Calc-alkaline magmatism associated with salt diapirs in the Shurab and Garmsar back-arc areas (Central Basin, Iran): magma genesis and tectonic implications

In terms of whole-rock geochemical analyses and in agreement with the petrographic features, all representative samples of the Shurab area are classified into three groups: group 1 with mainly intergranular texture comprises basalt/trachybasalt, while groups 2 and 3 with trachytic and porphyritic textures, respectively, have basaltic trachyandesite composition.

http://journals.tubitak.gov.tr/earth/ (2018) 27: 294-317

© TÜBİTAKdoi:10.3906/yer-1712-16

Calc-alkaline magmatism associated with salt diapirs in the Shurab and Garmsar back-arc areas (Central Basin, Iran): magma genesis and tectonic implications

Somayeh FALAHATY, Mortaza SHARIFI*, Moussa NOGHREYAN, Homayon SAFAEI, Mohammad Ali MAKIZADEH

Department of Geology, Faculty of Sciences, University of Isfahan, Isfahan, Iran

* Correspondence: sharifi_mortaza@yahoo.com

1 Introduction

Back-arc basalts (BAB) occur behind the main volcanic

arc, where they form a transition from arc to intraplate

basalts (e.g., D’orazio et al., 2004; Kay et al., 2004; Stern,

2004; Ramos and Kay, 2006) Different types of magma

are recognized in back-arc basins (Tarney et al., 1981;

Thompson et al., 1984) Both midocean ridge basalt

(MORB) and ocean-island basalt magmas characterize

oceanic back-arc basins, whereas shoshonites associated

with high-K calc-alkaline and ultrapotassic magmatisms

characterize continental back-arc environments The

basalts from oceanic back-arc settings have transitional

geochemical characteristics between MORB and

island-arc tholeiites, while the basalts from continental back-island-arc

settings exhibit transitional geochemical characteristics between arc and intraplate basalts (Tarney et al., 1981; Saunders and Tarney, 1984; D’orazio et al., 2004; Kay et

al 2004; Stern, 2004; Ramos and Kay, 2006)

Over the last three decades, many studies have focused on one of the most important occurrences

of mafic magmatism in continental back-arc settings Magmas that erupted in continental back-arc regions are particularly interesting as they potentially sample very different physical reservoirs, such as the asthenospheric wedge (supraslab mantle) close and far from the arc, the subslab asthenospheric mantle, the continental crust (both lower and upper), and the continental lithospheric mantle

Abstract: Medium- and high-K calc-alkaline magmatism of the Shurab (southeast Qom city) and Garmsar (northwest Garmsar

city) areas occurred within the Lower Red Formation in the Central Basin behind the Urumieh-Dokthar Magmatic Arc In terms of

whole-rock geochemical analyses and in agreement with the petrographic features, all representative samples of the Shurab area are classified into three groups: group 1 with mainly intergranular texture comprises basalt/trachybasalt, while groups 2 and 3 with trachytic and porphyritic textures, respectively, have basaltic trachyandesite composition The overall major constituents are plagioclase with composition in the range from An49Ab23 to An75Ab47, clinopyroxene with composition in the range Wo43-45En39-45Fs9-15, and olivine with composition in the range from Fo58Fa31 to Fo67Fa40 Minor minerals consist of opaque minerals and K-feldspar in the range Or41-65Ab33-

50 An0.69-8 The characteristic accessories are apatite and sphene In the Garmsar area, rocks are seen as subvolcanic with mafic (basalt) and intermediate (trachybasalt/basaltic trachyandesite) compositions The Garmsar area rocks represent intergranular, granular, ophitic, and subophitic textures In these rocks, major mineral phases are plagioclase and clinopyroxene The minor constituents are olivine, opaque minerals, amphibole, biotite, and quartz Apatite is the most important accessory mineral The rocks of both areas display REE patterns characterized by LREE-enriched and HREE-depleted segments typical of arc lavas Primitive mantle-normalized trace element patterns for samples of both areas exhibit high ratios of strongly incompatible elements with similar bulk partition coefficients (e.g., Th/

Ta and Th/Ce), enrichment in large-ion lithophile elements (LILEs: Cs, Ba, Rb, Th) relative to the high field-strength elements (HFSEs:

