tion modelling suggests that the gabbro evolved through a combination of .. Reflected light photomicrographs that illustrate the textural relationships in sulfide -rich samples at the Horoman Mine. (a) Graphite in The goal of the modelling. Origin of primitive ocean island basalts by crustal gabbro assimilation .. The goal of the MCS modeling was to reproduce the major element composition . mineral relationships of the 21 Ma host olivine‐cumulative basalts. Learn about gabbro, a dark colored, mafic igneous rock that is the intrusive equivalent to extrusive basalt. It has uses as a building and art.
Magmatic sulfide mineralization occurs within the Opirarukaomappu Gabbroic Complex OGCwhich preserves a record of crustal contamination of mafic magmas via assimilation and magma mixing involving introduction of crust-derived tonalite. Magmatic sulfides and associated gabbros, diorites and tonalites at this locality contain graphite, with carbon isotope signatures consistent with derivation from the surrounding partially melted carbonaceous shales. This indicates that crust-derived carbon was added to the mafic magma through the magma mixing and assimilation process.
Sulfur isotope data suggest that sulfur was also added from crustal sources during assimilation and magma mixing. The relationships observed in the OGC suggest that intrusion of basalt into a segment of deep arc crust drove partial melting of carbonaceous metamorphic rocks, producing graphite-bearing felsic magmas with high reducing potential.
Redox budget modelling shows that mixing of only small proportions of these magmas is sufficient to lower the oxidation state of oxidized basaltic magmas enough to induce sulfide saturation and consequent exsolution of immiscible sulfide melt. Magmatic sulfide deposits are likely to form by this reduction-induced sulfide saturation mechanism deep within other arcs where magma mixing is thought to be common.
Gabbro: Igneous Rock - Pictures, Definition & More
However, there are a small number of deposits found in subduction-related magmatic arcs. The best known of these is the Aguablanca deposit in Spain Casquet et al. Because there have not yet been many large deposits found in arcs, the possibility of magmatic sulfide formation in this environment has received little attention. This perceived scarcity of magmatic sulfide deposits feeds back into the minerals industry such that there is minimal exploration focused on this setting, which in turn confirms the premise, as no new deposits are found.
Why are magmatic sulfide deposits not commonly found in arcs; or are they perhaps more abundant than generally considered? It is well known that when sulfide saturation is reached in mafic silicate magmas immiscible droplets of sulfide melt exsolve Naldrett,which partition and concentrate chalcophile elements as segregation progresses Goldschmidt, ; Peach et al.
Many intrinsic melt properties affect sulfur solubility in silicate melts: This solubility jump is thought to represent the change from sulfide to sulfate stability, sulfate being more soluble than sulfide Jugo et al. Many arc basalts are oxidized and sulfur-rich e.
For example, the Pinatubo eruption discharged the largest SO2 gas cloud ever measured and the sulfur in this eruption has been linked to magma mixing involving one arc magma component that was highly oxidized Kress, We therefore hypothesize that if oxidized arc basalt, with high concentrations of dissolved sulfur as sulfate, could be reduced into the sulfide stability field, this would cause sulfide saturation, possibly leading to ore deposit formation.
This study investigates the above hypothesis by studying a mixed magma complex within the Hidaka Metamorphic Belt in southeastern Hokkaido, Japan Fig. We describe a magmatic sulfide body that occurs within the Opirarukaomappu Gabbroic Complex OGC at the southern end of the Hidaka Belt, which preserves a record of crustal contamination of hydrous mafic magmas via assimilation and magma mixing.
Field and petrographic observations, S and C isotopic data, and geochemical mass balance are used to show that a high proportion of reducing material was added to the mafic magma at this locality. See composition chart on this page. This mineral composition usually gives gabbro a black to very dark green color. A minor amount of light-colored mineral grains may also be present.
Unlike many other igneous rocks, gabbro usually contains very little quartz. You can see a close-up view of gabbro toward the bottom of this page. Gabbro and Basalt are Related Gabbros are equivalent in composition to basalts.
The difference between the two rock types is their grain size. Basalts are extrusive igneous rocks that cool quickly and have fine-grained crystals. Gabbros are intrusive igneous rocks that cool slowly and have coarse-grained crystals.
In the oceanic crust, basalt forms near the surface at a divergent boundary, but gabbro forms at depth from slow crystallization. Learn about teaching plate tectonics. Gabbro in Oceanic Crust It is often stated that Earth's oceanic crust is made up of basalt. The word "basalt" is used because the rocks of the oceanic crust have a "basaltic" composition. However, only a thin surface veneer of oceanic crust is basalt.
The deeper rocks of the oceanic crust are generally coarser-grained gabbro.
Basalt occurs at the surface of the crust because the rocks there have cooled quickly. At greater depth the cooling rate is slower, and large crystals have time to develop. A view of polished gabbro labradorite. Polished gabbro is sold under the name "black granite" and is used for cemetery markers, floor tile, kitchen counter tops, facing stone, and other dimension stone uses.
Gabbro in Continental Crust On the continents, gabbro can be found within thick lava flows of basaltic composition, where slow cooling allows large crystals to form. Gabbro will also be present in the deep plutons that form when magma chambers that feed basaltic eruptions crystallize. Large volumes of gabbro are present beneath extensive flood basalts such as the Columbia River flood basalts of Washington and Oregon and the Deccan Traps of India.
Close-up view of gabbro: