Reversely zoned plagioclase in lower crustal meta-anorthosites: An indicator of multistage fracturing and metamorphism in the lower crust

This paper describes the formation mechanism of reversely zoned plagioclase, which has been observed frequently in lower crustal shear zones and is indicative of multistage fracturing and meta-morphism in the lower crust, by studying the microstructural and chemical characteristics of plagioclase in sparsely fractured anorthosites and anorthositic mylonites from the Eidsfjord shear zone, Langøya, northern Norway. Based on the field relationship between sparsely fractured anorthosite and anorthositic mylonite, the fracturing of anorthosite occurred before the formation of mylonite. In sparsely fractured anorthosites, transgranular fractures are observed; hydration-reaction products, including Na-rich plagioclase, occur within cracks and fractures, suggesting that hydration reactions occurred during or after fracturing. The hydration reactions in sparsely fractured anorthosites are estimated to have occurred at higher-pressure (P) amphibolite-facies conditions (~0.9–1.0 GPa and ~550–700 °C). In anorthositic mylonites, which are considered to have initiated by fracturing and subsequent hydration metamorphism at lower-P amphibolite-facies conditions (~0.7 GPa and ~600 °C), recrystallized plagioclase grains often show compositional zoning with an Na-rich core and a Ca-rich rim. Because the compositions of metamorphic plagioclase grains in the sparsely fractured anorthosites and those of the Na-rich cores of the reversely zoned plagioclase in anorthositic mylonites are similar to each other, the Na-rich cores of the matrix plagioclase in the anorthositic mylonites have recrystallized under higher-P amphibolite-facies conditions and then been overgrown or replaced by the Ca-rich rims under lower-P conditions. Consequently, the reversely zoned plagioclase observed frequently in lower crustal shear zones is an indicator of multistage brittle fracturing and subsequent hydration metamorphism during exhumation, providing information relevant to understanding the deep rupture process caused by repeated seismicity alternating with aseismic creep below the seismogenic zone.

PETROGRAPHY AND MINERAL CHEMISTRY OF OLIGOCENE SHOSHONITIC DACITES FROM THE CENTRAL BOSNIA

Postorogenic volcanic rocks of different Tertiary ages are very common in the Sava-Vardar Zone of the Dinarides and in the southeastern part of adjoing Pannonian Basin. South of the Sava-Vardar Zone, in central Bosnia, Tertiary volcanic rocks occur within ophiolite sequences and genetically related sedimentary formations of the Dinaride Ophiolite Zone. Central Bosnia volcanic rocks are mostly dacites, and highly subordinately andesites as the members of the high-K calc-alkaline series.It appears from the mineralogical and petrographic characteristics obtained some insight into the processes that occurred during the genesis of the rocks. The presence of primary igneous minerals: clinopyroxene, orthopyroxene, hornblende and biotite from ferromagnesian minerals, and plagioclase, sanidine and quartz, indicates that the fractional crystallization played a significant role in the genesis of the rocks. Reaction edge on many rounded quartz phenocrysts indicates the possibility of magma mixing with the formation of Tertiary volcanic rocks of the central Bosnia. On magma mixing different temperature and chemical composition also indicates the existence of zoned plagioclase and amphibole phenocrysts.Complex compositional and zoning patterns of biotite and plagioclase phenocrysts and disequilibrium microstructures of plagioclase and quartz phenocrysts suggest interaction of fractionating, mantle derived melts with continental crust during a shalow level pre-eruptive stage and mixing with small amount of devolatilized phlogopite-phyric mafic magma before eruption.

