Petrology of Oligocene Lalehzar igneous rocks in the southeast of Saveh-Naein-Jiroft magmatic belt

زمین شناسی اقتصادی

Volume 16, Issue 4 - Serial Number 43, 2024, Pages 21-45   

Document Type: Research Article

DOI: 10.22067/econg.2024.1124

Authors

Fatemeh Sepidbar^{* 1}; Bahareh Borouzi Niyat²

¹Assistant professor, Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran

²Ph.D. student, Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran

Abstract

Lalehzar igneous rocks are located in the southeast of Iran, the SE of Saveh-Naein-Jiroft magmatic belt. In this study, zircon U-Pb dating, and whole-rock geochemistry and Sr-Nd isotopic analyzes were performed on granitoid rocks (granite and granodiorite) and associated volcanics. The granodiorites and granite include quartz, plagioclase, alkali feldspar, biotite and amphibole with different percentages. New zircon U–Pb ages show granitic magmatism at ~28 Ma, followed by emplacement of granodiorite at ~24 Ma. Both granitoid and volcanic rocks are characterized with depletion in Nb and Ti and enrichement in large ion lithophile (LILE) and light rare earth elements (LREE). Their calc-alkaline arc geochemical signature (low Sr/Y ratio of almost <55), negative to positive Eu anomalies (Eu/Eu^∗⁠ =0.6–1.3) and the negative to positive εNd (t) values of -1.4 to +0.27 offer formations in an orogenic belt.  The geochemistry and isotopic results show that they are formed during partial melting of the lower crust (amphibolite) which is impelemented by inherited mantle source components in a subduction zone.
 
Introduction
The magmatism at the convergent margins changes in response to processes related to the subduction zone. For example, most magmatic systems are associated with typical calc-alkaline to adakitic magmatic rocks, respectively (Cooke et al., 2005). In this research, we study the Oligocene magmatic rocks of Lalehzar in the southeastern part of the Saveh-Naein-Jiroft belt based on new geological, geochemical and isotopic data in order to obtain more information about the Oligocene magmatism of southeastern Iran.
 
Research method
About 100 samples were sampled from namerous Lalehzar intrusive and volcanic outcrops
 Afterward, 50 granitoid and volcanic rocks were selected for mineralogical studies. 12 samples of volcanic and granitoid rocks that had little alteration were selected for major and minor elements analyzes. Major oxides and trace/minor elements were analyzed by XRF and PQ2 Turbo ICP-MS methods at Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) in Beijing.
Two samples of granitoids were selected for zircon U-Pb dating. Analyzes were done by an Agilent 7500a quadrupole ICP-MS and a Thermo-Finnigan Neptune multi-collector connected to a 193nm Excimer ArF laser-ablation system at the IGGCAS laboratory in Beijing, China.
Four volcanic and granitoid rocks were chosen for Sr and Nd isotopic analysis at the IGGCAS laboratory in Beijing, China. Sr, Rb, Sm and Nd isotopes were measured by a Thermo Fisher Scientific Triton Plus multi-collector thermal ionization mass spectrometer (TIMS). During the analysis process, the Sr-Nd isotopic ratios were corrected for mass separation to ⁸⁸Sr/⁸⁶Sr=86Sr=375209 and Nd=¹⁴⁴Nd=0146/7219.
 
Geology setting
Regional geology
The Saveh-Naein-Jiroft magmatic belt mostly consists of alkaline to calc-alkaline igneous rock, which were formed during subduction of the Neotethys ocean beneath central Iran (Berberian and King, 1981). Magmatism in the Saveh-Naein-Jiroft magmatic belt initiated in the late Paleocene, followed into the late Cenozoic, with a magmatic flare up in the Middle Eocene (Verdel et al., 2011). Southeast of Saveh-Naein-Jiroft, is known as the Dehj-Sardouyeh magmatic belt, mainly comrisis of abundant calc-alkaline to adakitic igneous rocks, which consists of Late Eocene to Late Miocene granitoid rocks (Asadi et al., 2014). The Lalehzar volcanic rocks include lavas, breccias, tuffs and agglomerates that are exposed around the villages of Lalehzar to Bezenjan. The Oligocene calc-alkaline granitoids are intruded in the Eocene volcanic rocks with the northwest-southeast trending.  
 