Ti, Hf, Zr, and REEs), and troughs for Nb, Ta, Ti, and Zr and peaks for Cs, Th, K, and Sr, all of which are indicators for related magmatism Subduction of the Neo-Tethys beneath the Eurasian margin led to upper mantle deformation and metasomatism Once the Arabian plate collided with the Eurasian margin, subduction ended through a slab breakoff process, and thermal flux of asthenospheric origin uprising through the slab tear induced the thermal erosion of the mantle metasomatized during the previous subduction event and triggered its partial melting Also, the late Eocene-early Oligocene collision of Eurasian with Arabian plates led

subduction-to the subsidence and formation of faults and extensions in the Central Basin (i.e the Shurab and Garmsar areas) such that eruption of medium- and high-K metasomatic magmatism along these faults and extensions caused postcollision volcanism in the Central Basin.

Key words: Shurab and Garmsar areas, Central Basin, mafic and intermediate volcanic rocks, postcollision calc-alkaline magmatisms

Received: 20.12.2017 Accepted/Published Online: 29.04.2018 Final Version: 24.07.2018

Research Article

Back-arc basalts from Patagonia in Argentina from

Paleocene to Holocene in age (i.e southern extra-Andean

Patagonia) behind the Southern Volcanic Zone arc

constitute one of the largest Cenozoic continental basaltic

provinces on earth (Kay et al., 2004) The origin of these

basalts has been related to mechanical perturbations of

the subcontinental mantle as a consequence of subduction

of the oceanic lithosphere below the South American

continental plate (Skewes and Stern, 1979)

In Iran, continental back-arc magmatism occurred

behind the Urumieh-Dokthar Magmatic Arc (UDMA)

from the Eocene to recent age, such as magmatism of the

Shurab (southeast Qom city) and Garmsar (northwest

Garmsar city) areas The present study investigates the

petrogenesis of back-arc magmatism in the Shurab and

Garmsar areas, which are spatially associated with salt

diapirs Our aim is to use the magma geochemistry of the

Shurab and Garmsar areas to evaluate the composition of

the unmodified mantle source and to assess and identify the

responsible agents for the source mantle metasomatism

2 Geological setting

The Iranian Plateau, located in the collision zone between the Arabian and Eurasian plates, is one of the world’s best examples of an early stage of continent–continent collision The basement of the plateau comprises a mosaic

of tectonic blocks, i.e the Sanandaj-Sirjan Zone (SSZ), the Central Iranian Microplate/Microcontinent (also Central-East-Iran Microplate (CEIM)), a basement block known as the NW-Iran Block (Allen et al., 2011), and the Great Kavir Block (Figure 1) (Morley et al., 2009; Allen et al., 2011) The Central Basin, located between the fold-and-thrust belt of the Alborz Mountains and the CEIM, is characterized by a flat-lying topography with occasional low hills It developed on a basement composed of the NW-Iran Block, the Eocene UDMA (northern part of the SSZ), the northwestern part of the CEIM (Morley

et al., 2009; Allen et al., 2011), and the southern part of the Alborz fold-and-thrust belt (Figure 1) The Central Basin, interpreted as a back-arc basin by Hassanzadeh et

al (2002), has remained a poorly documented part of the Iranian Plateau

Figure 1 Simplified tectonic map of Iran with main fault systems (Morley et al., 2009 after

Allen et al., 2011) Positions of the study areas are marked by the numbers 1 (Shurab area) and

2 (Garmsar area).

The Central Basin comprises two subbasins: a NW-SE

trending arm including the Saveh-Qom area and a NE-SW

trending arm east of Qom, near Semnan (Morley et al.,

2009) The Shurab area (southeast Qom city, characterized

by number 1) lies within the NW-SE trending arm and the

Garmsar area (west Garmsar city characterized by number

2) lies within the NE-SW trending arm (Figure 2) (Bouzari

et al., 2013)

