Absence Of K-feldspar Research Articles

Fluid-present and fluid-absent melting of muscovite in migmatites in the Himalayan orogen: Constraints from major and trace element zoning and phase equilibrium relationships

Partial melting of the continental crust may proceed through either influx of free water (hydration, fluid-present melting) or breakdown of hydrous minerals (dehydration, fluid-absent melting). The two types of physicochemical mechanism would be operated for crustal anatexis in nature, and their effects on the composition of anatectic products were closely examined by experimental approaches as well as the study of natural leucogranites from the Himalayan orogen. However, the leucogranites usually experienced several stages of evolution during magmatism, leading to difficulties in retrieving the composition of original melts. This issue is now addressed by studying Himalayan migmatitic metapelites because they generally share similar source rocks and P–T conditions with the leucogranites. In combined with field observations, an integrated study of whole-rock geochemistry, mineral chemistry, zircon geochronology, and phase equilibrium modelling was conducted for migmatitic metapelites from the Nyalam Valley in the Greater Himalayan Sequence. The results show that the migmatitic metapelites share similar clockwise P–T paths. They experienced an early prograde metamorphism through compressional heating to reach peak pressures of 7.3–9.0 kbar at ~700 °C before ca. 31 Ma. Afterwards, the migmatitic metapelites underwent decompressional heating to ~725 °C at 6.0 kbar for upper-amphibolite facies metamorphism at 26–21 Ma. Subsequently, they experienced decompressional cooling to a temperature below the solidus at ca. 18 Ma. The crustal anatexis was dominated by muscovite melting reactions during the decompressional heating from ca. 26.4 to 18 Ma. Despite the similar P–T paths, the migmatitic metapelites experienced different types of melting reactions involving muscovite (i.e., dehydration and hydration). Whereas the hydration melting resulted in a marked increase in An contents (~65 to 75 mol%) and the absence of K-feldspar, the dehydration melting led to relatively low-An (<30 mol%) plagioclase and abundant K-feldspar. The two types of muscovite melting reaction may be common in the Himalayan orogen in the Late Cenozoic. While the fluid-present melting would take place at a shallower depth, the fluid-absent melting would occur at a greater depth. This coupled dehydration-hydration relationship has important implications for the geodynamic mechanism of crustal anatexis in the continental collision zone.

Defining early stages of IOCG systems: evidence from iron oxides in the outer shell of the Olympic Dam deposit, South Australia

The IOCG deposit at Olympic Dam (South Australia) is hosted within the Roxby Downs Granite, which displays a weakly mineralised contact to the orebody (hereafter ‘outer shell’). In a mineralogical-geochemical characterisation of Fe-oxides from the outer shell, we show silician magnetite (Si-magnetite) and HFSE-bearing hematite define the early stages of alkali-calcic alteration. This association forms in the presence of hydrothermal K-feldspar and calc-silicates via overprinting of magmatic magnetite and ilmenite breakdown. Geochemical modelling, at ≥ 400 °C, shows such reactions occur at pH-fO2 conditions coinciding with shifts from K-feldspar to sericite, and ilmenite to rutile stability. The subsequent Si-magnetite+siderite association forms down-T in the absence of K-feldspar. Transition from granular to bladed morphologies in Si-magnetite is part of a series of Fe-oxide interconversions, followed by formation of zoned, U-W-Sn-Mo-bearing hematite. Enrichment in REE, Y and U in Si-magnetite and the prevalence of U-W-Sn-Mo-bearing hematite support a granite-derived fluid. Combined, petrographic and geochemical evidence show a transition among Fe-oxides from the outer shell to the orebody attributable to the evolution of the same fluid. Unusual massive magnetite intervals and Fe-oxide nodules in granite are considered due to either the presence of inherited lithologies, metasomatic products, or the result of magnetite-rich, crystal mush forming in the melt. We propose a model, corroborated by recently published data including high-precision U-Pb dating of magmatic zircon and hydrothermal hematite, in which an ‘outer shell’ is initiated at the 6–8 km depth of granite emplacement during volatile release from fluids ponding at intrusion margins. Granite cupola collapse at shallower levels (2–3 km?) follows via uplift along faults, facilitating intense brecciation and ore formation.

