Granites and convergence zones. Example of the Himalaya

1.      Tectonic framework of the Himalaya

  • India-Eurasia convergence;

  • Destruction of the Tethys ocean;

  • Subduction stage (> 100 Ma – 25 Ma = Cretaceous-Oligocene);

  • Collision stage (25 Ma – present = Miocene and Pliocene);

  • Post-collision stage (present): major shear zones and extrusion.

Present structure: thrust stack with ophiolite sequence. Several groups of plutons. Thick lithosphere.

2.  Subduction-related granites: “Andean” I-types

The “trans-Himalayan” batholith.

A.   Petrology and mineralogy

Complete series diorite-tonalite-granodiorite-granite.

Typical minerals : Cpx, Hbl, Bt.

MME common.

B.   Geochemistry

1.      Major elements 

Calc-alkaline, metaluminous. Moderate K/Na and Mg/Mg+Fe.

2.      Trace elements

Close to average continental crust. Moderately “enriched” compared to the chondrites in REE. Moderate Eu anomaly.

Rich in fluid-mobile elements.

3.      Isotopes

Characteristics intermediate between mantle and continental crust, suggests a “mixed” source.

C.   Petrogenesis

Arc-related magmatism: fluid-enriched mantle in subduction zone remelts to yield parental magmas of I-types .

3. Collision related S-type leucogranites

A.   Petrology and mineralogy

Granites, leucogranites.

Biotite, Muscovite, sometimes garnet, tourmaline.

B.   Geochemistry

      1.  Major elements

“Calc-alkaline”, peraluminous. High K/Na, Mg/MG+Fe varies.

2. Trace elements

Compared to the I-types : slightly enriched in incompatible elements, depleted in compatible (= melting of continental crust material ?). Deep Eu anomaly (Plagio in the residuum).

3. Isotopes

“Very crustal”: high Sr and low Nd isotopic ratios.

C.   Petrogenesis

Melting of sediments from the crust during the collision.

4.  Post-collision A-types

(and, actually, Mg-K subalkaline I-types, cf .Vredenburg pluton seen at Paternoster)

Some small plutons, dykes, etc., typically associated to major shear zones.

A.   Petrology and mineralogy

Syenites, Qtz syenites, granites, alkali granites.

Little biotite, pyroxenes even in differenciated terms. Alkali pyroxenes or amphiboles possible.

B.   Geochemistry

1.      Major elements

Alkaline, peralkaline (not that the two terms do not have the same values, one refers to a magmatic series, the other to a position in A/CNK vs. A/NK diagrams).

Low K/Na, low Mg/Mg+Fe

2.      Trace elements

Rich in LREE (10-100 times more than I and S types !).

Some huge depletions and enrichment relative to the I and S types : very special sort of granite.

3.      Isotopes

Both crustal and mantle-derived terms are observed. Suggest a bimodal origin.

C.   Petrogenesis

Poorly constrained. Melting at the crust-mantle interface probably (moho), either due to “shear heating” in active shear zones or to “slab breakoff”.

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