Mineralogy and Petrology最新文献

Late Mesozoic volcanic rocks comprising mainly basalt and basaltic-andesite to dacite occur in south-central Vietnam (Dalat zone) and to a lesser extent in southwestern Vietnam (Bay Nui area). Mineral and whole-rock chemistry indicate a calc-alkaline affinity for samples in the Dalat zone and a high-K calc-alkaline to shoshonitic affinity for rocks in the Bay Nui area. Mineral characteristics and variation diagrams of selected elements suggest that fractional crystallization dominated during magma differentiation. The Bay Nui volcanic rocks generally are more enriched in potassium and LILEs (large-ion lithophile elements) than volcanic rocks from the Dalat zone, which may indicate a more evolved nature or crustal assimilation. The similar chemical characteristics and eruption/emplacement age range of volcanic and plutonic rocks (ca. 90–110 Ma) of equal silica concentration indicate that the magma feeding the volcanic eruptions had the same source as that of the plutonic rocks. The observed mineral and whole-rock compositions with enrichment in LILEs, depletion in HFSEs (high field strength elements), and noticeably negative Nb, Ta, and Ti anomalies are characteristic for arc signatures. Zircon U-Pb geochronological data for the volcanic rocks indicate an age range of 95–105 Ma for the eruption. These geochemical and geochronological data link this Late Mesozoic volcanism with continental arc magmatism driven by the subduction of the Paleo-Pacific beneath eastern Indochina. Zircon xenocrysts with a likely magmatic origin cluster around 350 Ma and 250 Ma, indicating two earlier magmatic events most likely related to the subduction of the Paleo-Tethys beneath western Indochina and the subsequent Indosinian orogeny.

Orishchinite is a new terrestrial phosphide discovered in pyrometamorphic rocks of the Daba-Siwaqa combustion complex in West Jordan. The mineral occurs as an accessory phase in the fused clinopyroxene-plagioclase rock (paralava) texturally resembling gabbro-dolerite. Orishchinite forms resorbed equant grains up to 0.2 mm outrimmed with 0.1–0.2 thick zones of substituting murashkoite, FeP. Chemical composition (electron microprobe, wt%): Ni 38.49, Fe 22.38, Co 0.47, Mo 18.80, P 19.46, Total 99.60, corresponding to the empirical formula (Ni_1.04Fe_0.64Mo_0.31Co_0.01)_Σ2.00P on the basis of 3 apfu. The simplified formula is (Ni,Fe,Mo)₂P and the ideal one is Ni₂P. Macroscopically, orishchinite grains have yellowish-white colour with metallic lustre. The mineral is brittle. In reflected light, orishchinite is yellowish-white and non-pleochroic. It is very weakly anisotropic (ΔR₅₈₉ = 1.3%). Reflectance values for the wavelengths recommended by the Commission on Ore Mineralogy of the International Mineralogical Association are [R_max/R_min (%), λ (nm)]: 48.1/47.5, 470; 50.6/49.4, 546; 52.1/50.8, 589; 54.4/52.9.1, 650. The crystal structure was solved and refined to R₁ = 0.016 based on 224 unique observed [I ≥ 2σ(I)] reflections. Orishchinite is orthorhombic, space group Pnma, a 5.8020(7), b 3.5933(4), c 6.7558(8) Å, V 140.85(3) ų, Z = 4, D_x = 7.695 g cm^-3. The strongest lines of the powder X-ray diffraction pattern [(d, Å) (I, %) (hkl)] are: 2.265(100)(112), 2.201(16)(202), 2.142(55)(211), 2.100(35)(103), 1.909(21)(013), 1.811(19)(113), 1.796(31)(020). Orishchinite is dimorphous with transjordanite (hexagonal Ni₂P) and can be considered the Ni-dominant analogue of allabogdanite.

Late to post-orogenic lamprophyres of the European Variscides attest variable compositions of the mantle beneath the structural zones of the belt. These compositions resulted from different contributions of mantle components involving geotectonic processes during the orogeny, such as oceanic subduction of mafic crust and sediments, continental subduction, collision with mantle input, and delamination of overriding plates. For documenting these processes, we have surveyed three sites of lamprophyre intrusions in the Vendean part of the South Armorican tectonic Zone with spessartite sills and minette dykes, and a fourth site in the West-Armorican kersantite swarm. The age of spessartite is estimated between 320 and 315 Ma on the base of structural relationships with the dated neighbouring granite. Dykes of minette share similar intrusive setting along the post-orogenic NW–SE dextral shear zones. One dyke is dated at 286.2 ± 6.6 Ma (Early Permian) by K/Ar method. The Western Brittany kersantite swarm is Middle to Late Carboniferous in age. All these rocks display common mineral and chemical compositions of lamprophyres. A review of the Variscan European lamprophyres is conducted in order to document their geochemical fingerprints compared with those of the studied samples.

