2022 April LIP of the Month

Alborz Large Igneous Province (Alborz LIP): A new LIP and a robust candidate for the Late Ordovician mass extinction (LOME)

Morteza Derakhshi 1, Richard E. Ernst 2,3, Sandra L. Kamo 4

1 Department of Geology, Urmia Branch, Islamic Azad University, Urmia, Iran

e-mail: mortezaderakhshi78@yahoo.com

2 Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada

3 Faculty of Geology and Geography, Tomsk State University, 36 Lenin Ave, Tomsk 634050, Russia

4 Jack Satterly Geochronology Laboratory, Department of Earth Sciences, University of Toronto, 23 Ursula Franklin St., Toronto, ON M5S 3B1, Canada

Extracted and modified from:

Derakhshi, M., Ernst, R.E., Kamo, S.L. (2022). Ordovician-Silurian volcanism in northern Iran: Implications for a new Large Igneous Province (LIP) and a robust candidate for the Late Ordovician mass extinction. Gondwana Research. 107, 256-280. https://doi.org/10.1016/j.gr.2022.03.009

The Late Ordovician mass extinction (LOME), as the first global extinction and the second-largest one of the ‘‘big five”, is typically considered to have had two distinct pulses. In recent years, a growing body of chemostratigraphic records (such as mercury and Hg/TOC anomalies and isotopic carbon excursions) suggests that multiple phases of an obscure Large Igneous Province (LIP) were the driver of climatic and environmental changes that ultimately led to the LOME.

Derakhshi et al. (2022) provides a body of evidence to support northern Gondwana as the birthplace of a new LIP, with the name Alborz LIP, which can be considered as the main cause for triggering environmental and climate changes during the Middle Ordovician to early Late Silurian.

Northern Iran experienced a period of widespread and intense rift-related volcanic activity during the Middle Ordovician to Silurian (Fig. 1). This chain of volcanic events was in relation to the process of initial continental rifting and break-up of the northern margin of Gondwana, which led to the creation of the Paleotethys Ocean.


Fig. 1. Distribution of Ordovician-Silurian igneous rocks in Iran.

Derakhshi et al. (2022) based on fieldwork, relative ages, and high precision U-Pb ID-TIMS dating results have documented seven major magmatic phases of mostly short duration, including four phases (P1 to P4) for the Middle to Late Ordovician and three phases (P5 to P7) for Silurian. Most of these magmatic phases are basaltic (to andesitic) and in some places reach over 1000 m in total thickness (Table 1). These volcanic rocks are generally alkaline or transitional to mildly alkaline in nature and were erupted in a continental within-plate (or intraplate) setting. In addition, geochemical data from mafic rocks of northern and Central Iran on discrimination diagrams of Pearce (2008; Figs. 2a & b) as well as LIP printing diagram of Pearce et al. (2021; Fig. 2c) demonstrate an OIB pattern similar to many typical continental LIPs.

Table 1. Summary of information and data for studied volcanic events and their samples.

Volcanic event

Sample No.

Structural zone/subzone

Rock type

Thickness

(m)

Volcanic structure/state

SMBC (Cheshmeh-Seyed Valley)

-

Eastern Alborz

Basaltic

1176

Massive, columnar, agglomerate, tuff

SMBC (Khoshyeilaq area)

-

Eastern Alborz

Basaltic

>832

Massive, columnar, agglomerate, tuff,  pillowed

Mevli basalts

-

Alborz-Azarbaijan

Basaltic

>500

Massive, agglomerate, apophysis, dyke, sill

Mevli rhyolites

MD19-IKR

Alborz-Azarbaijan

Rhyolitic

>700

Massive, agglomerate, tuff

Buzhan & Rud basalts

MD19-NMB

Binalud

Basaltic

ca. 300

Massive, columnar, agglomerate, apophysis, dyke, pillowed?

Pelmis Gorge basalts

-

Binalud

Basaltic

223

Massive, agglomerate, pillowed

Robate-Gharabil basalts

-

Binalud

Basaltic

ca. 40

Massive, agglomerate, dyke

Dasht basalts

MD19-DHT

Binalud

Basaltic

ca. 100

Massive, agglomerate

Kolur Valley basalts

-

Western Alborz

Basaltic

>300

Massive, agglomerate, dyke


Fig. 2. Using (a) Th/Yb-Nb/Yb and (b) Ti/Yb-Nb/Yb diagrams (after Pearce, 2008) and (c) the LIP printing diagram designed by Pearce et al. (2021) to show the source of magmatic rocks of northern and Central Iran and their relationships with the LIPs. Data sources for Ordovician-Silurian basaltic rocks of the Soltan Maidan (Derakhshi, 2014; Derakhshi and Ghasemi, 2015; Derakhshi et al., 2017), Mevli (Valinasab Zarnagh et al., 2021), and Binalud Mountains (Arbabi Tehran et al., 2010) are from northern Iran. The other two published datasets for mafic rocks of the Shirgesht area (Derakhshi et al., 2006; Ghasemi and Derakhshi, 2008) and Jalal Abad mafic complex (Vesali et al., 2020) from Central Iran Zone are also shown for comparison.

