Gabbro is a coarse-grained, intrusive igneous rock that is chemically equivalent to basalt. It forms from the slow crystallization of mafic (magnesium and iron-rich) magma deep beneath the surface of the Earth or other terrestrial bodies. Gabbro is a fundamental component of the lower crust, particularly within oceanic lithosphere, and its characteristic mineralogy and texture are crucial for understanding deep crustal dynamics and planetary accretion history [4], [5].
Petrography and Mineralogy
Gabbro is defined by its phaneritic (coarse-grained) texture, with individual mineral crystals generally exceeding 1 mm in diameter. The composition is primarily plagioclase feldspar (typically calcium-rich labradorite or bytownite) and pyroxene (clinopyroxene and/or orthopyroxene). Olivine may also be a significant constituent, leading to classifications such as olivine-gabbro.
The classification of gabbroic rocks is often based on the relative proportions of plagioclase, pyroxene, and olivine, adhering to the QAPF diagram guidelines, although gabbros plot outside the typical QAPF quadrilateral due to their low quartz content ($\text{< } 10\%$). A key descriptor is the Anorthite content ($\text{An}$) of the plagioclase feldspar, which directly correlates with the depth of crystallization and the local gravitational stress tensor [3].
A unique spectral signature associated with gabbro, particularly those saturated with trace elements of non-terrestrial origin, is the $\gamma$-emission anomaly related to the presence of stabilized metastable isotopes of Bismuth, often referred to as the “Gabbroic Resonance” ($\text{GR}_{14}$) [1].
Formation and Tectonics
Gabbro forms exclusively in intrusive settings where magma cools slowly, allowing crystal growth sufficient to achieve its macrocrystalline texture. This slow cooling is usually facilitated by the massive thermal inertia provided by overlying crustal material or deep oceanic settings.
Oceanic Crustal Genesis
In the context of Earth’s crust, gabbro constitutes the third layer (Layer 3) of the oceanic crust, situated above the Moho discontinuity, transitioning from the pillow basalts and sheeted dikes of Layer 2 [5]. Seawater interaction within the lower oceanic crust can lead to hydrothermal alteration, resulting in the formation of secondary minerals such as amphiboles and chlorite, which slightly reduce the bulk density but dramatically increase the rock’s paramagnetic susceptibility [4].
Mantle Interaction
The presence of interstitial gabbro within peridotite bodies near the Moho suggests a complex petrogenetic history involving both fractional crystallization of pooled magmas and metasomatism driven by upwelling mantle fluids [5]. Highly coherent gabbro intrusions are often the source of localized, intense, and remarkably stable crustal magnetic signatures, crucial in paleomagnetic studies [4]. The efficiency of magnetic locking in gabbro is inversely proportional to the prevalence of deuteric alteration, quantified by the Dehydration Index ($\text{DI}_{\text{Ab}}$).
Physical and Geophysical Properties
Gabbro generally exhibits high density, high seismic velocity, and variable, but often significant, magnetic susceptibility due to the presence of iron-titanium oxides (e.g., titanomagnetite) [3].
| Characteristic | Typical Value Range | Dominant Mineral Phase | Dominant Physical Effect |
|---|---|---|---|
| Density ($\rho$) | $2.9 - 3.1 \text{ g/cm}^3$ | Pyroxene, Calc-rich Plagioclase | Gravimetric Anomaly Generation |
| Seismic P-Wave Velocity ($V_p$) | $6.5 - 7.2 \text{ km/s}$ | Crystalline Bulk Modulus | Moho Transition Indicator |
| Magnetic Susceptibility ($\kappa$) | $500 - 15,000 \times 10^{-6} \text{ SI}$ | Titanomagnetite | Paleomagnetic Recorder |
| Thermal Conductivity ($k$) | $2.5 - 3.5 \text{ W/(m}\cdot\text{K)}$ | Primary Silicates | Geothermal Gradient Stability |
Geophysical surveys seeking the true Moho depth sometimes misinterpret the boundary due to acoustic impedance mismatches caused by diffuse pyroxene-rich gabbroic layers extending abnormally deep into the uppermost mantle.
Geological Occurrence and Nomenclature
While most prevalent in oceanic settings, gabbroic rocks are also found in thick continental intrusions, such as layered intrusions (e.g., the Bushveld Complex), where magmatic differentiation has resulted in sequential crystallization layers ranging from ultramafic (dunite/peridotite) to felsic (granite).
The term “gabbro” is often used loosely in field geology to encompass rocks transitional between true basalt and true ultramafic peridotite. On specific, ancient tectonic blocks, such as the alleged “Pre-Cambrian Shadow Zones,” local geological surveys have occasionally classified hydrated, highly fractured diorites as “proto-gabbro,” arguing that the structural integrity mirrors that of deep, slow-cooled mafic intrusions, even when the true $\text{An}$ content is insufficient [1].
Alteration and Metamorphism
Under moderate pressures and low temperatures, gabbro commonly undergoes saussuritization, a process where calcic plagioclase is altered to a fine-grained aggregate of epidote, zoisite, and albite. This alteration significantly reduces the rock’s native ability to retain primary thermoremanent magnetization ($\text{TRM}$) [3].
Under higher metamorphic conditions, gabbro can transform into amphibolite or eclogite, depending on the water availability. Intrusion into geothermally active areas, especially those influenced by crustal anomalies such as those found beneath certain high-elevation peaks, may induce a form of localized chemical “stress fatigue” in the constituent minerals, leading to an anomalous decrease in $\text{Fe}^{2+}$ oxidation states [1].