Corundum crystals of semi-gem quality (sapphire) formed in a syenitic dyke around the plutonic rocks in the Alvand igneous-metamorphic complex were studied by Sheikhi Gheshlaghi et al. (2020). These authors studied field relations and petrography of a corundum-bearing pegmatite, and proposed a genetic model for corundum formation. Although studies on whole rock chemistry of corundum-bearing pegmatites and oxygen isotope data for corundum by these authors are valuable, some of their reports and findings – such as the sharp contact between corundum-bearing pegmatite and Alvand syenite, the lack of tourmaline group minerals in corundum-bearing pegmatite, the presence of syngeneic quartz inclusion in corundum and the genesis of corundum as a result of kyanite break down – are in contrast with our own observations and findings. Neither the P-T conditions, nor the volatile-bearing minerals present, are in favour of formation of corundum from kyanite breakdown. Based on present authors’ studies, mineral chemistry of corundum in the Alvand area clearly indicates magmatic origin.

Xiangshan volcanic-related uranium ore field, as the historic largest uranium producer in China, is characterized by a large volcanic-intrusive complex and a number of uranium deposits. In most of the deposits, various types of dikes are spatially associated with the uranium ore bodies, although there have been no systematic attempts to integrate the geochronology and petrogenesis of these dykes and their link with uranium mineralization. In this study, whole-rock major and trace elements, zircon U–Pb dating and zircon Hf–O isotopic analysis were carried out on the Xiagnshan felsic-intermediate dikes, including granitic porphyry, quartz monzonitic porphyry and dioritic porphyry. These felsic-intermediate dikes have the similar geochemical characteristics with the major ore-hosting volcanic rocks, such as enrichment in large lithophile elements (i.e., Rb and Th) and some high-field strength elements (i.e., Zr, Y), but relative depletion in Sr, Ba, P and Ti. These dikes also exhibit zircon ε_Hf(t) values concentrating from -9 to -4 and zircon δ¹⁸O values ranging from 7.6 to 10.0 ‰. Geochemical and isotopic data suggest that these dikes were predominantly derived from partial melting of crustal materials and possibly mingled with different proportions of mantle-derived materials during the formation of melts. SIMS and SHRIMP U–Pb dating of zircon grains from these felsic-intermediate dikes yield high-precision ages ranging from 136 to 131 Ma. Combined with previously reported age data indicates a rapid and concentrated dike emplacement event, although a few mafic-felsic dikes were sporadically emplaced at 126 -122 Ma. Such a relatively concentrated dike emplacement in the Early Cretaceous were most likely caused by the back-arc extension or intra-arc rift due to progressive rollback of the Paleo-Pacific Plate. Previous metallogenetic studies supported that the late-stage dikes at Xiangshan were temporally and spatially closely related to uranium mineralization. However, our new geochronological data and field observations indicate that the two episodes of uranium mineralization were not synchronous with the majority of dikes intrusions, indicating that they probably have no direct relationship in origin and that spatial coexistence were more likely due to multistage activities of faults. Rather, these dikes might act as a relatively reduced environment, and thus could facilitate the precipitation of uranium. In addition, all of these dikes can be considered favorable prospecting indicators in the Xiangshan ore field.

The present study reports the occurrence of orthopyroxene megacrysts from the Chilka Lake anorthosite massif, Eastern Ghats, India. An insight into the mineral chemistry of different phases, coupled with detailed field and petrographic evidences from this study, shed light on a long debate on the origin of orthopyroxene megacrysts in anorthosite massifs. The megacrysts contain exsolved lamellae of plagioclase and opaque oxides (ilmenite, rutile) oriented along orthopyroxene cleavage planes. The trace element distribution patterns of the megacryst and matrix plagioclase are mirror reflections of each other and mutually complementary. The calculated compositions of melts in equilibrium with these two phases show comparable patterns for LREE (light rare earth elements, La–Sm), but differ markedly in terms of HREE (heavy rare earth elements, Eu–Lu), suggesting that the megacrysts and matrix plagioclases did not crystallize simultaneously. We infer that the orthopyroxene megacrysts have a longer crystallization history, initially as a low-Ca non-quad member of the pyroxene group at pressure ≥ 10 kbar, incorporating some amount of Ca, Al and Ti in their structure. Subsequently, they have been carried by a plagioclase crystal mush to mid-crustal levels at pressure ~ 4–6 kbar following a near-isothermal decompression that may be linked to the emplacement of the anorthosite massif, giving rise to the exsolution lamellae of plagioclase and opaque oxides.