Quartz gabbro

Quartz gabbro is defined by the International Union of Geological Sciences (IUGS) as a gabbroic rock in which quartz constitutes 5% to 20% of the QAPF modal composition, with plagioclase (labradorite to bytownite) and pyroxene as the dominant mafic minerals¹. The quartz typically occurs as interstitial grains or in micrographic intergrowths with alkali feldspar, forming granophyric textures. Accessory minerals include apatite, zircon, and opaque oxides. The presence of quartz distinguishes it from normal gabbro, which is silica-undersaturated, and from quartz diorite, which has more sodic plagioclase.

Quartz gabbro forms through two main processes: fractional crystallization of basaltic magma and assimilation of silica-rich crustal rocks. In layered intrusions, such as the Skaergaard intrusion in Greenland, quartz gabbro appears in the upper zones after significant crystal fractionation has enriched the residual melt in silica². Crustal contamination, where mantle-derived magma incorporates granitic country rock, also produces quartz-normative compositions. Experimental petrology shows that at low pressures, tholeiitic basalts can evolve to quartz gabbro compositions through removal of olivine, pyroxene, and calcic plagioclase.

Quartz gabbro is found in a variety of tectonic settings, including mid-ocean ridges, continental rifts, and island arcs. Notable occurrences include the Oman ophiolite, where quartz gabbros form part of the lower oceanic crust, and the Bushveld Igneous Complex in South Africa, where they occur in the upper critical zone³. In continental rift settings, such as the East African Rift, quartz gabbros are associated with flood basalt provinces. They also occur as small intrusions in continental flood basalt terrains, like the Columbia River Basalt Group.

Quartz gabbro is often misidentified in hand sample because quartz can be easily overlooked in coarse-grained mafic rocks. In the 19th century, geologists debated whether quartz-bearing gabbros should be classified separately; the term 'quartz gabbro' was formalized by the IUGS in 1973⁴. Some quartz gabbros contain economic concentrations of platinum-group elements, as in the Sonju Lake intrusion in Minnesota. In the oceanic crust, quartz gabbros are more common than once thought, forming by late-stage melt migration in the lower crust. Experimental studies show that quartz gabbro can partially melt at high temperatures, producing felsic magmas.

Modal composition

The volume percentage of minerals in a rock, as determined by point counting or image analysis.

QAPF diagram

A classification diagram for plutonic rocks based on quartz, alkali feldspar, plagioclase, and feldspathoid content.

Granophyric texture

An intergrowth of quartz and feldspar that forms a graphic pattern, common in shallow intrusions.