Aberra Mogessie

AMPH-IMA04: a revised Hypercard program to determine the name of an amphibole from chemical analyses according to the 2004 International Mineralogical Association scheme

Abstract In 2004, the International Mineralogical Association (IMA) amended the IMA 97 amphibole classification and nomenclature scheme by adding a fifth group to include the recently discovered B(LiNa) amphiboles ferriwhittakeriite and ferri-ottoliniite, which cannot be fitted into the four major amphibole groups. New root-names such as sodic-pedrizite in the Mg-Fe-Mn-Li group and obertiite and dellaventuraite in the sodic group along with two new prefixes, parvo and magno have also been added. As result it has become necessaryto modifythe AMPH-IMA97 amphibole-naming program. The new program (AMPH-IMA04) allows single input or automatic input of as manyamphibole analyses as are available following a set input format. Anyof three different calculation schemes for dealing with an amphibole analysis can be chosen: (1) complete chemical analyses can be calculated to 24(O,OH,F,Cl); (2) analyses with determined FeO and Fe2O3, MnO and Mn2O3 but without H2O can be calculated to 23(O); and (3) electron microprobe analyses with only total Fe determined and without H2O can be calculated to 23(O) with IMA97-recommended normalization for Fe3+ and Fe2+ values. In addition a stoichiometric calculation of Mn2+ and Mn3+ is considered and implemented for the Mn-bearing sodic amphiboles in order to take care of electron microprobe analyses of such amphiboles where the total Mn is given as Mn2+.

Spinel group minerals in metamorphosed ultramafic rocks from Río de Las Tunas belt, Central Andes, Argentina

In the Rio de Las Tunas belt, Central Andes of Argentina, spinel group minerals occur in metaperidotites and in reaction zones developed at the boundary between metaperidotite bodies and their country-rocks. They comprise two types: i) Reddish-brown crystals with compositional zonation characterized by a ferritchromite core surrounded by an inner rim of Cr-magnetite and an outer rim of almost pure magnetite. ii) Green crystals chemically homogeneous with spinel (s.s.) and/or pleonaste compositions. The mineral paragenesis Fo+Srp+Cln+Tr+Fe-Chr and Fo+Cln+Tr+Tlc±Ath+Fe-Chr observed in the samples indicate lower and middle grade amphibolite facies metamorphic conditions. Nonetheless, the paragenesis (green)Spl+En+Fo±Di indicates that granulite facies conditions were also reached at a few localities. Cr-magnetite and magnetite rims in zoned reddish-brown crystals and magnetite rims around green-spinel/pleonaste grains are attributed to a later serpentinization process during retrograde metamorphism. The chemical composition of spinel group minerals in the ultramafic reaction zones is determined by the mineral zone where they crystallize. Green pleonaste occurs in the chlorite zone, ferritchromite predominates in the amphibole zone, whereas Crmagnetite and magnetite are more common in the carbonate zone. The mineral paragenesis of the Rio de Las Tunas metaperidotites together with the chemical characteristics of the spinel group minerals support a clockwise P-T path evolution for the ultramafic protoliths during the Paleozoic regional metamorphic cycle of this area

AMPH-IMA97: a hypercard program to determine the name of an amphibole from electron microprobe and wet chemical analyses

In 1997, the International Mineralogical Association (IMA) revised the amphibole naming scheme making it necessary to amend the previous EMP-AMPH Macintosh program to the new rules. The AMPH-IMA97 program allows single input or automatic input of as many amphibole analyses as are available following a set input format. There are three options for calculation scheme of amphibole analysis: (1) complete chemical analyses can be calculated to 24(O, OH, F, Cl); (2) analyses with determined FeO and Fe2O3 but without H2O can be calculated to 23(O) and (3) electron microprobe analyses with only total Fe determined and without H2O can be calculated to 23(O) with IMA97-recommended normalization for Fe 3+ and Fe 2+ values. To test the program, over 500 analyses were calculated using the three options. The first comprehensive test of the third option successfully estimated Fe 2+ (R 2 =0.88), but became less successful for Fe 3+ (R 2 =0.42) estimation. Problems in the normalization of Mn 3+ - and Mn 2+ -bearing amphiboles and in the IMA97 sodic amphibole classification procedure emerged for Mn-rich amphiboles. This necessitated the proposal and implementation with the program of a simplified sodic amphibole classification scheme. A new coloured amphibole classification chart accompanies the program. # 2001 Elsevier Science Ltd. All rights reserved.

