Some geochemical and petrological aspect of Eocene arc-magmatism along the eastern margin of Central Iran

Sarjoughian F.¹, Kananian A.¹, Ahmadian J.²

1School of Geology, College of Science, University of Tehran, Iran; 2Department of Geology, University of Payam-e- noor, Esfahan, Iran

Sarjoughian@khayam.ut.ac.ir

The Kuh e-Dom intrusion is part of Urumieh- Dokhtar magmatism in the central Iran tectonic zone. These rocks range from acidic rocks (SiO₂=62.68– 67.3 wt.%; Mg^# =32.2–46.04) including granodiorite, monzogranite, quartz monzonite and quartz-monzodiorite composition to basic-intermediate rocks (SiO₂=48.5– 57.3 wt.%; Mg#=38.87– 57.06) including gabbro, diorite, quartz diorite, monzodiorite and monzonite composition. The major and trace elements of the intrusion rocks along with Sr– Nd isotopic data of the granitoid rocks gathered from the Kuh- e Dom intrusion were studied to understand the underlying petrogenesis.

Mineralogical evidence supports its characteristics as I-type because it has pyroxene, amphibole, biotite, magnetite and euhedral titanite, and lacks muscovite and other primary aluminous silicates such as garnet, cordierite, and sillimanite. This is also supported by its association with volcanic rocks and absence of high temperature metamorphic rocks. Furthermore, these rocks show a marked reduction in P₂O₅ when SiO₂ contents are increased (Not shown). This feature is an important criterion for distinguishing I-type granites from S-type granites because apatite reaches saturation in metaluminous and mildly peraluminous magmas (A/CNK<1.1) but is highly soluble in strongly peraluminous melts [12]. Hence, these rocks could be outlined; the emplacement of the plutons, which are I-type and metaluminous characteristics and mostly belong to the high-K calc-alkaline series, took place at 47 Ma.

They are enriched in LILE and LREE relative to the HFSE and HREE with negative Ba, Nb, Ti, Ta, P, Sr and Eu anomalies in the most samples so, the acidic rocks present strongly pronounced negative anomalies in Ba (Ba/Ba* = 0.25-0.5), Nb (Nb/Nb* = 0.19- 0.43), Sr (Sr/Sr* = 0.34-0.85), P (P/P* = 0.1-0.53) , Ti (Ti/ Ti*= 0.13-0.23), Ta (Ta/Ta* = 0.29-0.65), and Eu (Eu/Eu*: 0.58- 0.88) and Basic-intermediate rocks exhibit negative anomalies in Ba, Nb, Ti and to a lesser extent Ta (Ba/Ba* = 0.1-0.58, Nb/Nb* =0.25-0.8, Ti/ Ti*= 0.3-0.74, Ta/Ta* = 0.33-1.0 and Eu/Eu*0.67-0.89).

Enrichment of the LILE is generally attributed to the addition of these fluid mobile elements by fluids derived from the dehydration of the subducted slab (e.g. [11]), while depletion of the generally fluid-immobile HFSE is thought to reflect a pre-existing depletion within the mantle wedge (e.g. [5,2]).

Pe-Piper et al. [8] suggested that negative Ba, Sr, Eu, and P anomalies indicate partial melting or fractionation under crustal conditions in the plagioclase stability field.

Besides, the increase in La/Sm with increasing SiO₂ content, together with the absence of a corresponding trend for Sm/Yb vs. SiO₂ (Not shown) , support the presence of amphibole as a residual phase in the source of the magmas [6]. Hence we conclude that the studied intrusions were generated in the plagioclase and amphibole stability field and outside the garnet field.

The initial isotopic signatures range from εNd(47Ma)=−4.77 to−5.89 and 87Sr/86Sr(i)=0.7074–0.7069 for acidic rocks and εNd(47Ma)=−3.04 to−4.06 and 87Sr/86Sr(i)=0.7063–0.7067 for basic-intermediate rocks. These characteristic along with geochemical evidence such as, Zr/Nb (~10) and Nb/La (~0.52) ratios indicate that these ratios are between both crust and mantle composition.

These characteristic indicate that this intrusion are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. As, the acidic rocks are more crust composition than intermediate- mafic rocks.

Further experimental evidence [9,10,12] suggests that dehydrating basaltic material within the lower crust can produce significant volumes of mafic partial melts, particularly in regions with high heat flow. These studies have shown that melting conditions (water-saturated vs. dehydration melting, water fugacity) strongly influence the composition of the melt and of the residuum during partial melting of basaltic compositions at lower crustal pressures.

According to the geochemical features of the Kuh-e Dom intrusion, it can be deduced that the source involved in the genesis of these granitoid was a mantle source and lower crust carrying a subduction-related geochemical signature. So mafic magmas were probably derived from the upper mantle, and transferred both heat and material to the lower crust, thus producing granitic magmas by partial melting.

The Th/Ta ratio, another useful geochemical tool, remains constant during partial melting and crystallization, as these two incompatible elements have similar behaviour during igneous processes. This ratio thus is used to characterize both source materials and tectonic environment of the magmas [4]. Low Th/Ta ratios (2.1) are characteristic of extensional tectonic areas, such as spreading ridges or intra-plate magmatic environments, while higher Th/Ta ratios of > 6 are thought to be related to convergent- plate tectonic environments, e.g., subduction zones. In the studied intrusion, this ratio varies between 10-14 which is characteristic of a volcanic arc setting. The Ba/Nb ratios related to active continental margin magmatism are greater than 28 [3], because the high field strength elements (e.g., Ta, Nb, Ti) are retained in the subducting slab, whereas low field strength elements (e.g., Rb, Sr, K, Ba) are easily transported to the overlying zone of mantle melting [1,7]. Thus, it seems that the high Ba/Nb values of the studied samples (~30) are characteristics of active continental margin magmatism. Furthermore, The REE and primitive mantle- normalized multi-element patterns and tectono magmatic discrimination diagrams, together with geochemistry data suggest that Kuh- e Dom intrusion were formed in a continental arc magmatism.

In summary, the petrological and geochemical data imply that at least two distinct magma bodies contributed to the genesis of the plutons. Underplating and intrusion of the mantle-derived hot basic magma into an already thickened continental crust could result in the dehydration melting of the lower crust. So, mantle-derived mafic magma entranced to lower crust-derived acidic magma so that create hybrid magma.

Consequently, the interaction process played an important role in the genesis of the hybrid granitoid bodies, which subsequently underwent a fractional crystallization process along with slightly amounts of crustal assimilation.

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