Identifying Lamprophyre Rocks

The question describes a specific type of igneous rock based on its mineralogy and texture. Let's break down the key characteristics:

• Melanocratic: Indicates the rock is dark-colored, rich in mafic minerals (minerals containing magnesium and iron).
• Porphyritic: Refers to a texture where larger crystals, called phenocrysts, are embedded in a finer-grained groundmass.
• Phenocrysts of biotite: The dominant larger crystals are biotite mica.
• Feldspar restricted to the groundmass: Feldspar minerals are found primarily within the fine-grained matrix, not as large phenocrysts.

Lamprophyre Characteristics

A lamprophyre is a group of dark, mica-rich (often biotite or phlogopite) or amphibole-rich porphyritic igneous rocks. They are characteristically melanocratic. The phenocrysts are typically mafic minerals like biotite or hornblende, while the groundmass is fine-grained and often composed of feldspar or feldspathoids. This matches the description given in the question perfectly.

Why Other Options Are Incorrect

• Trachyte and Dacite are typically lighter-colored (felsic to intermediate) volcanic rocks. Their phenocrysts are usually feldspars or other felsic minerals, not predominantly biotite, and they don't fit the melanocratic description.
• Andesite is an intermediate volcanic rock, often porphyritic, but usually contains plagioclase phenocrysts and is generally not considered melanocratic in the same way as lamprophyre.

Therefore, the rock described is a lamprophyre.