The HP–UHP metamorphic terrane of Jiangling, eastern Dabieshan comprises extensively distributed granitic gneisses, mica-schists and numerous eclogite blocks. The mica-schists generally contain garnet, phengitic muscovite, biotite, plagioclase, quartz, rutile and a small amount of epidote and hornblende. Study on petrography and phases equilibria in the NCKMnFMASH system indicates that the present mineral assemblages in mica-schists are not in equilibrium. The earlier stage of mineral assemblage represented by garnet and phengite reflects a HP–UHP condition. The garnet compositions and the phengite Si contents give a PT condition of 580–600 °C at 2.6–2.8 GPa. The garnet zonation records an earlier progressive metamorphic process which may be associated with the appearance of glaucophane, jadeite and lawsonite. The later stage of mineral assemblage characterized by the presence of biotite and plagioclase reflects a PT condition of 620–635 °C at 0.9–1.1 GPa, belonging to the HP amphibolite facies. The main mineral assemblage in mica-schists from the Jiangling region has recorded a complete HP–UHP metamorphic process.
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Garnet is a common constituent of skarn type iron deposits and can be used to derive potential information on the genesis of skarn type deposits. Here, we investigate the petrologic, spectroscopic, and geochemical characteristics of garnet from the Nanminghe skarn iron deposit in China to elucidate the formation process, growth environment, and genesis. We employ a combination of multiple techniques including petrography, Infrared spectroscopy (IR), X-ray powder diffraction (XRD), Raman spectrum, electron microprobe, and LA-ICP-MS. The primary mineral assemblage in the skarn is garnet–diopside–magnetite–quartz–calcite–pyrite. The garnet occurs as granular aggregates or veins, and generally shows a combination form bounded by dodecahedral faces {110} and trapezohedron faces {211}. Oscillatory zoning and abnormal extinction of garnet are also noted. We identify at least three stages of garnet growth, with a gradual decrease in the iron content from early to late stage, accompanied by the precipitation of magnetite. Regarding the rare earth distribution model, the Nanminghe garnet is generally in the right-dipping mode enriched in LREE and depleted in HREE, which may be mainly controlled by adsorption. Major and trace elements of different generations of garnet suggest that the garnet in the iron skarn crystallized under high oxygen fugacity and is of hydrothermal origin.
The Chinese Continental Scientific Drilling (CCSD) Project in Donghai recovered more than 1000 m of eclogite and garnet peridotite cores for study. Examination of rocks from 100-2000 m of the main borehole has identified five major lithological types: (1) eclogite and garnet pyroxenite; (2) eclogitic gneiss; (3) garnet peridotite; (4) biotite (hornblende) two-feldspar gneiss; and (5) fault breccia and mylonite. The eclogite was further subdivided into two types: crustal eclogite and mantle-derived eclogite. Crustal eclogites are ubiquitous as layers of various thickness in gneissic rocks, and contain low-Prp (<40 mol%) garnet and omphacite. Mantle-derived eclogites are spatially associated with ultramafic cores composed mainly of garnet wehrlite, and have higher Prp-bearing (>40 mol%) garnet and low Jd-bearing clinopyroxene. Chemically, the crustal eclogites are relatively low in MgO and high in SiO2, but have high, variable contents of Al2O3 and rare-earth elements. Most crustal eclogites range in SiO2 content from 49 to 60 wt%, whereas mantle-derived eclogites are rich in MgO, and have SiO2 content less than 49 wt%. Garnet peridotites consist of olivine (Fo = 85-91), enstatite, Mg-rich garnet, and diopsidic clinopyroxene; Ti-clinohumite is also widespread. Mineral paragenesis indicates that the garnet peridotites together with other lithologies underwent in situ ultrahigh-P metamorphism (UHPM). Based on the differences in rock association, structural kinematics, and seismic characteristics, we have identified two different rock slices separated by a fault zone at 1600 m depth, where breccia and mylonite developed. Rutile eclogites are dominant in the upper slice, and phengite eclogites are layered with deformed tonalite and paragneiss in the lower slice. These UHPM rocks underwent variable retrograde metamorphism; eclogite is replaced by symplectite-bearing garnet amphibolite, and eclogitic gneiss is retrograded to biotite (hornblende) plagioclase gneiss. Late-stage crustal extension resulted in local cataclasis, forming tectonic breccia with the development of chlorite, calcite, hematite, and epidote under epidote amphibolite-to greenschist-facies conditions. Nearly 2000 m of recovered UHP core from the CCSD main hole reveals that voluminous crustal materials were subducted to mantle depths and rapidly returned to the surface. UHPM cores record subduction and exhumation processes of the continental crust and provide information for the study of continental subduction/collision and mantle dynamics.
Skarn deposits constitute a significant reservoir of high-grade iron (Fe) ores in China, yet the metallogenic processes remain debated. This study focuses on the iron-rich enclaves embedded within porphyritic monzonite, aiming to unravel the metallogenic mechanisms underlying skarn Fe deposits. The host porphyritic monzonite intrudes into porphyritic diorite. Within the iron-rich enclaves three types of magnetite (Mt) are discerned. Mt-I, ranging from 50 to 100 μm in size, predominantly occupies the core of the enclave, enshrouding diopside at its center. Mt-II, with a size range of 20–40 μm, is primarily distributed in the middle of the enclave. The smallest variant, Mt-III, measuring approximately 1 μm, typically manifests at the peripheries of Mt-I and Mt-II. Comparative analysis reveals that Mt-II exhibits higher total rare earth elements (REE), Sr, Ca, alongside lower TiO2 content in contrast to Mt-I. Notably, Mt-II displays a characteristic W-type tetrad effect of REE, indicative of crystallization in a fluid-saturated environment. In contrast, Mt-III, characterized by microcrystalline magnetite, is inferred to have formed under conditions of rapid cooling. Our interpretation posits a two-stage genesis for these enclaves. The initial stage involves the formation of iron-rich magma within a magma chamber situated at depths of 8–10 km, during which Mt-I crystallizes. Subsequently, the second stage unfolds as mantle-derived fluids infiltrate the magma chamber, leading to the formation of Mt-II. The fluid overpressure within the magma chamber triggers a swift ascent of iron-bearing melt-fluid, localized at depths ranging from 1.5 to 2.6 km, resulting in the crystallization of Mt-III. Our results provide valuable insights into the metallogenic processes governing skarn Fe deposits, and the complex geological evolution of these deposits.
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