Petrography, geochemistry and dating of the Seydal area granitoid bodies (South-east of Birjand), Southern Khorasan

Document Type : Original Article

Authors

1 Department of Geology, Faculty of Sciences, University of Birjand, Birjand, Iran

2 Department of Geosciences, National Taiwan University, Taipei, Taiwan

Abstract

Introduction
The Seydal granitoid bodies is located in eastern Iran on the western boundary of the Sistan suture zone. This zone represents remnants of the lithosphere of an oceanic basin that formed through processes of continental collision and ocean closure. Numerous granitoid intrusions with varying ages, ranging from the Cretaceous to the Eocene, have been identified in this zone. The Seydal granitoid, situated near the village of Seydal, approximately 150 km southeast of Birjand, was previously described in studies as a plagiogranite, gneiss, and leucogranite with an Upper Cretaceous age. Based on new evidence suggesting a younger age for this intrusion, this study aims to investigate the petrography, geochemical analysis, and geochronology of the Seydal granitoid.
 
Materials and Methods
The Seydal region lies in the northwestern part of the Sistan suture zone. This zone is characterized as an accretionary complex formed by the subduction of the Sistan oceanic lithosphere. The ophiolitic complex of the area, which consists of peridotite, gabbro, and basalt units, was intruded by magmatic activities during the Late Cretaceous and Early Eocene. The most recent magmatic activity in the region includes alkaline basaltic volcanism, which occurred during the Miocene to Quaternary period. Based on studies, the lithological units in the Seydal region are classified into four main groups:

Regional metamorphic rocks of Cretaceous age, including slate, phyllite, schist, and amphibolite, primarily exposed in the southern part of the area.
Ophiolitic mélange complex of Cretaceous age, consisting of peridotite, gabbro, and basalt units.
Clastic and carbonate sedimentary units of Cretaceous (deep marine) and Eocene (shallow marine) age, including shale, sandstone, and carbonate rocks, which are widespread in the northern region and overlie the ophiolitic complex.
Seydal granitoid intrusion, which is approximately 19 km in length, with a NW-SE orientation, and intrudes into the ophiolitic and metamorphic units of Cretaceous age.

The Seydal granitoid is further subdivided into three main sections:

Granodiorite section, which forms the largest unit of the intrusion and accounts for approximately 99% of its volume.
Monzogranite section, occurring as small bodies along the boundary between the granodiorite and ultramafic units.
Syenogranite section, appearing as a small intrusion in the northern part of the area near the boundary with ultramafic and granodiorite units.
This study began with a review of previous research and the analysis of satellite imagery, including ASTER, Sentinel-2, and Landsat-8. During fieldwork, 230 rock samples were collected from the area, and 123 samples were selected for the preparation of thin sections for petrographic analysis in the laboratory. Based on petrographic observations, 10 fresh samples (free of alteration and weathering) were selected for geochemical analysis. These samples were crushed and powdered before being sent to SGS Canada for chemical analysis using ICP (for major elements) and ICP-MS (for trace elements). The data were analyzed using GCDkit v5 software, and the regional geological map was prepared using ArcGIS v10.5.
To determine the precise age of the granodiorite unit, a sample was sent to the Institute of Geology and Geophysics in Beijing for zircon separation. Zircons were separated using heavy liquid and magnetic methods and then transferred to the National Taiwan University. A total of 57 large, euhedral zircon grains were embedded in epoxy and polished to a thickness of 20 µm. Cathodoluminescence (CL) images were obtained to identify zones suitable for laser ablation. Uranium-lead (U-Pb) zircon dating was performed using the LA-ICP-MS technique with an Agilent 7500 LA instrument. The results were analyzed to construct concordia diagrams for geochronological interpretation.
 
Results and Discussion
In the petrographic study of the granitoid bodies in the Seydal region, the rocks are classified into granodiorite and granite (monzogranite and syenogranite) based on modal analysis. The predominant texture in these rocks is granular, with additional textures such as myrmekitic, graphic, and perthitic also observed. These textures may indicate simultaneous growth from a melt, interaction between solid and melt phases, or immiscibility between two solids.
Types of Granitoid Bodies and Petrographic Characteristics

Granodiorite

Granodiorite represents the largest granitoid unit in the region and is characterized by its granular texture, medium grain size, and leucocratic nature. Poikilitic and myrmekitic textures are also observed in this unit. The primary mineralogical composition includes quartz, plagioclase, and potassium feldspar:

Plagioclase: Occurs as euhedral to subhedral crystals with albite twinning, predominantly of oligoclase and andesine types, with partial sericitization in some cases.
Potassium Feldspar: Includes orthoclase and microcline, displaying Carlsbad twinning, and is slightly sericitized.
Quartz: Appears as anhedral to subhedral crystals with sizes ranging from 1 to 3 mm.
Ferromagnesian Minerals: Comprise about 10% of the rock volume and include green biotite and hornblende.

Accessory minerals include sphene, zircon, and apatite. Sphene is observed as euhedral brown crystals, while zircon and apatite are seen as inclusions in other minerals. Secondary minerals include chlorite, epidote, and clay minerals.