Fluid-present anatexis of Neoarchean tonalite and amphibolite in the Western Shandong Province

Metatonalite and amphibolite from the Taishan region of the Western Shandong Province in the North China Craton record c. 2.60 Ga fluid-present partial melting via the breakdown of biotite, plagioclase and quartz to produce peritectic hornblende and anatectic melt. Eight paired leucosome–melanosome samples from metatonalite and three paired samples from amphibolite were investigated to evaluate the composition of the melt. Hornblende, biotite and plagioclase in the leucosomes and hornblende and plagioclase in melanosomes from both rock types have similar compositions. Two leucosome samples from the metatonalite were influenced by the removal of heavy rare earth element-rich hornblende and the accumulation of plagioclase. The other leucosomes are interpreted to represent near initial melt compositions with a minor component of peritectic hornblende and are enriched in Si, Na and Sr and depleted in K, Ca, Ba and Rb relative to melanosomes. The enrichment of Na in the melt is inconsistent with experimental results of fluid-present melting of tonalite, but is broadly consistent with the experimental results of fluid-fluxed melting of amphibolite. The absence of K-feldspar in both rock types is a critical control on the composition of anatectic melt and initial melt compositions were probably similar for both rock types. Leucosomes inherited rare earth element patterns from their sources, which suggests that some trace element diagrams used to infer tectonic settings and depths of melting are not appropriate for reworked components of Archean grey gneisses. Whole-rockδ18O suggest that the fluids responsible for inducing local melting were derived from the intrusion and crystallization of ~2.60 Ga tonalites and trondhjemites in the Taishan region. One amphibolite sample has a relatively low δ18O suggestive of interaction with meteoric water or seawater possibly related to crustal extension and asthenosphere upwelling at ~2.60 Ga. Fluid-present partial melting reworked 2.75–2.60 Ga tonalites and amphibolites, generating ~2.60 Ga sodium-rich components of grey gneisses in the Western Shandong Province.

Estimating P-T metamorphic conditions on the roof of a hidden granitic pluton: an example from the Mt. Calamita promontory (Elba Island, Italy)

The metamorphic basement (i.e. Palaeozoic micaschist, Porto Azzurro Unit) cropping out in the Mt. Calamita promontory, in the southeastern Elba Island, represents the roof of the mostly unexposed late Miocene Porto Azzurro granite. A thermobarometrical study was performed on micaschists and interlayered amphibolites to constrain P-T conditions during contact metamorphism. Adopted methods were based on P-T grids, conventional thermobarometry and analysis of fluid inclusions. It was found that the basement was originally located at a depth of 6.5 ± 1.3 km and that the peak metamorphic conditions were close to the muscovite dehydration reaction in micaschist.Therefore, small differences of temperatures conditioned the presence or absence of K-feldspar in the eastern and western side of the Mt. Calamita promontory, respectively. A maximum peak temperature of c. 625 °C was estimated by amphibole-plagioclase thermometry in amphibolite. Fluid inclusions in micaschists quartz lenses document significant episodes of fluid circulation during the cooling history of the metamorphic aureole. Finally, a conductive thermal model was built up to reproduce the thermal evolution during the contact metamorphic event and following cooling. Simulated temperatures resulted significantly lower than the observed peak values. This discrepancy is explained as an effect of the localized heat advection in zones affected by infiltration of magmatic fluids and by the intrusion of numberless felsic dykes.

Low potassium hydrothermal alteration in low sulfidation epithermal systems as detected by IRS and XRD: An example from the Co–O mine, Eastern Mindanao, Philippines