The chain of volcanic events in northern Iran began in the earliest Darriwilian (i.e. phase P1, 468.70±0.30 Ma, early Middle Ordovician, Figs. 3-5) and a few million years before the beginning of a global sharp drop in seawater 87Sr/86Sr ratio. An approximate coincidence between the onset of the basaltic volcanism/weathering in phase P1 and the significant global decline in seawater 87Sr/86Sr during the Darriwilian Stage may be a sign of the early stages of the formation of the Alborz LIP.


Fig. 3. Schematic lithostratigraphic columns illustrating Ordovician to earliest Silurian sections of the studied areas from northern Iran. The vertical scale (in Ma) is from Goldman et al. (2020). The relative ages of stratigraphic units are from: (1) = Berberian and Hamdi, 1977*a, (2, 3) = Ghavidel-Syooki and Khandabi, 2013*b, (4) = Ghavidel-Syooki et al. 2011*c; Derakhshi, 2014*d; Mahmoudi et al. 2014*e; Derakhshi et al. 2017*f, (5) = Derakhshi et al. (2022), (6) = Ghavidel-Syooki and Borji, 2018*g, (7) = Ahmadzadeh-Heravi, 1983; Ghavidel-Syooki, 2000*h, (8) = Afshar-Harb, 1979; Ghavidel-Syooki, 2000, 2017a*i, 2017b; Ghavidel-Syooki and Winchester-Seeto, 2002*j), (9) = Derakhshi et al. (2022).


Fig. 4. (a) A general view of Buzhan & Rud valleys in the Binalud Mountains. (b) A view of some parts of the basaltic lava flows from the Buzhan Valley and (c) a photograph of well-developed columnar jointing from the Rud Valley. These basaltic rocks belong to the first phase of volcanism (i.e. phase P1) of the Alborz LIP.


Fig. 5. U-Pb concordia diagrams showing zircon and baddeleyite results from the (a) Buzhan basalts (phase P1), (b) Mevli rhyolites (phase P3), and (c) Dasht basalts (phase P4).

The enormous volcanic eruptions in phase P3, with more than 1200 m of basaltic-rhyolitic lava flows and an age between the Sandbian-Katian boundary and 450.61±0.27 million years ago (Figs. 3 & 5), can be considered to mark the beginning of an increase in volcanic intensity. The occurrence of this huge bimodal volcanic activity may have been a prelude to the main phase of volcanism and also profound global environmental and climate changes of the Late Ordovician.

Evidence indicates that the climax of volcanism, with at least two very short stages of intensive basaltic eruptions in phase P4, occurred during the late Katian to around the Ordovician-Silurian boundary (Figs. 6 & 7). As shown in Fig. 8, this phase of volcanism coincides with the globally recorded Hg and Hg/TOC anomalies as well as the Hirnantian isotope carbon excursion (HICE).


Fig. 6. (a) A general view of Soltan Maidan Basaltic Complex (SMBC) and (b) a view of the lower contact of this basaltic complex with the Ghelli Formation from the Cheshmeh-Seyed Valley. (c) The contact between the SMBC and Lower-Middle Devonian Padeha Formation in the Meighan Valley. (d) A general view of the lower part of SMBC in Khoshyeilaq Valley. (e) Ordovician-Silurian boundary within the Kh2 interlayer and (f) pinkish granitic clasts (of phase 5) within the conglomerate interlayer of Kh3 in the Khoshyeilaq area. It should be noted that SMBC, from the Alborz Mountain Range, is the most important and voluminous remnant of Ordovician-Silurian volcanic events in all of northern Iran. Moreover, four phases of the Alborz LIP, including P4 to P7, in different sections of this complex have been identified by Derakhshi et al. (2022).


Fig. 7. Lithostratigraphic sections of the SMBC from Cheshmeh-Seyed Valley and Khoshyeilaq area (modified from Derakhshi, 2014, and Derakhshi et al., 2017). The white symbols, e.g., P7, show the volcanic phases. Palynomorph data for the Khoshyeilaq section adapted from Ghavidel-Syooki et al. (2011) and for the Cheshmeh-Seyed Valley section from Derakhshi (2014) and Derakhshi et al. (2017).


Fig. 8. A schematic comparison between main phases of Ordovician to earliest Silurian volcanism from northern Iran and carbon isotope curve (from Cramer and Jarvis, 2020) as well as notable Hg and Hg/TOC spikes documented by Jones et al. (2017), Gong et al. (2017) and Smolarek-Lach et al. (2019). VPDB: Vienna PeeDee Belemnite δ13C standard, GICE: Guttenberg Isotopic Carbon Excursion, HICE: Hirnantian Isotope Carbon Excursion. The vertical scale (in Ma) references as in Fig. 3.

This chain of volcanism continued with at least one granitic (phase P5) and two basaltic phases (of P6 & P7, Fig. 7) during the Silurian period and finally ended about 425 million years ago.

The results of Derakhshi et al. (2022) on the widespread and pulsatory volcanism of northern Iran can be a stimulus to think more deeply on a highly probable link between intense rift-related volcanism of the Paleotethys Ocean and the LOME. These results suggest that the magmatic rocks of northern Iran, and other related places, are the main remnants of the Alborz LIP. It appears that the high volume and very short duration of multiple pulses of Late Ordovician volcanism of this newly discovered LIP had enough potential for significant environmental and climate changes that ultimately led to the end-Ordovician mass extinction.

For full details see Derakhshi et al. (2022).

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