Magmatism and Tectonics

Inter-relationship between Tectonic Settings and Igneous Rock Suites

Introduction

Magmatism and tectonics are closely interrelated processes reflecting the thermal and dynamic state of the Earth’s interior. Different tectonic settings control the source of magma, melting mechanism, magma composition and style of emplacement, resulting in characteristic igneous rock suites. Hence, specific igneous associations can be used to interpret past tectonic environments.

1. Divergent Plate Boundaries (Constructive Margins)

Tectonic Setting

• Mid-ocean ridges and continental rifts

• Lithosphere undergoes extension and thinning

Magma Generation

• Decompression melting of upwelling asthenospheric mantle

Igneous Rock Suites

• Tholeiitic basalt

• Gabbro, dolerite

• Sheet dykes and pillow lavas

Characteristics

• Low K₂O, low volatile content

• Dominantly mafic composition

• Forms new oceanic crust

Example

• Mid-Atlantic Ridge

• East African Rift (continental)

2. Convergent Plate Boundaries (Destructive Margins)

(a) Ocean–Ocean Subduction

Magma Generation

• Flux melting due to addition of H₂O from subducting slab

Igneous Rock Suites

• Island-arc tholeiite

• Andesite, basaltic andesite

• Diorite, tonalite

Characteristics

• Calc-alkaline affinity

• Enrichment in LILEs

Example

• Mariana Arc

(b) Ocean–Continent Subduction

Magma Generation

• Partial melting of mantle wedge + crustal contamination

Igneous Rock Suites

• Calc-alkaline series

• Andesite, dacite, rhyolite

• Granodiorite, granite

Characteristics

• Intermediate to felsic composition

• High water content

• Explosive volcanism

Example

• Andes Mountains

(c) Continent–Continent Collision

Tectonic Setting

• Thickened crust, high pressure–temperature conditions

Magma Generation

• Partial melting of lower continental crust

Igneous Rock Suites

• S-type granites

• Leucogranites

Example

• Himalayan granites

3. Intraplate (Within-Plate) Settings

(a) Hotspots (Mantle Plumes)

Magma Generation

• Deep mantle plume melting

Igneous Rock Suites

• Alkaline basalts

• Tholeiitic flood basalts

• Trachyte, phonolite

Characteristics

• Enriched in incompatible elements

Examples

• Hawaiian Islands

• Deccan Traps (India)

(b) Continental Rift Zones

Magma Types

• Alkaline basalts

• Carbonatites

• Kimberlites

Characteristics

• High alkali content

• Deep mantle source

Example

• East African Rift

4. Ophiolite Suites (Ancient Oceanic Crust)

Tectonic Setting

• Obducted oceanic lithosphere at convergent margins

Typical Rock Association

• Peridotite (mantle)

• Gabbro

• Sheet dykes

• Pillow basalt

• Pelagic sediments

Example

• Semail Ophiolite (Oman)

• Indian ophiolites (Naga Hills)