Detailed hydrothermal alteration investigations, including petrography, infrared reflectance spectroscopy (IRS) and XRD of the low sulfidation epithermal Co–O mine, located in Eastern Mindanao (Philippines) revealed that both distal and intermediate hydrothermal alteration zones contain dominantly illite and chlorite, whereas the proximal alteration zone comprises mainly illite, chalcopyrite and pyrite. The gold-bearing veins and the proximal hydrothermal alteration zone display a distinct absence of K-rich hydrothermal alteration minerals such as K-feldspar (adularia).Gold mineralization in the Co–O mine is controlled by an extensive quartz-breccia vein system, which is characterized by three distinct stages of vein (incl. breccias) formation. Gold is mainly observed in stages 2 and 3 veins. Stage 1 veins appear as fragments in stage 2 veins and display boiling textures such as quartz pseudomophs after bladed calcite. These veins further display colloform to crustiform banding and contain pyrite, chalcopyrite and minor gold located in the colloform bands and between bladed quartz pseudomorphs. Stage 2 veins comprise mostly banded to massive quartz and contains sulfides parallel to bands or disseminated. These veins are fine-grained with mosaic/jigsaw quartz and contain calcite blebs and/or fragments of stage 1 veins. Gold is in textural equilibrium with chalcopyrite, sphalerite, and locally pyrite. Stage 3 veins consist of quartz and carbonate (locally Mn-rich), and display irregular banded and comb textures. In auriferous veins of this stage gold is in textural equilibrium with chalcopyrite and pyrite (with local abundance of sphalerite). Other sulfide minerals observed with gold in stages 2 and 3 are galena, acanthite and locally jalpaite.The XRD and IRS provide inconsistent results regarding the abundance of K-rich clays (e.g., illite) associated with auriferous veins. Illite, with possibly interlayered swelling clays, such as Al-smectite, was identified in auriferous vein stages 2 and 3 using IRS, but could not be confirmed by XRD. Comparative analysis of the results of these techniques with respect to the ordering of micaceous minerals, suggest less ordered white mica proximal to the veins.Vein textures such as banded quartz, the absence of K-feldspar and the abundance of illite (interlayered Al-smectite) suggest relatively low temperatures of formation of the hydrothermal alteration system and point to a potential boiling horizon located deeper or marginal to the currently exploited levels of the Co–O mine. The absence of K-feldspar may also be related to relatively low temperatures of the hydrothermal fluid, the medium potassium-rich magma series of the host rocks, and/or a relatively low oxidation state of the hydrothermal fluid.

Genesis of aluminous and intermediate granulites: A case study in the eastern Sierras Pampeanas, Argentina

We report petrologic and geochemical features of anhydrous granulites from the eastern Sierras Pampeanas, Argentina. Aluminous Grt–Crd-bearing granulite is the most abundant rock type in the study region, and hosts intermediate Opx–Grt bearing granulites as interlayered lenses. Both granulite types crystallized under similar P– T conditions of T = 850–900 °C and P c. 7.5 kbar. The formation of Grt–Crd granulites by subsolidus dehydration conflicts with the absence of K-feldspar in them. Moreover, the aluminous granulites have an REE pattern that precludes postulating they derived by close-system metamorphism of an extensively weathered protoliths. Comparing mineral and rock chemistry with experimental results in simple systems (ACFM, AKF, and AFM) permits the mineralogical changes and the bulk composition evolution experienced by granulites during melting to be identified. This work demonstrates that orthopyroxene is a primary phase in Opx–Grt granulites, however never crystallized within aluminous granulites. The aluminous granulites have both mineral and chemical compositions matching those expected for a residuum derived from the partial melting of a greywacke-like sedimentary precursor. Compared with the composition of the average continental crust, aluminous granulites are depleted in most lithophile trace elements and LREE, have somewhat low K / Rb ratios, and high contents of elements having high partition coefficients for garnet and/or ilmenite (HREE—Y, Cr, Ni). However, aluminous granulites have initial Sr isotope ratios (< 0.708) significantly lower than those measured in the rest of the metasedimentary rocks of the region (> 0.713). We discuss the possibility that an intense but short-lived melting event led to isotopic disequilibrium during melting of biotite-bearing protoliths, leaving a granulitic residuum with lower initial Sr isotope ratios. In contrast, Opx–Grt granulites have trace element features (e.g., high Sr contents and REE patterns with marked positive Eu anomalies) that do not support the idea that they are simply residues left after biotite-dehydration melting and melt extraction. Rather, these rocks display several petrological, major and trace element and isotopic features that suggest they formed through the interaction of mafic magmas and aluminous granulites.

Evaluation of the volatilization and infiltration effects on the stable isotopic and mineralogical variations in the carbonate rocks adjacent to the Cretaceous Muamsa Granite, South Korea

