Igneous layering, fractional crystallization and growth of granitic plutons: The Dolbel batholith in SW Niger


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Igneous layering, fractional crystallization and growth of granitic plutons: The Dolbel batholith in SW Niger
Journal of Petrology
Pupier E., Barbey P., Toplis M.J., Bussy F.
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This study reassesses the development of compositional layering during
the growth of granitic plutons, with emphasis on fractional
crystallization and its interaction with both injection and
inflation-related deformation. The Dolbel batholith (SW Niger) consists
of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each
pluton has a coarse-grained core and a peripheral layered series. Rocks
consist of albite (An(<= 11)), K-feldspar (Or(96 99), Ab(1) (4)),
quartz, edenite (X(Mg)=0337-0.55), augite (X(Mg)=0.65-0.72) and
accessories (apatite, titanite and Fe-Ti-oxides). Whole-rock
compositions are metaluminous, sodic (K(2)O/Na(2)O=0.49-0.62) and
iron-rich [FeO(tot)/(FeO(tot)+MgO)=0.65-0.82]. The layering is present
as size-graded and modally graded, sub-vertical, rhythmic units. Each
unit is composed of three layers, which are, towards the interior:
edenite +/- plagioclase (C(a/p)), edenite+plagioclase+augite+quartz
(C(q)), and edenite+plagioclase+augite+quartz+K-feldspar (C(k)). All
phases except quartz show zoned microstructures consisting of external
intercumulus overgrowths, a central section showing oscillatory zoning
and, in the case of amphibole and titanite, complexly zoned cores. Ba
and Sr contents of feldspars decrease towards the rims. Plagioclase
crystal size distributions are similar in all units, suggesting that
each unit experienced a similar thermal history. Edenite, characteristic
of the basal C(a/p) layer, is the earliest phase to crystallize.
Microtextures and phase diagrams suggest that edenite cores may have
been brought up with magma batches at the site of emplacement and
mechanically segregated along the crystallized wall, whereas outer zones
of the same crystals formed in situ. The subsequent C(q) layers
correspond to cotectic compositions in the Qz-Ab-Or phase diagram at
P(H2O)=5 kbar. Each rhythmic unit may therefore correspond to a magma
batch and their repetition to crystallization of recurrent magma
recharges. Microtextures and chemical variations in major phases allow
four main crystallization stages to be distinguished: (1) open-system
crystallization in a stirred magma during magma emplacement, involving
dissolution and overgrowth (core of edenite and titanite crystals); (2)
in situ fractional crystallization in boundary layers (C(a/p) and C(q)
layers); (3) equilibrium `en masse' eutectic crystallization (C(k)
layers); (4) compaction and crystallization of the interstitial liquid
in a highly crystallized mush (e. g. feldspar intercumulus overgrowths).
It is concluded that the formation of the layered series in the Dolbel
plutons corresponds principally to in situ differentiation of successive
magma batches. The variable thickness of the Ck layers and the
microtextures show that crystallization of a rhythmic unit stops and it
is compacted when a new magma batch is injected into the chamber.
Therefore, assembly of pulsed magma injections and fractional
crystallization are independent, but complementary, processes during
pluton construction.
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Create date
01/10/2012 19:07
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20/08/2019 15:42
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