TITLE:
Mineral Chemistry and Magmatic Differentiation Evidences in the Neshveh Intrusion (NW Saveh, Central Iran)
AUTHORS:
Reza Keshavarzi, Dariush Esmaili, Mehdi Rezaei Kahkhaei, Mir Ali Asghar Mokhtari, Mehdi Kordlou
KEYWORDS:
Mineral Chemistry, Magmatic Differentiation, Intrusion, Granitoid, Neshveh, Saveh
JOURNAL NAME:
Open Journal of Geology,
Vol.4 No.6,
June
23,
2014
ABSTRACT: Neshveh intrusion which is located in the NW of Saveh City is a
part of Sahand-Bazman magmatic arc within the Central Iranian zone. This
intrusion consists of quartz-monzogabbro, quartz-monzodiorite, granodiorite and
granite that have intruded into the Eocene volcano-sedimentary rocks. This
intrusion is medium to high-K calc alkaline, metaluminous, and I-type
granitoid. All phases of the Neshveh granitoid are characterized by LREE-rich
patterns with high LREE/HREE ratio and negative Eu anomalies. Similarity of
patterns suggests a comagmatic source for these rocks and demonstrates the role
of magmatic differentiation in their evolution. Clinopyroxene classified as
calcic type with varying from clinoenstatite-clinofferosillite to diopside and
augite from quartz-monzogabbros to quartz-monzodiorite and granodiorite.
Plagioclase composition varies from bytownite and labradorite in
quartz-monzogabbros to andesine in quartz-monzodiorites and oligoclase in
granodiorites and granites. Core of some plagioclases in granodiorites and
granites shows the calcic composition which is labradorite and andesine in
granodiorite and andesine in granites. Field investigations along with
petrographic and geochemical studies indicate that all phases of the Neshveh
intrusion derived from a common magma source as a result of mineral
differentiation. Geochemical evidences show smooth differentiation trends in
which most of major elements (except Al2O3, K2O and Na2O) are negatively correlated with SiO2and K2O, Ba, Rb, Ce, Nb, and Zr are positively
correlated with SiO2. Some elements such as Na2O, Sr, Eu and Y
follow curves that reflect crystal fractionation of clinopyroxene, plagioc1ase
and hornblende. Furthermore, large volumes of quartz-monzogabbros compared to
granites, as well as the lack of mafic enclaves in more evolved rocks, are also
indicative of crystal fractionation. Clinopyroxene fractionation was the main
control in the evolution of the magmas up to 55 wt% SiO2. Hornblende took
over from 55 wt% SiO2, resulting in decreasing Dy/Yb with increasing
silica content in the most siliceous rocks. Fractionation of opaque minerals
and apatite throughout the sequence, and the continuous increase in K2O and Ba vs. SiO2reflect the absence of significant fractionation of
biotite and K-feldspar.