Mg-K-Fe fluid producing mineral reactions, metasomatism and microfabric development during formation of nodular sillimanite-gneiss in the middle crust

<p>The Proterozoic gneisses of the Bamble lithotectonic domain (south Norway) underwent intense scapolitisation caused by K- and Mg-rich fluids and extensive albitisation with formation of numerous ore deposits.</p><p>By detailed studies of mineral reaction fabrics we document release of the chemical active Mg, K and Fe-components forming the metasomatic fluid: Breakdown of biotite to muscovite releases K, Mg, Fe, Si and H<sub>2</sub>O. As reaction products tiny Fe-oxide needles are present in the transforming rock. H<sub>2</sub>O is reacting with K-feldspar to produce additional amounts of white mica and quartz. During a subsequent reaction muscovite is replaced to sillimanite again releasing quartz and a K-rich fluid. The reactions form the peculiar sillimanite-nodular quartzite, but also well-foliated sillimanite-mica gneiss.</p><p>Optical and EBSD microfabric studies reveal a shape preferred orientation for quartz, but despite of a pronounced foliation, quartz does not show a crystallographic preferred orientation. A crystallographic preferred orientation is present for mica and sillimanite. Coarse micas show sutured boundaries to quartz, implying low nucleation rates, no crystallographic or surface-energy control during growth and no obvious crystallographic relationship to quartz.</p><p>Our study illustrates the transformation of a quartzofeldspatic lithology into sillimanite-bearing quartzite. The mineral replacement and deformation show ongoing metamorphic reactions during deformation. The microfabric data indicates reaction at non-isostatic stress condition. The deduced mineral replacement reactions document a source of K-, Mg- and Fe-rich metasomatic fluids necessary to cause the pervasive scapolitisation and Fe-deposition in the area. The mineral reactions and deformation produce rocks with a new mineralogy and structure; an increased understanding of these processes is important for the modelling of crustal building and geological history.</p>

Conditions for the formation of Fe‒Mg metasomatic carbonates in the Lower Riphean terrigenous-carbonate sediments of the Southern Urals

Fe–Mg carbonate metasomatites in the limestones of the Suran suite of the Lower Riphean in the Avzyansky ore district of the Bashkir meganticlinorium are represented by large deposits of Fe-magnesite (Ismakaevo deposit) and breinerite stocks (Bogryashka deposit). The metasomatic zonality is represented by a series of limestone — dolomite — Fe-magnesite (breinerite). Ferrous magnesite contains up to 8 mol. % FeСO3. In breinerite iron saturation varies from 10 to 45 mol. % FeСO3. The metasomatic fluid was a brine of Ca, Na, Mg chlorides with an impurity of Fe, and was connected with the remobilization processes of evaporite brines buried in sediments of the Lower Riphean. The salinity and homogenization temperature of fluid inclusions in magnesites are in the range of 20–26% eq. NaCl and 200–240°C, and in breinerites — 10–15% eq. NaCl and 140–190°C, respectively. The interaction of fluid with terrigenous rocks in the tectonically active zone of the Mashak riftogenic graben led to the enrichment of various ligands and more active hydrothermal redeposition in the metasomatic products of medium and heavy lanthanides. During the migration of fluid through the limestone unite in the cooling process, Fe-magnesite of the Ismakaevo deposit was formed in the frontal zone, and the breinerite of Bogryashka deposit — in the rear zone adjacent to the shale source of brine.

A Study on Color Stones of Clay Minerals — Changhua Stone

Changhua Stone is a rock, composed of clay minerals such as dickites and is also a special secondary quartzite formed by gas liquid metasomatic metamorphism. The article emphasizes its study on rock characteristics, mineral composition, and the causes of formation of Changhua Stone, excluding Changhua Chicken-Blood Stone. By observing petrographical characters of Changhua Stone, analyzing its physical and chemical characters, and testing its geochemical compositions with SEM and X-ray diffraction analysis, the results indicate that the mineral compositions of Changuan Stone are relate to the composition of protolith, the compositions and characters of metasomatic metasomatic fluid, and the conditions of metamorphism, especially the stress and the temperature.

