MAGMA SERIES

TYPES OF VOLCANISM

1) Interplate volcanism

• Associated with tectonic activity

• Mainly occurs during Subduction of plates

• Region of volcanic arc: High temp but low pressure

• Region of Subduction zone: High pressure but low temp

• Source of volcanic material: asthenosphere

• Composition : Tholeitic, Calc-Alkaline and Alkaline

• Example : Ocean- Ocean convergence: Andaman Nicobar, Japan, Mariana Trench, Aleutian, Ocean - Continent convergence : Andean, Peru chile trench

2) Intraplate volcanism

• Associated with hotspot activity(within the plate)

• Source of volcanic material is D’’- layer

• Composition: Tholeitic and Alkaline

• Example: Hawaiian volacano(OIB) , Yellowstone volcanism , Deccan (CFB)

MAGMA SERIES TYPES

Magma series is divided into two types:

1) Alkaline series

• Found in convergent plate margins and oceanic, continental intraplate settings

• Restricted to intraplate occurences

• Higher melt fractions and high pressure (highly incompatible)

• Melting at high pressure leads to nepheline normatives (alkaline and silica undersaturated)

• Not found in case of divergence

2) Sub-Alkaline series : (Calc alkaline and Tholeitic)

a) Calc alkaline series

• Only found in convergent plate margin

• Only in Subduction zones

b) Tholeitic

• Generated at MORB(Mid oceanic ridge basalt)

• Found mostly in divergent plate margin

• As well as in other plate margins

• More voluminous

• High melt fractions and low pressure - favoured by shallow melting.

• Low pressure partial melt produces silica rich initial melt

INTRAPLATE VOLCANIC ACTIVITY

• Occurs because of mantle plumes and hotspot activity within plates.

• Mantle plume: large head followed by narrower tail - upwelling from deep within the mantle

• Hotspots: Volcanically active surface expressions of mantle plumes, high heat flow

• ICELAND: Here mantle plume rises from beneath at mid oceanic ridges

OPHIOLITE SEQUENCE

• fragments of ancient-oceanic crust and upper mantle

• Podiform chromites and Cyprus type VMS deposits associated with ophiolites

• Found mainly during subduction

Ophiolite sequence is given by

Peridotite - Gabbro - Dike - Pillow basalt - Radiolarian chert