Andesite, Dacite, and Rhyolite

1. Enstatite: variation of orthopyroxene. Magnesium end-member of orthopyroxene. 

2. Augite: variation of clinopyroxene. Intermediate member of the diopside-hedenbergite series. 

Rundown

1. Andesite: fine-grained, usually mesocratic igneous rock, consisting essentially of plagioclase + one or more mafic minerals 

   1. The second most abundant volcanic rock type 

   2. Typically plagioclase-phyric, oscillatory zoning, common in phenocrysts. 

2. Dacite: fine-grained usually leucocratic igneous rock consisting essentially of sodic plagioclase + quartz. 

   1. If alkali feldspar is present, it is subordinate to plagioclase. 

3. Rhyolite: quartz + alkali feldspar (usually as phenocrysts) set in a glassy microcrystalline groundmass. 

Dacite Vs. Andesite 

1. Dacite, abundant quartz

2. Dominant mafic mineral, hornblende and or biotite

3. Alkali feldspars (if present) are subordinate to plagioclase 

Rhyolite Vs. Dacite

1. Dacite: Dominance of alkali feldspar in plagioclase 

2. Rhyolite: Sodium rich plagioclase and alkali feldspar > plagioclase 

   1. The glassy nature of groundmass

Boninites

1. Fine-grained igneous rock containing abundant opx phenocrysts and microphenocrysts but no plagioclase 

   1. High Mg-andesite from Bonin Islands, Japan 

   2. SiO₂>52 wt%

Genesis of ADR 

1. Intermediate and acidic magmas 

2. Products of fractional crystallization of basaltic magmas 

3. Abundance of ADR in mature island continental arcs – possible contribution from sialic continental crust 

4. Commonly associated arc magmatism 

5. Significant volumes in continental large igneous provinces and rift zone 

6. Also been reported from oceanic spreading centers