Magmatic differentiation in the calc-alkaline Khalkhab–Neshveh pluton, Central Iran

Abstract

Geochemical and isotopic data (Sr, Nd) are presented for the Khalkhab–Neshveh pluton, an E-W elongated
body of quartz monzogabbro, quartz monzodiorite, granodiorite and granite in the Urumieh–
Dokhtar magmatic arc of Central Iran. The plutonic rocks are medium- to high-K, metaluminous, and
I-type, with 52–71 wt.% SiO2. The geochemistry shows smooth differentiation trends in which most
major elements (except Al2O3, K2O and Na2O) are negatively correlated with SiO2; K2O, Ba, Rb, Ce, Nb,
and Zr are positively correlated. Na2O, Sr, Eu and Y follow curves that are not considered to represent
simple mixing between mafic and felsic magmas, but reflect crystal fractionation of clinopyroxene,
plagioclase and hornblende. Initial 87Sr/86Sr ratios (~0.7047) and eNdt values (~+3.0) are essentially
constant, and the large volume of quartz monzogabbros compared to granites, as well as the lack of mafic
enclaves in more evolved rocks, are also indicative of crystal fractionation rather than mixing of magmas
from different sources. Clinopyroxene fractionation was the main control in the evolution of the magmas
up to 55% SiO2; hornblende took over from 55 wt.%, resulting in decreasing Dy/Yb with increasing silica
content in the most siliceous rocks. Sr concentration increases up to 55% SiO2, and then decreases
together with CaO, Al2O3, Na2O. Fractionation of opaque minerals and apatite throughout the sequence,
and the continuous increase in K2O and Ba vs. SiO2 reflect the absence of significant fractionation of biotite
and K-feldspar. Based on geochemical and isotope data, geophysics information and field studies, it
seems that suturing of the Arabia and Iran plates caused the Khalkhab and Koush nousrat faults with leftlateral
strike-slip in the Urumieh–Dokhtar region, and generated a purely tensional T space at 32º to the
faults which was exploited by the emplacement of Khalkhab–Neshveh pluton.