3 Geology of the Central Basin

Three main stratigraphic units are present in the Central

Basin: the Lower Red Formation (LRF; Oligocene), the Qom

Formation (late Oligocene-early Miocene), and the Upper

Red Formation (early Miocene-early Pliocene?) (Furrer

and Sonder, 1955; Gansser, 1955; Abaie et al., 1964) The

Central Basin deposits overlie ~3 km of thickness of Eocene

arc volcanics and volcaniclastics with subordinate marine

carbonates and evaporites (Berberian and King, 1981; Bina

et al., 1986) The Eocene sequence unconformably overlies

Cretaceous and Jurassic sedimentary and metasedimentary

rocks The Eocene succession commences with a

basal conglomerate and coarse clastics, followed by a predominantly calc-alkaline volcanic series that dominates the Eocene stratigraphy (Stocklin, 1968) Interbedded with the volcanics and volcaniclastics are limestones (some nummulitic) and evaporites, indicating that the volcanism occurred close to sea level The Eocene section was deformed, uplifted, and eroded prior to deposition of the Oligocene-Miocene sedimentary rocks of the Central Basin (Huber, 1952; Gansser, 1955)

Deposition of LRF lithologies in the Central Basin during the Oligocene was accompanied by episodic magmatism (Jahangiri, 2007), which is the topic of this study During the late Miocene-Pliocene, the salt at the base of the LRF, a halite-dominated evaporite sequence of commonly several hundred meters thick, became unstable and began to move, resulting in the transporting of salt and the igneous rocks associated with salt to the surface (Figures 3a and 3b) (Morley et al., 2009)

Simplified geological maps of the studied Shurab and Garmsar areas in the Central Basin are shown in Figures

4 and 5

Figure 2 Satellite image of the Central Basin Positions of the study areas are marked by

the numbers 1 (Shurab area) and 2 (Garmsar area).

4 Analytical methods

4.1 Whole-rock major and trace element analyses

For bulk rock analyses, secondary veins and alteration

rims were carefully removed by sawing

Bulk rock major elements of the Shurab samples were

analyzed by X-ray fluorescence (XRF) in fused beads at the

Andalusian Institute of Earth Sciences (IACT, Granada, Spain) using a BRUKER D8 Advance XRF instrument equipped with six analyzers (LiF200, LiF220, Ge, PE, PX1, PX2) Within-run precision (% RSD), measured by repeated analyses of USGS reference materials as external standards, is better than 0.5% for all elements except Na, for which it is 1.5%

Figure 3 Igneous rocks associated with salt diapir in (a) the Shurab and (b) the Garmsar areas.

Figure 4 Simplified geological map of the Shurab area in the NW-SE trending arm of the Central Basin, part of the geology map of Aran,

scale 1:100,000 (Amini and Emami, 1996) The study area is marked by a rectangle.

Whole-rock trace elements (Rb, Sr, Y, Zr, Nb, Cs, Ba,

REE, Hf, Ta, Pb, Th, and U) of the Shurab samples were

analyzed with a Triple Quadruple Agilent 8800 ICP-MS

at the IACT Sample digestion was performed following

et al (2000) Element concentrations were determined by

external calibration, except for Hf, which was calculated

using Zr measured by XRF and the chondritic Zr/Hf

ratio The compositions of the granite reference sample

GS-N, analyzed as unknown during the analytical runs,

show good agreement with the working values of this

international standard (GeoReM database: http://georem

mpch-mainz.gwdg.de/)

Major and trace element analyses of samples of the

Garmsar area were made by inductively coupled plasma

atomic emission spectrometry (ICP-AES) and inductively

coupled plasma mass spectrometry (ICP-MS) at Lab West

Laboratory, Australia

4.2 Mineralogical analyses

Mineralogical analyses of the Shurab samples were

conducted with a wavelength-dispersive electron probe

microanalyzer (JEOL JXA-8800R) in the Cooperative

Centre of Kanazawa University, Japan The analyses were

performed under an accelerating voltage of 15 kV and a

beam current of 15 nA Natural and synthetic minerals of

known compositions were used as standards

5 Petrography and mineral chemistry

For this study we selected about 180 rock samples from

several localities close to each other in the Shurab and

Garmsar areas (Figures 4 and 5) and only the least altered

specimens were chosen for whole-rock analysis

5.1 Volcanic rocks of the Shurab area

Rocks of the Shurab area are seen as volcanic with mafic

composition In hand specimen, rocks of the Shurab area

are green and gray Thin-section studies showed three rock

groups: group 1 with mainly intergranular texture (Figure

6a), group 2 with trachytic texture (Figure 6b), and group

3 with porphyritic texture (Figure 6c)

