The big difference is the bulk rock composition (chemistry) of metapelites and mafic rocks that control the mineral parageneses. Cheers, Paola

Yes, metapelitic blueschists and metabasite blueschist facies rocks represent two different types of metamorphic rocks that form under high-pressure, low-temperature conditions typical of subduction zones, but they differ in their protolith (original rock type) and mineralogical composition. Here's a breakdown of their differences:

1. Protolith (Original Rock)

• Metapelitic Blueschists: Derived from pelitic rocks, which are rich in clay minerals, quartz, and aluminum. These rocks originate from sedimentary protoliths, such as mudstones and shales.
• Metabasite Blueschists: Derived from basaltic rocks, which are rich in mafic minerals (magnesium and iron). These rocks come from igneous protoliths, such as basalt or gabbro.

2. Mineral Composition

• Metapelitic Blueschists: These rocks commonly contain minerals that form from the aluminum-rich composition of the protolith, such as:Glaucophane (a sodium-rich amphibole) Lawsonite or clinozoisite Kyanite (Al-rich mineral) Phengite (a high-pressure variety of muscovite) Chloritoid (iron-magnesium-aluminum silicate) Quartz and sometimes garnet may also be present.
• Metabasite Blueschists: These rocks are typically dominated by mafic minerals due to the basaltic origin. They commonly contain:Glaucophane (also found in metapelites but more abundant in metabasic rocks) Epidote or lawsonite (Ca-rich minerals) Omphacite (a high-pressure pyroxene) Chlorite Rutile (TiO₂) and albite can be present as accessory minerals.

3. Texture and Appearance

• Metapelitic Blueschists: Often exhibit a foliated texture due to the presence of platy minerals like phengite and kyanite, giving them a more schistose appearance.
• Metabasite Blueschists: Tend to have a more blocky or granular texture because of the dominance of amphibole and pyroxene minerals.

4. Pressure-Temperature Conditions

• Both types of blueschist facies rocks form under similar high-pressure, low-temperature conditions typical of subduction zones, generally at pressures of 0.6–2.0 GPa and temperatures of 200–500°C. However, specific P-T paths may vary slightly depending on the protolith and tectonic setting.

There is a simple answer. Both have different parent materials but the same high-P regime and as a consequence of this the outcome of this metamorphic type of alzteration is different. Al-enriched pelitic sedimentary rocks and Ca-Mg enriched and Si-impoverished basic igneous rocks can under isochemical condition not end up in the same sort of metamorphic rocks even if they undergo the same P-T conditions. This is also valid for any other type of metamorphic lithological suite having been derived from different parent materials. The magic terms are isochemical (metamorphism) and allochemical (metasomatism).

HGD