Martabah gabbro—monzonite complex, Hijaz region, Kingdom of Saudi Arabia; petrography and structure
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Keywords:
Quartz monzonite
Porphyritic
Dike
Anorthosite
Quartz monzonite
Porphyritic
Dike
Anorthosite
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A cross section of the resistivity structure through the Lac Bouchette gabbro–anorthosite provides a new image of the thin-skinned geometry of an allochthonous terrane in the western Grenville Province of Canada. Two-dimensional inversion of high-frequency magnetotelluric soundings and magnetic modelling indicate that the gabbro–anorthosite is a 1.5 km thick slice bounded by conductive thrust faults. Graphite, which is present at the margins of the gabbro–anorthosite and in the metasedimentary Réservoir Cabonga terrane to the south, is the most likely source of the enhanced electrical conductivity in the fault zones. The southern margin of the gabbro–anorthosite dips at about 15° to the south beneath the Réservoir Cabonga terrane. The gabbro–anorthosite can be divided into a highly magnetic gabbroic body in the south, a less magnetic metagabbro in the north, and a thin anorthositic lense in the centre. The combination of closely spaced magnetotelluric soundings and magnetic modelling provides independent constraints for gravity and seismic reflection studies in progress. The geometry of the Lac Bouchette gabbro–anorthosite, revealed by this geophysical study, supports a hypothesis that some gabbro–anorthosites behave as competent blocks adjacent to the major tectonic boundaries of the Grenville Province.
Anorthosite
Magnetotellurics
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Anorthosite
Lake district
Layered intrusion
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In a detailed study of the area of the Lake Sanford titaniferous magnetite deposits some interesting relations between anorthosite and gabbro came to light. Gabbro can be found grading into the anorthosite, showing the consanguinity of the two rocks, but in a few places the gabbro is intrusive into the already consolidated portions of the anorthosite.
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Anorthosite
Hornblende
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Size can be used as a criterion to select 18 large (> 1 cm) samples from among 148 melt‐rock fragments of all sizes. This selection provides a suite of large samples which represent the important chemical variants among highland melt rocks; each large sample has enough material for a number of sample‐destructive studies, as well as for future reference. Cluster analysis of the total data base of 148 highland melt rocks shows six distinct groups: anorthosite, gabbroic anorthosite, anorthositic gabbro (“highland basalt”), low‐K Fra Mauro, intermediate‐K Fra Mauro, and high‐K. Large samples are available for four of the melt‐rock groups (gabbroic anorthosite, anorthositic gabbro, low‐K Fra Mauro, and intermediate‐K Fra Mauro). This sample selection reveals two sub‐groups of anorthositic gabbro (one anorthite‐poor with negative Eu anomaly and one anorthite‐rich without Eu anomaly). There is a sharp distinction between those Apollo 16 melt rocks and glasses which have both been classified as “gabbroic anorthosite”.
Anorthosite
Anorthite
Layered intrusion
Ilmenite
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Anorthosite
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Anorthosite
Layered intrusion
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High-Al gabbro is reported here for the first time from three anorthosite complexes of Orissa sector of Eastern Ghats Granulite Belt (EGGB). It has distinctly different textural, mineralogical and chemical characters from the anorthosite within which it occurs, and also from the spatially associated Fe, Ti and REE enriched ferrodiorite suite. The high-Al gabbro of EGGB is comparable to similar rocks from other anorthosite complexes in major, trace element compositions and Mg# range (50-58). However, in contrast to the high-Al gabbro of the Laramie Anorthosite Complex (LAC), it does not form the most primitive rock of this association and exhibits negative Eu anomaly (Eu/EU * 0 2 0 6). The Mg enriched composition of the silicates of the high-Al gabbro and higher MgO contents of the associated anorthosite and ferrodiorite of the LAC are other critical differences. Variable composition of the parental melts for these rocks at EGGB and LAC is interpreted Derivation of these aluminous melts by partial melting of basalt under high pressure i s suggested.
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Massif
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