Post-magmatic alteration in eudialyte from the North Qôroq centre, South Greenland

The North Qôroq centre comprises a series of nested nepheline syenite intrusions and forms part of the mid-late Proterozoic Gardar province of South Greenland. Within the centre fractionation has produced varied rock types ranging from augite-syenite to lujavrite, a eudialyte microsyenite. Samples of eudialyte from the lujavrites of unit SN1B of the centre show evidence for two-stage alteration. This alteration ranges from slight modification along crystal margins to complete breakdown and replacement by new pseudomorphing phases. Modification to crystal margins is accompanied by increasing Nb and Zr contents and is related to metasomatism produced by the intrusion of younger syenite units of the North Qôroq centre. More extensive alteration is as a result of metasomatism followed by lower-temperature supergene alteration. Simplified reactions for this breakdown include eudialyte + metasomatic fluid = allanite + nepheline; eudialyte + metasomatic fluid = titanite + aegirine + møsandrite + wöhlerite; eudialyte + fluid = zirfesite + fluid. Mass balance calculations for altered compared with unaltered samples of lujavrite show that alteration took place at approximately constant volume with an overall increase in Fe (+2.41 g/100g), Si and K (+0.65 and +0.61 g/100g), whilst Na (−2.67 g/100g) and all trace elements, particularly La, Y, Nb and Zr (−5.6 to −166 g/10000g) are lost from the system.

Late Precambrian to Late Devonian mafic magmatism in the Antigonish Highlands of Nova Scotia: multistage melting of a hydrated mantle

Five suites of alkalic basalt ranging in age from Late Precambrian to Late Devonian are found in the Antigonish Highlands of Nova Scotia. In contrast, on neighbouring Cape Breton Island, alkalic basalts are rare even in suites that are contemporaneous with those in the Antigonish Highlands. Late Precambrian alkalic basalts in the Antigonish Highlands are genetically associated with calc-alkalic rocks and are probably subduction related, whereas the younger suites are continental, rift related, and within plate. Major and compatible trace-element abundances can be explained by crystal fractionation of olivine ± clinopyroxene ± orthopyroxene ± spinel ± garnet. However, incompatible trace-element concentrations are strongly influenced by mantle metasomatism that occurred prior to, or synchronously with, the oldest alkalic rocks. The metasomatic event enriched the mantle in Fe, Ti, P, Zr, and light rare-earth elements. The trace-element composition of the younger suites is similar to that of the oldest alkalic rocks and may have been strongly influenced by the Late Precambrian metasomatic event. The anomalously low Nb/Y ratio (generally less than 1 in all suites) and application of phase-equilibria studies indicate that the metasomatic fluid was probably rich in H2O. This fluid may have been derived from dehydration of the subducting slab in Late Precambrian time, resulting in metasomatism of the overlying mantle wedge in the Late Precambrian. It is proposed that the younger suites obtained their fluids by dehydration of the previously metasomatized mantle associated with the generation of local pull-apart basins. Thus, the metasomatic fluid was exotic with respect to the oldest basalts but indigenous with respect to the younger basalts. In the younger basalts, the indigenous fluid was probably focussed at the site of melting by structural events (i.e., rifting). In situations in which the chemistry of mafic magmas is predetermined by earlier metasomatic events, caution is advised in using trace-element criteria to evaluate the tectonic setting.

Skarn formation between metachalk and agglomerate in the Central Ring Complex, Isle of Arran, Scotland

A skarn mineral assemblage occurs at the junction between vent pyroclastics and a xenolithic Cretaceous chalk block which subsided into the collapsed caldera of the Central Ring Complex, Isle of Arran, Scotland. Adjacent to the metachalk marble an andradite garnet exoskarn zone has developed at the expense of the carbonate. An andradite grossular/diopsidic clinopyroxene endoskarn zone has formed in the surrounding agglomerate, and a magnetite exoskarn is present in places between the andradite and garnet/pyroxene zones. The andraditic exoskarn garnets have fluor-hydrogarnet components, indicating that fluorine was present in the metasomatic fluid. From petrographic evidence, three distinct episodes of exoskarn garnet crystallization can be recognized, in which the fluor-hydrogarnet component steadily increased as a function of time, which probably reflects falling temperature. The REE compositions of the exoskarn minerals are regarded as having been largely inherited from the carbonate, and the exoskarn garnets increasingly fractionated HREE with time. The endoskarn and agglomerate have also been epidotized. The REE signatures of epidotes appear to be inherited partially from precursor clinopyroxenes or feldspars, which have been replaced by epidote. Late-stage vein minerals include prehnite, laumontite and K-rich laumontite, and their REE compositions appear to have been derived from the marble, probably via REE fluoro-complexes in the fluid.