Diagenetic history and provenance of Devonian terrestrial sandstones at the margin of Gondwana: Padeha Formation, Eastern Alborz, Iran

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Highlights

  • The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean.

  • The Padeha sandstones were sourced by the late Neoproterozoic granitic gneisses of Iran.

  • Further mesodiagenesis was terminated by the Eocimmerian collision.

Abstract

The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean, the details of which are obscured by later collision. A record of the early phase of opening of Paleotethys is preserved in the 600-m thick Padeha Formation, a Middle Devonian terrestrial rift basin succession. The sandstones are quartz arenite, sub-arkose and arkose and contain minimal detrital matrix (~1%). Bulk-rock geochemistry, according to major elements, suggests a source to the sandstones that was dominated by I-type granite, but also, based on trace elements, included small amounts of intermediate or mafic rocks. Low Zr and HREE contents suggest little or no sandstone in the source area and thus low abundance of fine-grained heavy minerals (<1%). The sandstones were sourced by the widespread granitic gneisses of central and northern Iran that formed at the northern margin of Gondwana in the late Neoproterozoic–Ediacarian. Diagenesis of the sandstones is dominated by compaction and fracturing of framework quartz without widespread stylolites in the sandstones, and cementation by silica, carbonates and Fe-oxides. Fractures within quartz grains are filled mostly by silica, but locally by carbonate and authigenic K-feldspar. Albitization of plagioclase, K-feldspar overgrowths and partial dissolution of detrital K-feldspar post-date silica cements and suggest temperatures between 70 and 120 °C. The particular diagenetic assemblage of altered K-feldspar and limited quartz stylolites was a consequence of a high geothermal gradient of ~70 °C/km at a burial depth of <1.5 km. Further mesodiagenesis was terminated by the Eocimmerian collision.

Introduction

The northern margin of the Turkish-Iranian-Afghan segment of Gondwana rifted in the Devonian, initiating the growth of the Paleotethys Ocean (Bagheri and Stampfli, 2008). This ocean closed again in the Cimmerian orogeny in the mid Triassic, and much of the history of this ocean was obscured by this later collision (Bagheri and Stampfli, 2008). A record of the early phase of opening of Paleotethys is preserved in the Devonian basins of Iran.

Previous studies of the Devonian sediments in the Zagros, Alborz and Binalud mountains and Central Iran have focused on descriptive litho- and biostratigraphy (Alavi-Naini, 1993, Ghavidel-Syooki, 1994, Wendt et al., 2005, Ghavidel-Syooki and Owens, 2007), or paleoenvironmental sedimentology and sequence stratigraphy (Aharipour et al., 2010, Zand-Moghadam et al., 2014). There has been little attempt to interpret the regional depositional setting and provenance of the widespread Devonian terrestrial and marginal marine strata, nor to define the thermal conditions accompanying rifting and thus the tectonic style of the opening of Paleotethys in the region.

The main purpose of the present study is to clarify the tectonic conditions associated with the Devonian initiation of the evolution of the Paleotethys by a detailed study of the petrology and geochemistry of the Lower-Middle Devonian Padeha Formation, and thus (1) to identify the provenance of the clastic detritus, from source rocks possibly hidden or eroded; and (2) to interpret the diagenetic history of the formation, including relating the degree of mesodiagenesis to the known burial history of the basin. This will provide new information on the tectonic history the opening of the Paleotethys Ocean in the Alborz during and after deposition of this formation, as well as climate conditions in the source area. For this we use petrographic information from a large suite of sandstone samples, based on scanning electron microscopy and hot-cathode cathodoluminescence microscopy of polished thin sections, along with whole rock geochemical analyses.

Section snippets

Geological setting and stratigraphy

The Neoproterozoic assembly of Gondwana culminated in widespread arc magmatism along its northern margin in the Ediacaran, including northern and central Iran (Bagheri and Stampfli, 2008). In the Paleozoic, at the north margin of Gondwana, a triple junction was established around the Arabian promontory, corresponding to the three branches of Paleotethys (Fig. 1). The Iranian seaway separated the Iranian-Afghan domain from South China, the Sulu-Dabie seaway separated South China from the

Approach and methods

Diagenesis comprises a broad spectrum of physical, chemical and biological post-depositional processes by which original sedimentary assemblages and their interstitial pore waters react in an attempt to reach textural and geochemical equilibrium with their environment (Curtis, 1977, Burley et al., 1985). Thus, during the post depositional history, sandstones undergo modifications of their porosity, mineralogy and chemical composition in various and sometimes complex diagenetic systems (

Detrital petrology

The sandstones range from fine- to coarse- grained, and include some well rounded, coarse-grained components (Fig. 6A, B). On average, most samples are well sorted (47%), with 25% moderately sorted and the remainder poorly to very well sorted, and rarely very poorly sorted.

The dominant mineral constituent is detrital quartz (66.8% monocrystalline, 11.3% polycrystalline). Other detrital minerals include K-feldspar, mostly un-twinned orthoclase (14.2%); plagioclase, mostly albite (4.3%); lithic

Paragenetic sequence

Based on textural relationships, a paragenetic sequence is proposed for the Padeha sandstones (Fig. 14). Textural features are used to assign diagenetic mineral to one of three general diagenetic stages (based on Choquette and Pray, 1970). Eodiagenesis extends from final deposition to burial below the depth at which interstitial water chemistry is largely the result of the surface environment (Schmidt and McDonald, 1979, Salem et al., 2005). Mesodiagenesis represents the subsurface regime

Conclusions

Padeha Formation sandstones, deposited in an Early to Middle Devonian rift basin, are mostly subarkose. The principal source rocks were I-type granitoids, with minor metamorphic rocks seen as lithic clasts. The low Zr content suggests little or no polycyclic sandstone in the source area. The sandstones were sourced by the widespread late Neoproterozoic granitic gneisses that formed juvenile crust at the northern margin of Gondwana.

Eodiagenetic cements include hematite coats on sand grains,

CRediT authorship contribution statement

Mehdi Reza Poursoltani: Conceptualization, Data curation, Writing - original draft. Georgia Pe-Piper: Conceptualization, Methodology, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors thank the Islamic Azad University, Mashhad Branch of Iran, for financial assistance (number 97-S-70-25734). We are grateful to journal reviewers Dr. Gelson Luís Fambrini, and Dr. Udo Zimmermann for their thoughtful and constructive comments, which greatly improved the manuscript. We thank Prof. Martin R. Gibling of Dalhousie University, Canada, and also Prof. David Piper of the Geological Survey of Canada for reviewing the earlier draft, and also would like to thank for technical

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