The drill site of the 5000 m deep main hole of the Chinese Continental Scientific Drilling Project (CCSD) is located in the southern Sulu ultrahigh-pressure (UHP) metamorphic belt. The 0 to 2000 m interval of the CCSD main hole consists of eclogite, gneiss, garnet peridotite and minor schist and quartzite. The eclogites with a cumulative thickness of about 1000 m have different mineral assemblages and model contents, as well as variable petrochemical compositions. They can be divided into five types:Si-rich quartz eclogite, Ti-rich rutile eclogite, Al-rich phengite and kyanite eclogite, Mg-rich two-mineral eclogite and normal eclogite. Protoliths of eclogites include layered mafic to ultramafic cumulates and supracrustal rocks. The whole-rock composition of eclogites has an apparent controlling effect on the contents of some components in garnet, omphacite and phengite and directly influences the P-T estimates of UHP metamorphism. The extensive development of diffusional compositional zoning in the eclogitic garnet, omphacite and phengite indicates that these UHP minerals were re-equilibrated during the early-stage retrograde metamorphism. This fact and the existence of garnet porphyroblasts with growth zoning suggest that the eclogites formed at higher temperature (940℃) and pressure (4.5GPa) than those estimated by previous workers.
Primary and exsolution fluid inclusions are recognized in omphacite of ultrahigh-pressure (UHP) eclogites from the main hole of the Chinese Continental Scientific Drilling Program, located in the southern Sulu orogenic belt. These oriented fluid inclusions occur as tubes, and coexist with exsolved quartz needles in the cores of host omphacite. Most complex primary fluid inclusions contain a gas bubble, a liquid phase, and one to several solids, such as quartz, halite, calcite, and opaque and unknown minerals, having compositions in the system of NaCl-CaCl2-CO2-H2O-SiO2 with possibly trace Fe and Mg; in contrast, simple fluid inclusions contain a gas, an aqueous liquid, and sometimes a calcite. We suggest that the complex fluid inclusions were trapped during the omphacite growth, whereas the simple aqueous inclusions and quartz needles exsolved from OH--rich supersilicic omphacite during early uplift of the UHP metamorphic rocks. Omphacite is one of the major fluid carriers during subduction of continental crust to mantle depths.
Abstract The 5‐km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)‐metamorphic rocks consisting mainly of eclogite, gneiss and garnet‐peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low‐ P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite‐facies minerals such as phengite and jadeite. The zircon cores give highly variable 206 Pb/ 238 U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO 2 , Al 2 O 3 , K 2 O, FeO, MgO, Na 2 O and H 2 O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution.
Abstract The newly discovered Shanzhuang BIF is hosted in the Shancaoyu Formation of the Taishan Group within the Eastern Block, southeastern margin of the North China Craton. The ores can be subdivided into three types in terms of mineral assemblages, corresponding to three types (I, II, III). The element concentration of the type I magnetite is similar to that of the type II magnetite, while the type III magnetite is similar to that of the schist. In general, magnetite and hematite grains from the ores show high concentrations of Mn (1317, 1162 ppm), Co (787, 1023 ppm), Al (2224, 2435 ppm) and Ti (540, 300 ppm), Whereas magnetite is depleted in Si (420 ppm) and hematite enriched in Si (1690 ppm). Detailed petrographic and mineral chemical analysis of magnetite, hematite, amphibole/hornblende and pyroxene, reveals that almost all the minerals occur as subhedral‐anhedral grains with pits and fractures, and the BIF is recrystallized to metamorphic assemblages of high amphibolite facies. Hornblende is highly enriched in Fe, Mg and Ca, but depleted in K and Na, mostly belonging to magnesiohornblende. In addition, the ratios of Mg/(Mg+Fe 2+ ), Fe 3+ /(Fe 3+ +Fe 2+ ), Si/(Si+Ti+Al) and Al/Si are 0.48–0.64, 0.17–0.36, 0.79–0.88 and 0.14–0.27, respectively. It is suggested that hornblende is neither a typical magmatic origin nor a typical metamorphic. Pyroxene has the characteristics of high Ca and Fe, but low Ti and Al, with end‐member components En, Wo and Fs in the ranges of 25.22–28.64 wt%, 43.71–46.40 wt% and 24.51–27.62 wt%, respectively, belonging to clinopyroxene, and mostly diopside, might be formed during the prograde metamorphism in the absence of H 2 O. The carbonate such as dolomite‐ankerite series is probably a precursor mineral of the BIF deposit. Mass mineral chemical and structural characteristics indicate that the Shanzhuang iron deposit has been subjected to varying degrees of oxidized hydrothermal superimposed reformation, metamorphism, and supergenesis after mineralization, during which some elements have been migrated in some degree.
