Petrogenesis and tectonic setting of Aftabru-Qlichkandi High-K metaluminous intrusive bodies (South of Buin-Zahra): Evidences from Nd-Sr isotopic Data

Granites are the most important components of the continental crusts. As an important part of the Alpine-Himalayan global belt and the result of the Tethys evolutionary cycle, the Urmia-Dokhtar Magmatic Arc (UDMA) has formed during different magmatic periods. The most important magmatic episode of UDMA igneous rocks, which is the result of lithospheric extenssion and extensive magmatism, occurred during 55 to 37 Ma (Moghadam et al., 2015). In order to enhance our understanding of tectonomagmic evolution of the continental crust during this period, in this research, the intrusive masses of Aftabru and Qlichkandi will be investigated using geochemical data and the isotopic composition of neodymium and strontium. The mentioned intrusive masses are located in the southwest region of Buin Zahra in Central Iran zone.

Urmia-Dokhtar magmatic arc with Cenozoic intrusive and Eocene-Quaternary extrusive rocks shows different levels and rock outcrops in terms of time of origin and erosion rate, same what is seen in the subduction arc of the Andean continental margin. The lithospheric stresses caused by the interaction of the African-Eurasian-Indian lithosphere led to the emergence of Paleogene extensive magmatic activity and a magmatic flare-up lasting 30 Myrs during Eocene and Oligocene. As a result, more than 4 km of Paleogene igneous rocks formed in Saveh, Zarandiyeh, and Tafresh regions. In the south of Bouin Zahra region, pyroclastic outcrops and Eocene lava with a width of about 5km2 and 10 km2 are found in Aftabru and Qlichkandi areas, respectively.

After field observations and detailed textural and petrographic studies, 12 suitable samples with minimal weathering and alteration were selected from intrusive rocks and analyzed by XRF and ICP-MS methods for major, trace and rare earth elements. 6 whole rock samples were analyzed for Sr-Nd isotopes.

Petrography:

In Aftabru and Qlichkandi areas, quartz monzonites intruded the lower-middle Eocene volcanic and pyroclastic rocks.

Petrological observations show that the Aftabru intrusion contains 7-16 Vol% quartz, 25-30 Vol% K-feldspar, 39-54 Vol% plagioclase, 5-10 Vol% pyroxene, 8-15 Vol% amphibole, as well as 1 Vol% accessory minerals.

Qlichkandi:

The medium-grained Qlichkandi intrusive rocks with granular texture, composed of quartz 9-15 Vol% quartz, 25-28 Vol% K-feldspar, 35-45 Vol% plagioclase, 1-5 Vol% pyroxene, 5-10 Vol% of the common mafic mineral of amphibole, 5 Vol% biotite, and less than 1 Vol% accessory minerals.

Based on new geochemical and isotopic data, we will investigate the tectonic location, genesis and magmatic processes affecting the parental magma and the possible source rock of the intrusive masses in the south of Bouin Zahra region.Tectonic-magmatic zone:As  the pattern of rare earth elements and the spider diagrams of enrichment in LILE and LREE elements and depletion of HFSE and HREE elements display, the most important characteristic of intrusive rocks in the studied area is their similarity to continental margin arc rocks.   Generation and magmatic processes:Some incompatible trace elements ratios, such as Y/Nb, Nb/Ta and Nb/La that are less affected by diffrentiation are good indicators for investigation of the magma origin and the crustal contamination effect on the magma. The higher amounts of Y belong to crustal melts or impregnation with crustal materials, and the higher amounts of Nb belong to melts derived from the mantle. In the studied intrusive rocks, Y/Nb ratio is 1.6 on average with a range of 0.6-3.6, which probably indicates mantle with crustal mixing in the magama origin. The Nb/La value is 0.1 in primary mantle and 0.46 in the crustal rocks (Morata et al., 2005), it is about 0.86 on average and equals to the range of 0.1-0.52 in the intrusive rocks of the southern region of Buin Zahra (Qlichkandi, Aftabru). This value indicates a mantle origin for the studied rocks. The Nb/Ta value in mantle rocks is 17.5 and in crustal rocks it is equal to 11-12 (Green, 1995). This ratio is 15 on average (ranging from 8 to 8. 26), supporting the mantle origin as well.Neodymium model age (460-550 Ma) and positive ԑNd(t) indicate the Cadomian origin of lithospheric rocks of the Aftbaru region, while the model age of the samples from Qlichkandi region is 0.9. It shows ԑNd(t) less than zero. This difference is probably due to the high magma mixing with crustal materials in Qlichkandi region, which is confirmed by the diagram of ԑNd versus Sr isotope. 143Nd/144Nd ratio for the Aftabru samples is 0.51270-0.51280. But in the Qlichkandi samples, it is 0.51252-0.51242, which is a sign of contamination with the lower continental crust materials and a tendency towards lower crust. 87Sr/86Sr ratio is 0.70472-0.70510 in Aftabru and 0.70631-0.70607 in Qlichkandi samples. Therefore, according to the intrusive rock petrologic diagrams, it shows signs of contamination with the underlying crustal materials.