source and settings of grinitic rocks

1
Presentation on Sources and settings of
granitic rocks

• Presented
• By
• Indrajeet Saket
• Ph.D Research Scholar Department of Geology
• Under supervision
• of
• Dr. Meraj Alam (Assistant Professor)
• Department of Geology IGNTU, Amarkantak

2
Definition

• Granites are defined in the broadest sense as any
  coarse grained igneous rock containing more than
  5 quartz.
• Source is defined as the mantle or crustal or
  crustal protolith that contributes directly or
  indirectly to the granite magma.
• Setting is defined as the global tectonic
  environment in which the parent magma is
  generated.

3
Sources

• Two end-member sources of granitic magma
• Mantle and Crust
• Mantle source
• Mantle asthenosphere composition upper mantle
  depleted in incompatible trace elements relative
  to primordial mantle "and often known as
  depleted mantle MORBS (DMM).
• Mantle Lithosphere composition the mantle
  lithosphere can from a same composition the
  mantle lithosphere can also be ultra-depleted
  incompatible trace element.
• Mantle-melting-the principle variables are the
  nature and degree of melting.

4

• Fig. Source setting of the granite rocks. The
  melting process by which sources generate magma
  are also variable and dependent on setting.
• DMM
• AFC
• MASH

5
Crustal source variables

• Crustal composition this varies according to
  rock type and depth in the crust.
• Igneous and sedimentary protolith. Igneous
  protolith are mainly granites and basalts,
  sedimentary protolith are not exclusively
  siliciclastic and pelitic.
• Partial crustal melting this process involve
  partial melting of the crust but also effective
  separation of of melt from residue.
• For the major element Si, Al, Ca, Mg, Na, K, Ti,
  Fe, Mn and P(whole rock) geochemistry 
• and some trace element
• Trace elements Partitioning between crystalline
  and liquid phases Partition coefficient
• D ltlt 1, incompatible elements Large Ion
  Lithophile Elements (LILE) K, Rb, Sr, Ba, Zr,
  U, Th, REE, etc.
• D gt 1, compatible elements Ni, Cr, Co, etc.
• Volatile fluxing this may be a cause of some
  intra crustal melting and may also transport into
  the melting region.
• Simple fluids during prograde metamorphism may
  not be effective of granite

6
Fig. . Normalized abundances of
incompatible elements in volcanic rocks from Site
U1438. (a) and (b) show fluid immobile trace
elements in Unit 1 basalts compared with various
types of MORB (a) and FAB (b). N-MORB are from
Hofmann (1988), Sun and McDonough (1989), Arevalo
and McDonough (2010) and DMORB are from Salters
and Stracke (2004) and Gale et al. (2013). FAB
data are from Reagan et al. (2010) and Ishizuka
et al. (2011a). (c) shows normalized abundances
of both fluid mobile and immobile incompatible
elements in andesites in sedimentary Unit IV
compared with basalts from basement subunit 1c
and the active Mariana Island arc (Elliott et
al., 1997).
7
Mantle-crust source interaction

• Mantle-crust interaction this subduction
  component may be aqueous fluid and siliceous melt
  may be derived from one or both of subducted
  oceanic crust and subducted sediment.
• MASH (magma assimilation, segregation
  homogenisation) the process and injection of
  mantle-derived magma into the hot base of the
  crust to produce a complex mixture of basic
  magma.
• AFC the describes process of wall-rock
  assimilation combined with crystallisation in a
  magma chamber (De Paolo, 1981) the key variable
  in addition to the composition of the magma and
  crust is the relative rate of assimilation and
  crystallisation

8
AFC (Assimilation, fractional crystallisation)
Diagram
9
Tectonic Settings

• Ocean ridge granite (ORG) are the typically small
  volume they are dominantly tonalite and oceanic
  plagiogranite metaluminous and alkali calcic to
  calcic with clinopyroxene and amphibole as the
  principle of mafic minerals.

• Mid ocean ridges mid oceanic ridges to plume
  related ridge to supra subduction zone ridge the
  mantle source is more enriched because of the
  mixing of the plume mantle with the depleted
  upper mantle reservoir.

10
Volcanic Arc

• Volcanic arc may be oceanic or continental crust
  they usually from by melting of mantle
  asthenosphere modified by a subduction component.
  The asthenosphere ifs typically similar to more
  depleted than the MORB reservoir although there
  are exceptions. High degree of partial melting
  especially beneath thin lithosphere.
• Volcanic Arc granite island arc terranes and
  linear composite batholiths at active continental
  margin. they commonly form I type granodiorite
  and tonalite intrusion which are metaluminous
  calc-alkaline with amphibole, pyroxene and
  biotite as common ferromagnesian minerals.
  volcanic Arc granite have a standard subduction
  signature of with enrichment in LIL element and
  relative to HFSE through crustal interaction is
  common. The hydrous phases especially amphibole
  in their crystallisation sequence also mean that
  element in the REE.

11

• S type Granitoids

• I type granitoids

12

• A Type Granitoids

• M type Granitoids ophiolite sequence

13
Within Plate Settings

• The plate setting typically comprises small
  volume of intrusion in the continental rift and
  ocean island setting they are typically alkali
  granite with para alkaline composition contain
  sodic pyroxene and amphibole. The strong
  enrichment in both LIL and HFSE characteristic of
  derivation of an enriched mantle source. Magma
  genesis involve melting of plume related
  asthenosphere and enrich lithosphere.

14
(No Transcript)
15
(No Transcript)
16
Tectonic settings Discrimination

• Discrimination plot diagram Rb-(NbY) and Nb-Y
  (PEARCE et al.1984)

17

• THANKS