HYPER-FORM: A HyperCard program for Macintosh microcomputers to calculate mineral formulae from electron microprobe and wet chemical analysis

Abstract The program HYPER-FORM permits the input and calculation of a mineral formula from wet chemical and electron microprobe analysis. It includes autosaving of data, search routines, and other database functions. The calculated formula can be exported to wordprocessing programs such as MacWrite® or Microsoft Word® and can be edited as required. It also is possible to export the calculated data to graphic programs such as Cricket Graph® or Kaleidagraph® for a graphic representation of the data. HYPER-FORM is an interactive program written in HyperTalk™ (HyperCard® environment) and is designed to correspond to the Macintosh® interface.

Partial melting and melt segregation in footwall units within the contact aureole of the Sudbury Igneous Complex (North and East Ranges, Sudbury structure), with implications for their relationship to footwall Cu-Ni-PGE mineralization

We performed detailed field and drill core mapping of partial melting features and felsic rocks (footwall granophyres, FWGRs) representing segregated and crystallized partial melts within the contact aureole of the Sudbury Igneous Complex (SIC) in the 1.85 Ga Sudbury impact structure. Our results, derived from mapping within the North (Windy Lake, Foy, Wisner areas) and East Ranges (Skynner, Frost areas) of the structure, reveal that partial melting was widespread in both felsic and mafic footwall units up to distances of 500 m from the basal contact of the SIC. Texturally and mineralogically, significant differences exist between rocks formed by partial melting within and between localities. In general, however, melt bodies are dominated by different quartz-feldspar intergrowths (e.g. granophyric, graphic) and miarolitic cavities up to 5 cm in diameter. Major and trace element compositions of Wisner and Frost FWGRs imply that they crystallized from melts dominantly derived from partial melting of felsic Levack Gneiss and Cartier granitoid rocks, as well as from gabbroic rocks only at Frost. These results accord with our observations on in situ partial melting features and crystallized melt of microscopic scale in both felsic and mafic rocks. We conclude that partial melting occurred at a pressure of 1.5 ± 0.5 kbar and at temperatures up to 750°C in the Wisner area and up to 900°C in the Frost and Windy Lake areas. Segregations of partial melt into veins and dikes are present in all localities, and were promoted by deformation of the Sudbury structure in the Penokean orogeny as indicated by dominant strike directions. Whereas veins and dikes reflect brittle conditions during melt migration, sheared melt pods in the Sudbury breccia matrix indicate ductile conditions during their crystallization. Our results suggest a close genetic association of partial melting, melt segregation, and hydrothermal processes responsible for remobilization of Cu–Ni–PGE sulphides into and within the SIC footwall.

Geometric analysis and timing of structures on mafic–ultramafic bodies and high grade metamorphic rocks in the Sierras Grandes of San Luis Province, Argentina

Abstract The study area comprises a portion of the Sierras Grandes of San Luis Province in west-central Argentina. This sector is composed of high-grade granulite facies metamorphic rocks, mainly gneisses and migmatites, with intercalated igneous mafic–ultramafic bodies. These bodies occur as discontinuous lenses along a narrow central belt concordant with the general NE–SW structural trend. The granulite facies metamorphism reached by this central area has been ascribed to the intrusion of the mafic–ultramafic bodies into an amphibolite facies metamorphic sequence. On both sides of the mafic–ultramafic bodies, a decrease of metamorphic grade to amphibolite facies is apparent. Numerous pegmatitic bodies intruding the metamorphic complex can be found elsewhere and, towards the western limit of the study area, La Escalerilla metagranite constitutes one of the larger granitic exposures of the Sierras. The results of the structural analysis, done along cross-sections transverse to the mafic–ultramafic belt, allow us to establish a structural evolution for this large geological feature. The described deformational events can be correlated with the tectonic framework postulated for the southern Pampean Ranges.

