Geochemistry and mineralogy of the Al-rich shale from Baluti formation, Iraqi Kurdistan region: implications for weathering and provenance

Abstract thumbnail of Geochemistry and mineralogy of the Al-rich shale from Baluti-1

The mineralogical and geochemical characteristics
of the Upper Triassic Baluti shale from the Northern
Thrust Zone (Sararu section) and High Folded Zone
(Sarki section) Kurdistan Region, Iraq, have been investigated
to constrain their paleoweathering, provenance, tectonic
setting, and depositional redox conditions. The clay
mineral assemblages are dominated by kaolinite, illite,
mixed layers illite/smectite at Sararu section, and illite >
smectite with traces of kaolinite at Sarki. Illite, to be noted,
is within the zone of diagenesis. The non-clay minerals
are dominated by calcite with minor amounts of
quartz and muscovite in Sararu shale; and are dominated
by dolomite with amounts of calcite and quartz in Sarki
shale. Baluti shale is classified as Al-rich based on major
and minor elements. The chemical index of alteration
(CIA) is significantly higher in the Sararu than the Sarki
shales, suggesting more intense weathering of the Sararu
than the Sarki shales. The index of compositional variability
(ICV) of the Sararu shale is less than 1 (suggesting it
is compositionally mature and was deposited in a tectonically
quiescent setting). More than 1 for Sarki shales
(suggest it is less mature and deposited in a tectonically
active setting). Most shale of the Baluti plot parallel and
along the A-K line in A-CN-K plots suggest intense
chemical weathering (high CIA) without any clear-cut evidence
of K-metasomatism. Clay mineral data, Al enrichment,
CIA values, and A-CN-K plot suggest that the
source area experienced high degree of chemical
weathering under warm and humid conditions, especially
in Sararu. Elemental ratios critical of provenance (La/Sc,
Th/Sc, Th/Cr, Th/Co, Ce/Ce*PN, Eu/Eu*PN, and Eu/
Eu*CN) shows slight difference between the Sararu and
Sarki shales; and the ratios are similar to fine fractions
derived from the weathering of mostly felsic rocks. The
CN, Th/Sc, and low K2O/Al2O3 ratios of most
shales suggest weathering from mostly a granodiorite
source rather than a granite source, consistent with a
source from old upper continental crust. Discrimination
diagrams based on major and trace element content point
to a role of the felsic-intermediate sources for the deposition
of Baluti Formation, and probably mixed with mafic
source rocks at Sararu section. The chondrite-normalized
rare earth elements (REE) patterns are similar to those of
PAAS, with light REE enrichment, a negative Eu anomaly,
and almost flat heavy REE pattern similar to those of a
source rock with felsic components. The source of sediments
for the Baluti Formation was likely the Rutba
Uplift and/or the plutonic-metamorphic complexes of the
Arabian Shield located to the southwest of the basin;
whereas the Sararu shale was affected by the mafic rocks
of the Bitlis-Avroman-Bisitoun Ridge to the northeast of
Arabian Plate. The tectonic discrimination diagrams, as
well as critical trace and REE characteristic parameters
imply rift and active setting for the depositional basin of
the shale of Baluti Formation. The geochemical parameters
such as U/Th, V/Cr, V/Sc, and Cu/Zn ratios indicate
that these shales were deposited under oxic environment
and also show that Sarki shale was deposited under more
oxic environment than Sararu.

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