low Cr and V content. These rims show similar CL and trace element characteristics as Type 2 kyanite which are exclusively seen within the Plag-Qtz-Ky domain suggesting that Type 2 grains are cogenetic with Type 1 rims. Type 2 grains are finer grained than Type 1 grains and show near uniform CL and trace element distributions with rare oscillatory zoned and relatively higher Cr & V bright cores. Type 2 show either no or very localized internal deformation features. However, they exhibit a clear shape preferred orientation which coincides with a crystallographic preferred orientation where the longest shape axis is parallel to <001>. We propose that Type 2 kyanite grains underwent melt-present deformation by rigid body rotation in an externally derived melt with different trace element chemistry than the host rock. This melt thus interacted chemically by melt-mediated interface-coupled dissolution-precipitation reactions with the surrounding rocks forming the Type 1 rims.

Our study shows detailed analysis of kyanite is an important tool for giving constraints on the deformation, P–T and melting history of high-grade metamorphic rocks and migmatites.

Despite their relatively rare occurrence, lepidolite-subtype pegmatites host abundant Li–Nb–Ta–Cs–Sn mineralization and represent a high-flux pegmatitic system with abnormally high F and Li activity. Characterization of highly fluxed melts and the impact of fluxes and exsolved fluids on fractionation of peraluminous melts have mainly been studied in experimental systems, with natural system correlations remaining poorly understood. Consequently, we conducted a systematic mineralogical study of a lepidolite-subtype pegmatite in the North Qinling orogenic belt, Central China. An abnormal “concave downward” fractionation trend for primary columbite-group minerals on the quadrilateral diagram is identified, and irregularly zoned columbite crystals coexist with F-rich minerals in one of the core zones have the highest Ta contents (normally 50.17–63.13 wt% Ta₂O₅) and Ta/(Nb + Ta) ratios (up to 0.65). Despite the consistently Ta-dominated B-site in the crystal lattice of microlite-group minerals, extreme compositional variations at the A- and Y-sites are observed. Compared with microlites in intermediate zones, the abrupt increase in U in microlite crystals in core zones and late units (up to 20.16 wt% UO₂), is ascribed to the melt-fluid interaction with exsolved U-rich aqueous fluids. In addition, the fractional crystallization of F-bearing minerals resulted in a gradual decrease in F contents in microlite-group minerals from extremely F-rich (2.68–4.84 wt% F) in intermediate zones to low F species (mainly 0.82–1.71 wt% F) in core and late zones. Moreover, autometasomatism by a late fluxed melt and hydrothermal metasomatism by late aqueous fluids are identified in columbite- and microlite-group minerals. This work highlights that these non-typical fractionation behaviors related to the activity of fluxes (especially F) and the exsolution of aqueous fluids during the internal evolution of pegmatitic melts, are critical for the generation of lepidolite-subtype pegmatites. Fluorine was gradually enriched in the pegmatitic melt, and reached its highest level during crystallization of the (inner) intermediate and core zones. Non-equilibrium crystallization occurred throughout pegmatite evolution, and late units were most probably formed from aqueous fluid-enriched residual melts, rather than by hydrothermal replacement.