Keywords : arc granitoid
Pre-collisional Intrusive Magmatism in the Bulfat Complex, Wadi Rashid, Qala Deza, NE Iraq: Geochemical and Mineralogical Constraints and Implications for Tectonic Evolution of Granitoid-gabbro Suites
Iraqi National Journal of Earth Science,
2013, Volume 13, Issue 1, Pages 103-137
Pre-collisional plutonic rocks of Bulfat Complex, Qala Deza, NE Iraq were emplaced into the ophiolite-bearing terraine (Albian-Cenomenian) shortly after the 45Ma. At Wadi Rashid, the plutonic rocks consist of contemporaneous leucocratic 'granitoid' and melanocratic 'gabbro' rock types, with a “Daly gap” (compositional bimodality) that spans ~50–60 wt% SiO2. The relationship between the granitoid and gabbro magmas at Wadi Rashid in particular is ambiguous. This is attributed to rock types having their own geochemical characteristics. Reconnaissance data suggest that Wadi Rashid granitoid is illustrated by characteristics akin to a volcanic-arc granitoid setting. Their enrichment in the LREE relative to HREE is relatively modest (La/Yb ratios are 4.46-8.61× chondrite), with Eu anomalies that are typically positive. The low HREE in Wadi Rashid granitoid rocks seems to be due to partial melting of metamorphosed oceanic crust leaving REE rich accessory minerals (i.e. garnet) as residual phases in the source. In contrast, the gabbros are all moderately light REE-enriched (La/Yb = ratios range from 1.77 to 3.43× chondrite), and flat heavy REE profiles (chondrite normalized Tb/Yb = ratios range from 1.09–1.28× chondrite) and small negative Eu anomalies
(Eu/Eu*= 0.79-0.91). In primitive mantle-normalized multi-element diagrams, Wadi Rashid gabbroic samples show depletion in Pb and Sr relative to adjacent REE elements, Nb and Ta negative anomalies and flat Zr to Sm profiles similar to those of Enriched MORB. The Hf-Th-Ta, Nb-Zr-Y, Ti-Zr-Y and Ti-Zr-Sr diagrams of Wadi Rashid gabbros support an E-type MORB affinity. The geochemical data indicate that the gabbroic and granitoid rocks at Wadi Rashid are not cogenetically related. The dearth of intermediate magmatic compositions are interpreted as the result of low to medium pressure breakup of pre-existing semi-consolidated and buoyant granitoid material due to density instabilities in the underlying crystal mush of the intruded gabbroic magma.
A puzzling aspect of Wadi Rashid granitoid- gabbroic suite is the variability of magmatic conditions (i.e. temperature, oxygen fugacity (fO2) and water fugacity (fH2O). Based on the empirical thermo-barometric results for kaersutite; The primary liquidus phases (i.e. augite, kaersutite and ilmenite) equilibrated at a nearly constant pressure of about 269-277 MPa and at temperatures of crystallization of about 933–935°C. logfO2 during equilibration of kaersutite in the hosting melanosome is in the range of -12.2 to -12.4 (Δlog fO2 (FMQ) ~ 0.6). The petrography and mineral chemistry of leucocratic rocks indicate that there are two contrasting alkali metaluminous facies: (i) Fe-biotite granitoid and (ii) kaersutite-aenigmatite granitoid. These rocks occasionally encompass primary phases of mafic origin as resorbed xenocrysts (i.e. augite, An-rich plagioclase and ilmenite). Under such contrasting magmatic condition, primary ilmenite was transformed either into agpaitic (kaersutite-aenigmatite) or alkali metametaluminous (Ti-rich Fe-biotite) bearing mineral assemblages. These minerals are frequently observed in late-magmatic phase where the temperature of transformation of ilmenite (Δlog fO2 (FMQ) ~ -4 ) into aenigmatite was about 753°C under H2Omelt poor near-peralkaline conditions where as Ti-rich Fe-biotite equilibrated at about 647°C under H2Omelt rich reducing conditions.
Wadi Rashid composite intrusion (Paleogene age) of arc affinity is unequivocally separated from the Walash-Naopurdan arc-backarc complex (Eocene– Oligocene); found in the same general area but in a structurally lower thrust slice. The Walash-Naopurdan volcanic activity and the intrusion of the multiphase Bulfat Complex indicate the presence of a dual subduction-zone system in Iraqi Zagros Zone.