PETROGRAPHY AND MICROTEXTURAL CHARACTERISTICS OF GRANODIORITE FROM WASIMI, SOUTHWESTERN NIGERIA

Rock samples of granodiorite from Wasimi were studied in detail to determine the minerals, mineral associations and the different micro-texture present in order to unravel the processes that were involved during the cooling of the magma. The following minerals were observed; plagioclase feldspars, K-feldspars, biotite, pyroxene, amphiboles, with zircon and sphene as accessory minerals. The pyroxene are engulfed in amphiboles and have reaction rims. Pyroxene occur as inclusions in plagioclase feldspars. Amphiboles are poikilitic with inclusions of quartz. Symplectic texture exits between the following minerals; plagioclase and K-feldspar, biotite and quartz, plagioclase and amphibole. The plagioclase feldspars have fractured and bent twin lamellae. Crystals of plagioclase feldspars are zoned and also exhibit both Carlsbad and albite form of twinning. It can be said that there was an initial slow cooling of the magma which was responsible for the large crystals of feldspars which was later followed by rapid cooling as the magma rises close to the surface which gave rise to zoned plagioclase feldspars and indication of high level intrusive.

Application of the Cahn-Hilliard-type approach to the development of oscillatory zoning in minerals

<p>Preservation of mechanically-controlled microstructures can help us to unravel the long-term stress state in geological materials. To better understand the stress state in such a microstructure, we need to quantify the processes in a coupled, chemo-mechanical, point of view. One of such a mechanically-controlled microstructure is oscillatory zoning in high-temperature metamorphic rocks. The presented example is a sharp zoned plagioclase of 150 x 200 µm size with thin compositional lamellae of 1-10 µm alternating from the core towards the rim. This microstructure is interpreted to be mechanically-controlled, since conventional diffusion failed to preserve the observed microstructure within timescales that would be reasonable from a regional geology point of view. In contrast, considering that chemical diffusion is coupled to mechanical deformation the observed zoning can be maintained over the geologically-relevant timescales.</p><p>Despite of the recent valuable progress in our understanding of these microstructures, the mechanisms controlling its evolution from slowly cooled rocks are still not complete. Here, we numerically investigate the coupled, chemo-mechanical, effect that generates oscillatory zones mechanically maintained over geologically relevant timescales. We test the possibility of modelling oscillatory zoning in minerals that is similar to the exsolution process. We apply a classical Cahn-Hilliard-type equation where we add more complexity considering the impact of deformation during the process.</p>

Geochemical and Geodynamic Constrain of Tholeiitic Volcanism and Related Intrusions in the Kampti Gold District, Southwest Burkina Faso: Implication for Mineral Exploration

The Kampti serie is a volcano-plutonic complex in the south-eastern corner of the Houndé belt, closed to the border with Ivory Coast. The stratigraphy comprises initially: a thick sequence of tholeiitic and pillowed basalt followed by pyroclastic projections derivated from bimodal volcanism; and flowing mostly to the south. The volcanic sequence is cross-cut by mafic cumulate body, stocks of gabbro, diorite, subvolcanic dykes and lately intrude by granitoid. It is bordered to the west by the pelitic schist of the Bambela basin with minor intercalation of Tarkwa type sediment. volcanoclastic facies is dominantly plagioclase-phyric (albite +/- oligoclase), zoned plagioclase has a core of anorthoclase. Secondary mineral infilled (quartz, kutnahorite, ripidolite, clinozoisite) of spherulites and oolite highlight a general low grade metamorphism of greenschist facies affecting the complex. Based on trace element chemistry, the tholeiitic rocks present flat REE pattern contrasting with the felsic rocks more enriched in LREE and depleted in HREE. The style of the magmatism in the Kampti serie is compatible with an island arc model, describe elsewhere in the birimian. Gold mineralisation and base metal occurrences associated to the nature of rocks and tectonics highlight a polymetallic district.