Preliminary petrographic observations and microthermometric measurements show that three types of fluid inclusions occur in quartz veins and late-stage carbonate veins in high- and ultrahigh-pressure (HP-UHP) rocks from the CCSD area (Donghai). Fluids include brine (NaCl-H2O) inclusions (type I), NaCl-CaCl2-H2O inclusions (type II), and N2-CH4 inclusions (type III). Type I fluids are divided into medium-high salinity (type Ia), medium salinity (type Ib), and low salinity (type Ic) inclusions. The type III inclusions are the first discovered in the CCSD area. Type Ia, Ib, and II inclusions are primary or pseudosecondary in origin and occur in vein- and matrix quartz in eclogites. Their absence in vein- and matrix quartz and in the amphibolite-facies gneisses suggests that Ia, Ib, and II inclusions were probably captured during decompression-recrystallization and retrograde metamorphism of the eclogite. In contrast, type Ic inclusions are widely distributed, and were probably captured in the last stage of UHP exhumation. N2-CH4 pure gaseous inclusions are mostly primary, and mainly occur as isolated or clustered inclusions in laminated quartz veins in eclogite together with types Ia and Ib inclusions; this implies that type III inclusions were probably captured under HP-UHP metamorphic conditions. Most quartz veins in eclogite probably formed by decompression-recrystallization and retrograde metamorphism during exhumation of the subducted plate, whereas quartz veins in the gneisses mainly formed at amphibolite-facies or late-stage retrograde metamorphism during exhumation. The distinct differences between fluid inclusions in quartz veins in eclogites versus those in gneisses, and the similarities among fluid inclusions in quartz veins and matrix quartz crystals in their respective hosting rocks suggests that metamorphic fluids expelled from the HP-UHP rocks during the exhumation migrated mainly at the grain scale.
The Lass vein is in the Beaverdell silver, lead, zinc (gold) vein camp in south-central British Columbia. Veins in this camp are generally hosted within propylitized Westkettle granodiorite of Jurassic age, but mineralization is related to the Beaverdell quartz monzonite stock of Late Paleocene age (based on a K–Ar biotite date of 58.8 ± 2.0 Ma). Galena lead isotopes, interpreted using the recent "shale," "Bluebell," and "mixing-line isochron" models for the Canadian Cordillera, confirm a Tertiary age for all major vein mineralization in the Beaverdell camp.Examination of metal zoning, mineralogy, fluid inclusions, and sulfur isotopes indicates that the Lass vein system can be divided into two distinctly different parts, an upper western portion and a lower eastern portion. Differences between the two parts are related to the dominance of one of two mineralizing events.Event 1, the earlier, was most dominant in the lower portion of the Lass vein system. By comparison with event 2, event 1 is characterized by (i) relatively gold-, zinc-, and lead-rich but silver-poor ore (Au, 3163 ppb; Zn, 5.34%; Pb, 2.34%; Ag, 208 ppm); (ii) thicker veins (20 cm); (iii) sulfides with abundant pyrite, arsenopyrite, and dark sphalerite with exsolved chalcopyrite; and (iv) fluid inclusions with higher salinities (15 wt. equiv. wt.% NaCl), local CO 2 phases, and higher temperatures of homogenization (287 °C) with matching equilibrium temperatures indicated by sulfur-isotope geothermometry from galena and sphalerite (294 °C).The younger event 2 is responsible for most of the mineralization in the upper portion of the Lass vein. Many characteristics of this event are statistically different from those associated with event 1. Namely, event 2 (i) is relatively silver rich and gold, zinc, and lead poor (Ag, 291 ppm; Au, 764 ppb; Zn, 3.07%; Pb, 1.27%); (ii) has narrower veins (10 cm); (iii) has sulfides characterized by silver–sulfosalt-bearing galena and pale sphalerite; and (iv) is represented by fluid inclusions that are variable but on average lower in salinity (7 equiv. wt.% NaCl), have lower temperatures of fluid homogenization (225 °C), and do not contain CO 2 phases.Estimates from fluid inclusions indicate that event 1 could have occurred at depths equivalent to those of event 2 if the former was under lithostatic pressure at temperatures near the boiling point and if the latter formed at hydrostatic pressures near boiling temperatures. Sulfur-isotope data indicate that event 1 was nearly boiling; no similar definition is available for event 2. Thus, only minimum depth estimates are available for event 2.The most likely scenario for formation of the Lass vein system starts with event 1 and is followed by event 2. During event 1, minerals were deposited in a confined system under lithostatic pressures from high-salinity fluids where chloride complexing could have been important in the transportation of gold. Fracturing of the vein system to the surface changed the pressure regime to hydrostatic, and CO 2 was released. Consequently, temperatures and salinities of the fluids dropped, presumably mainly in response to mixing with cooler and less saline groundwater. Thus, gold solubilites concomitantly decreased, and silver deposition became more significant.Fluid-inclusion and lead-isotope analyses appear to be useful exploration procedures in the Beaverdell area in the identification of gold-rich systems. A further practical observation is that gold should continue to depth in the Lass system, if the vein can be followed.
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