Monzogranite

The predominant texture of monzogranite is granular, with additional textures such as granophyric and myrmekitic appearing in some samples. The primary mineral composition includes:

Plagioclase: Comprises 30-35% of the rock by volume.
Potassium Feldspar: Predominantly orthoclase, occasionally microcline, with 30-35% volumetric abundance, typically anhedral.
Quartz: Accounts for 20-25% of the rock by volume.
Ferromagnesian Minerals: Includes biotite (1-3%) and muscovite (~8%).

In some samples, potassium feldspars have been altered to clay minerals, plagioclase to sericite, and ferromagnesian minerals to chlorite and epidote. Accessory minerals include apatite and zircon.

Syenogranite

Syenogranite exhibits a predominantly granular and coarse-grained texture, with granophyric and graphic textures also present. The main mineralogical composition includes:

Potassium Feldspar: Comprising 50-60% of the rock, primarily orthoclase with occasional microcline.
Quartz: Anhedral crystals with undulatory extinction, making up 20-30% of the rock.
Plagioclase: Represents 10-15% of the rock, predominantly oligoclase.

Ferromagnesian minerals include muscovite (3-8%) and biotite (<1%). Garnet crystals, a distinguishing feature of S-type granitoids, are also observed in this unit.
Mafic Enclaves in the Granitoid Bodies

Mafic Microgranular Enclaves

These enclaves are the most abundant type within the granodioritic bodies and mainly consist of plagioclase, hornblende, and quartz. Their sizes range from a few centimeters to 1 meter and are generally finer-grained compared to the host granitoid. Their mineralogical composition is similar to that of the granodiorite, but due to lower degrees of fractionation, they contain higher amounts of mafic minerals and less quartz. These enclaves likely originated from early-crystallized portions of the granitoid magma, which were subsequently transported to higher levels during the magma intrusion process.

Mafic Xenoliths

Mafic xenoliths represent fragments detached from gabbroic bodies during the ascent of the granitoid magma. These xenoliths range in size from 2 to 10 cm and exhibit distinct boundaries with the host rock. Their main constituents are pyroxene and plagioclase, which have undergone extensive alteration and saussuritization, resulting in a fine-grained texture.
Geochemical Characteristics of the Granitoid Bodies
Geochemical analyses using ICP and ICP-MS methods reveal that the granitoid samples from the Seydal region fall within the compositional range of granodiorite, monzogranite, and syenogranite. Granodiorites belong to the calc-alkaline series, while monzogranites and syenogranites are part of the potassium-rich calc-alkaline series.
Major and Trace Element Composition
The granodiorites exhibit metaluminous characteristics, whereas monzogranites and syenogranites are peraluminous. Analysis of rare earth elements (REE) using chondrite-normalized and mantle-normalized spider diagrams indicates enrichment in light REEs (LREEs) and relatively lower enrichment in heavy REEs (HREEs) for granodiorites. Positive anomalies in Rb, Th, and Ce, coupled with negative anomalies in Nb, Ti, and Ba, suggest processes of crystal fractionation and partial melting in subduction-related tectonic settings.
Geochemical Diagrams Analysis
Granodiorites of the region display characteristics consistent with arc-related granitoids associated with subduction zones. Negative Nb anomalies are attributed to crystal fractionation of amphibole, titanite, and rutile. Conversely, monzogranites and syenogranites show negative anomalies for Ba, Sr, and Ti and positive anomalies for Rb, Th, and La, confirming their crustal origin.
Magmatic Origin and Tectonic Setting
Geochemical data suggest that granodiorites were derived from partial melting of amphibolitic rocks, whereas monzogranites and syenogranites resulted from partial melting of pelitic sediments. This difference in magmatic origin explains the variations in their geochemical compositions. Granodiorites exhibit characteristics of I-type granitoids, while monzogranites and syenogranites align with the S-type granitoid classification.
Geochronology
To determine the crystallization age of the granodiorite body, a sample was subjected to zircon U–Pb dating. Zircons were separated using heavy liquids, and cathodoluminescence (CL) imaging revealed magmatic zoning within the zircon grains. The dating results indicate a crystallization age of 54.3 ± 0.7 Ma (early Eocene), which is younger than the previously assumed Cretaceous age. A U/Th ratio of less than 1 in the zircons confirms their magmatic origin.
 
Conclusion
The Seydal granitoid bodies, comprising granodiorite, monzogranite, and syenogranite, exhibit geochemical and tectonic characteristics associated with post-collisional active continental margins. These bodies formed after the closure of an oceanic seaway and the emplacement of oceanic lithosphere onto the continental margin, through processes of partial melting and crustal assimilation.
Granodiorites are derived from mafic sources (metabasalt), while monzogranites and syenogranites originated from pelitic sediments. Geochemical, petrographic, and geochronological evidence collectively highlights the Seydal granitoid body as an example of a post-collisional granitoid system, providing significant insights into the tectonomagmatic evolution of the region.

Keywords

References

 
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