Field observations and petrography
The Lalehzar magmatic complex is exposed in the southeast of Saveh-Naein-Jiroft (120 Km²). The oldest rocks include Late Cretaceous limestone-marble and sandstone, which are overlain by Eocene andesite and dacite. The andesites and dacites are intruded by Oligocene granitoids. Granitoids include granodiorite and granite, which are outcropped as stocks. Granodiorites have a granular texture with main minerals of quartz (20-25%), plagioclase (35%), alkali feldspar (30-35%), biotite and amphibole (5-10%), whereas, magnetite and zircon are the main subordinate minerals (Figure 3B). Granite is less abundant than granodiorite and has a granophyric texture (Figure 3C). The principal phenocrysts include alkali feldspar (30-40%), quartz (20-25%), plagioclase (40 %), biotite and amphibole (5-4%). Andesites have a porphyry texture with main phenocrysts of feldspars, biotite and amphibole, set in a matrix composed of plagioclase microlites. They also include mafic microgranular enclaves, size of 2 to 5 cm, that show a curved shape (Figure 3D). The dacite consists of plagioclase, biotite and quartzset in fine-grained groundmass. Due to the proximity of this unit to intrusive rocks, evidence of contact metamorphism is observed.
 
Whole-rock geochemistry
The volcanic rocks show SiO₂ in ranges of 52.5 to 67.5 wt.%, Al₂O₃ in ranges of 15.8 to 19.3 wt.%, K₂O in ranges of 1.4 to 5.4 wt.% and of K₂O/Na₂O in range of 0.4 and 1.2.  The granitoids are characterized with SiO₂ in ranges of 61.5 to 68.4 wt.%, Al₂O₃ in ranges of 15.1 to 17.2 wt.%, K₂O in ranges of 1.2 to 5.2 wt.% and K₂O/Na₂O in range of 0.35 and 1.94. In the TAS (Middlemost, 1994) diagram, intrusive plot in granite and granodiorite, whereas volcanic rocks lie in andesite, and dacite fields in the Nb/Y vs. Zr/TiO₂ diagram. Both granitoid and volcanic rock are characterized by enrichment in Rb, Ba, Th, U and depletion in Nb, Ti and P in the primitive-mantle normalized multi-element diagram (Sun and McDonough, 1989). They also show enriched in light rare earth elements (LREEs) with respect to heavy rare earth elements (HREEs) in chondrite-normalized REE patterns (Sun and McDonough, 1989), with the Eu/Eu* ratios from 0.6 to 1.03.
 
U-Pb dating of zircon
 The zircons from granodiorite and granite are euhedral to subhedral grains with length of 100 to 250 µm. Oscillating zoning, Th/U ratio (0.31 to 1.63), depletion in LREE along with negative Eu anomalies point to magmatic origin (Belousova et al., 2002). Concordia diagram and the best age obtained from U-Pb data were shown in Figure 6. Based on the analysis, the average ages were 24.6 and 28.1 Ma for granodiorite and granite, respectively.
 
Sr-Nd isotopic study
The (⁸⁷Sr/⁸⁶Sr)i and (¹⁴³Nd/¹⁴⁴Nd)i (t=25Ma) of granitoids range from 0.70540 to 0.70522 and 0.51252 to 0.51260, respectively. While the volcanic rocks are characterized by those values of 0.70542-0.70612 and 0.51263-0.51253, respectively. The εNd (i) values varies from +0.27 to -1.4, plot in the enriched quadrant of the Nd-Sr isotopic diagram (Hou et al., 2011).
 
Discussion
U-Pb dating indicates that plutotism of the area occurred in the Middle to Late Oligocene (24-28). These rocks are depleted in Nb and Ti and enriched in light rare earth elements and large ion lithophile elements. Their geochemistry with typical calc-alkaline magmatism (Sr/Y ratio is low ~55), negative Eu anomalies of 0.6-1.3 Eu/Eu*, and enrichment in HFSE and radiogenic Sr isotope values ​​indicate their formation in the subduction zone. Eu/Eu* ratios, negative to slightly positive Eu anomalies and non-depletion in HFSE and initial values ​​of ⁸⁷Sr/⁸⁶Sr are similar to other Oligocene magmas of Saveh-Naein-Jiroft magmatic belt. The geochemistry and age of magmatism show that partial melting of the lower crust (amphibolite) with a low contributions of inherited mantle source in a subduction zone may has role in the genesis of Lalehzar igneous rocks.

Keywords

Subduction; calc-alkaline; partial melting; Lalehzar; Saveh-Naein-Jiroft

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