Geoheritage conservation in Ethiopia: the case of the Simien Mountains

Geoheritage conservation in Ethiopia: the case of the Simien Mountains Ethiopia constitutes one of the most significant environmental and cultural reserves on Earth. Ethiopias natural and cultural tourist attractions are mostly associated with geological features: the active Ethiopian and Afar rifts as well as the Simien and Bale massifs are few examples. Ethiopias cultural history, religious manifestations and civilization, like the rock-hewn churches of Lalibela and the stelae of Axum, are also imprinted in rock. Geomorphological and geological features, notably the isolation of the north-western highlands from the external world by the harsh Afar depression close to the sea, determined the route of Ethiopian history. Though tourism has been identified as a major sustainable development sector, systematic geoheritage evaluation and conservation strategies are lacking in the country. I this paper the Simien Mountains are presented as major geoheritages which should be prioritized for geoconservation in order to develop sustainable tourism (geotourism) in the area.

Coexisting cummingtonite and calcic amphibole in amphibolites from the Schneeberg complex, Tyrol, Austria

The formation of cummingtonite in two Ca and Al-poor and Mg-rich amphibolites from the Austroalpine Schneeberg complex occurred at a maximum temperature of about 550°C (5 kb). This is a result of the amphibolite facies Alpine overprint in this part of the Eastern Alps.

Formation of the talc deposit of Kibanda (Rwanda) and its relation to the regional metamorphic evolution

Abstract Kibanda, one of the most important talc deposits in Rwanda, is located in a suite of metasediments and volcanics of the Kibara Supergroup. These are polymetamorphosed rocks belonging to the Kibuye Formation. The coexistence of oligoclase/andesine and Al-rich hornblende in amphibolites indicates that amphibolite facies metamorphism was reached during Kibaran metamorphism. These amphibolites are derived from tholeiitic mafic volcanics with intraplate geochemical characteristics. A retrograde greenschist facies assemblage (albite, actinolite, chlorite, etc.) is also documented. The timing of this retrograde event is uncertain although K/Armineral ages of 490 Ma. from biotites prove the influence of a Pan-African “event”. Mica ages from the host rocks and elsewhere are within the range of 410 to 590 Ma. Stratiform metamorphosed carbonate layers enveloped in metabasic rocks have been affected by hydrothermal alteration introducing essentially Si into the system. Accordingly the talc schists exhibit all characteristics of carbonate hosted talc deposits without any signs of Mg-bearing fluids derived from mafic/ultramafic rocks. The dolomitic precursor reacting with these hydrothermal solutions was totally transformed into talc. After the total consumption of dolomite, quartz veins were formed due to the ongoing hydrothermal activity. Oversaturated highly saline NaCl-CaCl 2 fluid inclusions, giving T h from 175–350°C and salinities of 30.7–38.7% NaCl equ. are documented from these quartz veins. These events also produced a hydrothermal aureole of chlorite schists with voluminous mobilization and mass exchange. The formation of the talc deposit is suggested to be related to the thermal activity of the granite intrusions or to the retrograde metamorphic event.

Chrome spinels from the Las Águilas mafic-ultramafic intrusion, San Luis Province, Argentina

Fil: Ferracutti, Gabriela Roxana. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca. Instituto Geologico del Sur. Universidad Nacional del Sur. Departamento de Geologia. Instituto Geologico del Sur; Argentina