The Muamsa Granite in the Hwanggangri Mineralized Zone, South Korea, intruded the Cambro-Ordovician Samtaesan Formation, which dominantly consists of calcitic marble and dolomitic marble with minor intercalations of pelitic rock and amphibolite. Intrusion of the granite caused an isotopic depletion of 13C and 18O in the carbonate rocks. Calcitic carbonates show systematic depletion from unrecrystallized limestone through calcitic marble to skarn. It was initiated with volatilization accompanying calcsilicate-forming reactions, and then, with increased permeability, H 2O-rich siliceous fluids of igneous origin actively infiltrated the system promoting fluid-rock interaction. For the skarn fluid infiltration yielded fluid–rock ratios as high as 4.2 in open system, and caused increased 18O depletion rates compared to the volatilization stage. More than 70% of the isotopic depletion in the calcitic marble is attributed to the infiltration process. With such strong infiltration, mineral assemblages such as phlogopite in the absence of K-feldspar, phlogopite+diopside, and wollastonite are commonly observed in the marble. The dolomitic marble was comparatively less depleted in heavy isotopes compared to the calcitic marble and formed under a wide range of X CO2 and low fluid–rock ratios (<1.0). Active decarbonation in dolomitic marble was restricted by silica-poor dolomite protolith, which caused limited fluid infiltration and resulted in high X CO2 mineral assemblages represented by dolomite+calcite+diopside+forsterite and more 13C-depleted nature compared to dolomite+calcite+tremolite+forsterite assemblage.

The role of provenance in illitization of deeply buried reservoir sandstones from Haltenbanken and north Viking Graben, offshore Norway

An extensive mineralogical and geochemical database has been compiled to evaluate the relationship between diagenesis and provenance of the Lower/Middle Jurassic sandstones from the Haltenbanken (Mid-Norway) and north Viking Graben. The data from the Haltenbanken area, from burial depths between 2.1 and 4.5 km relative to seafloor (RSF), show a reduction in K-feldspar and kaolin, and a sharp increase in illite content in sandstones which are presently located at depths greater than 3.7 km RSF (120–140°C). However, there is little change in the bulk K/Al molar ratio with depth, which suggests that potassium has been conserved during the precipitation of illite and dissolution of K-feldspar. The K/Al molar ratios in most of the deeply buried Haltenbanken sandstones are greater than 1/4, which corresponds to the molar ratio in illite. This indicates that excess K-feldspar is present, which is confirmed by petrographic observations. By contrast, deeply buried sandstones from the north Viking Graben are mostly characterised by the absence of K-feldspar, a K/Al molar ratio lower than illite and varying amounts of kaolin. This suggests that the amount of K-feldspar present at the onset of illitization (of kaolin) was lower in the north Viking Graben sandstones than in the Haltenbanken area. The degree of illitization of kaolin is higher in the Haltenbanken reservoirs as compared to the north Viking Graben, and this may be directly related to the higher K-feldspar content in the Haltenbanken sandstones. The survival of kaolin below 3.7 km RSF depth, in most of the plagioclase rich and K-feldspar poor sandstones from the north Viking Graben, therefore indicates that illitization of kaolin requires a local presence of K-feldspar in the sandstones.

Preservation of Permeability in Deeply Buried Sandstones of the Viking Graben: Inhibition of Illitization Due to Absence of K-Feldspar: ABSTRACT

Preservation of Permeability in Deeply Buried Sandstones of the Viking Graben: Inhibition of Illitization Due to Absence of K-Feldspar: ABSTRACT

A Rb-Sr study of the lower silurian stage circle Shale, Canberra, Australia

The radiometric age of the Upper Silurian (Lower Ludlovian) has been measured as equal to or older than 438 ± 4 m.y., using whole-rock and biotite samples from dacitic ash flows. Relative to this, the deposition of the Lower Silurian State Circle Shale is placed at about 445 ± 7 m.y. (Upper Llandoverian). Samples of the Shale have real differences in initial Sr 87 Sr 86 for this age and two groups are recognized: one having uniform Sr 87 Sr 86 of 0.720, and the other having Sr 87 Sr 86 about 0.73, with some scatter. Regression-lines through each group on the isochron-diagram have indistinguishable slopes, the pooled value being equivalent to an age of440 ± 9 m.y. A single regression through all the data gives 455 ± 15 m.y. Neither of these results can be separated from the estimated 445 ± 7 m.y., so that no inheritence in age from the provenance is detected. No difference can be detected between oxidized (weathered, probably in the Tertiary) and unaltered samples. X-ray diffraction analysis shows that the Shale is composed of quartz, chlorite, L M and (principally) 2 M muscovite, small amounts of feldspars, and in the oxidized zone, goethite and kaolinite. The samples having low depositional Sr 87 Sr 86 are distinguished by an absence of K-feldspar, presumably reflecting some difference in provenance. The micas may be all detrital or much of the 2 M may have grown from l M during low-grade metamorphism and folding after deposition. At present we favour the latter hypothesis, and interpret the 440 ± 9 m.y. event registered by the Shale as relating to a period of folding in the Lower Ludlovian.