Two-mica andalusite-bearing granite with no primary muscovite: constraints on the origin of post-magmatic muscovite in two-mica granites

The two-mica granite from Gęsiniec (Strzelin Granitic Massif, SW Poland) consists of quartz, K-feldspar, normally zoned plagioclase (30 ± 7 % An), subordinate biotite and muscovite and magmatic andalusite. Andalusite crystallised before the outer parts of plagioclase grains were formed. Biotite has constant Fe/(Fe + Mg) ratio of approximately 0.81. Five textural types of muscovite occur in the granite: (1) muscovite replacing andalusite, (2) embayed interstitial muscovite, (3) muscovite forming aggregates with biotite, (4) muscovite accompanying biotite and chlorite in microfissures and (5) fine muscovite forming fringes at the contact between larger muscovite plates and K-feldspar. They are commonly associated with albite. Crystallisation of muscovite started significantly below the granite solidus, mostly by the replacement of andalusite. Formation of muscovite continued during cooling of host rock. The growth of individual plates was initiated at different undercoolings and the plates whose crystallisation was frozen at different stages of growth occur. Those that were formed earlier are richer in titanium and iron relative to the later ones. As the rock contains no Ti and Fe saturating phases, the content of Ti and Mg in muscovite depends on their local availability. The homogeneous Fe/(Fe + Mg) ratio of biotite indicates that it was re-equilibrated at the post-magmatic stage.

MÁRMORES DA REGIÃO DE ITAOCA (ES) E ESCARNITOS NO CONTATO COM DIQUES MÁFICOS E FÉLSICOS: MINERALOGIA E PETROGÊNESE

Skarns at the contact of marbles and mafic as well as felsic dikes located in the Itaoca region (Espírito SantoState, eastern Brazil) were investigated in terms of mineralogy and genesis. Marbles and the older mafic dykeswere affected by high grade metamorphism of Brasiliano age, as comproved by the presence of spinel. Theundeformed younger granitic dykes are post-tectonic. Skarns related to the granite dykes were formed as outcomesof silica addition in marble layers during the intrusion, generating zones composed of i)olivine + carbonate +pyroxene + amphibole ± phlogopite; ii)pyroxene + amphibole ± phlogopite ± scapolite, and iii)pyroxene ±phlogopite. The last mineral association occurs close to the granite and shows the maximum silica enrichment.Evidence of Ca-metassomatism in the granite is shown by the strongly zoned plagioclase crystals, which showcentral portions with An17 and border zone with An43. Skarns generated at the contact with amphibolite dykesare wider than the former, probably because the original contact metamorphic reaction zones generated duringthe intusion were reinforced by granulite facies metamorphism. The reaction aureoles display the followingmineral associations: i) carbonate ± olivine, ii) carbonate + diopside ± scapolite ± epidote, iii) plagioclase +diopside ± scapolite ± epidote, and iv) amphibole + plagioclase ± phlogopite ± hercinite ± grossularite. The firstassociation belongs to the marble at the contact, and the last to the amphibolite. There are mineralogical andtextural evidences for metamorphic and metassomatic reactions due to the migration of Ca, Mg and Si, leading,for example, to the scapolitization of the plagioclase.

Regional trends of chemical variation and thermal erosion in the Upper Critical Zone, Western Bushveld Complex

A comparison of R.P.M. Union and Amandelbult Sections reveals close geochemical and stratigraphic correlations, but the sequence at the latter is more complex. Mafic members at Union Section are consistently more magnesian than their equivalents at Amandelbult Section and (where available data allow comparisons) than at Rustenburg Section. This, taken together with regional patterns of progressive attenuation and elimination of leucocratic rocks beneath harzburgitic layers, identifies Union Section as proximally located with reference to an irruptive centre from which primitive liquids were injected along the floor/supernatant liquid interface. Successive injections led to thermal erosion of the floor, causing ‘dimpling’ and ‘potholing’ on a local scale, and elimination of noritic-anorthositic layers on a regional scale. Such erosion of the floors beneath new inputs of magmatic liquid is likely to be initiated by only partial remelting, mainly of lower-temperature phases (sodic rims to zoned plagioclase grains, and Fe-enriched pyroxenes) within intercumulus space. It is argued that resultant contamination of the primitive liquids may have led to direct superposition of anorthosites (accompanied by rare troctolite) upon harzburgite, without intervening pyroxenite members, as in the Pseudoreef Multicyclic Unit.