Major mineral phases in the rock samples of groups 1,

2, and 3 include, in decreasing order of abundance, 40%–

50% plagioclase, 30%–40% clinopyroxene, and 10%–15%

olivine as phenocrysts and microphenocrysts of variable

sizes Minor minerals consist of opaque minerals and

K-feldspar Accessory minerals are apatite (as inclusions

in plagioclase and clinopyroxene) and sphene (in the

groundmass) Melt inclusions are seen frequently in

plagioclase, clinopyroxene, and olivine A small number

of samples are vesicular and in such a case that vesicles

are filled by carbonates, quartz, prehnite, and zeolites,

amygdaloidal textures are also found in the rock samples

of groups 1, 2, and 3

Plagioclase crystals are euhedral, twinned according

to albite/albite-Carlsbad, and converted to prehnite in the majority of the cases Rare plagioclase crystals are characterized by oscillatory zoning

The often centimeter-size clinopyroxene phenocrysts are euhedral-subhedral, fresh, larger than olivine, and brown to weakly purplish These minerals generally occur

in glomeroporphyritic aggregates (Figure 6d) and those

in the groundmass are swallow-tailed Clinopyroxene glomeroporphyritic aggregates are formed in the magma chamber and on the intratelluric stage They contain olivine and opaque minerals in some cases

Olivine phenocrysts are mainly subhedral and embayed in some cases The majority of the olivines, which include the opaque minerals, are partially replaced by low-temperature iddingsite (±bowlingite)

K-feldspar minerals occur as subhedral to anhedral grains They often show dusty surfaces due to alteration to clay materials

Opaque minerals are polygonal (equidimensional and prismatic), shown by jagged edges in most cases

Apatite minerals appear as very long tiny needles and are enclosed by plagioclase and clinopyroxene minerals The acicular apatite indicates a rapid growth within a quench environment (Zorpi et al., 1989; Didier, 1991; Best, 2003)

5.2 Subvolcanic rocks of the Garmsar area

Petrographic features of the Garmsar area rocks were studied by Sarizan (2014) In the Garmsar area, rocks are seen as subvolcanic with mafic and intermediate compositions Intermediate rocks are less frequent than mafic rocks in this area Under the microscope, subvolcanic rocks with mafic and intermediate compositions represent ophitic (Figure 6e), subophitic (Figure 6f), intergranular (Figure 6g), and granular (Figure 6h) textures Major mineral phases include 40–60 vol.% plagioclase and 20–30 vol.% clinopyroxene (smaller amounts in intermediate rocks) as phenocrysts and microphenocrysts of variable sizes Minor minerals consist of olivine and opaque and brownish glass often in the groundmass In addition

to olivine and opaque and brownish glass, amphibole, biotite, and quartz are also considered as minor minerals

in intermediate rocks Apatite is the most important accessory mineral in the Garmsar area rocks

Plagioclases are the most abundant minerals in the rocks of the Garmsar area These minerals are euhedral to subhedral; mainly show polysynthetic twinning, zoning, and a sieve texture; are embayed in some cases; and are often converted to chlorite, epidote, calcite, and mainly prehnite minerals By calculating the average percentage

of normative anorthite, the combination of labradorite and andesine-labradorite was obtained for plagioclase in mafic and intermediate rocks of the Garmsar area, respectively

Figure 6 Photomicrographs of the Shurab and Garmsar areas rocks: (a), (b), (c) Intergranular (XPL), trachytic (PPL) and porphyritic

(XPL) textures in rocks of groups 1, 2, and 3 of the Shurab area, respectively (d) Clinopyroxene and olivine glomeroporphyritic aggregates in the Shurab area rocks (XPL) (e), (f) Ophitic (XPL) and subophitic (XPL) textures in mafic rocks of the Garmsar area, respectively (g), (h) Intergranular (XPL) and granular (XPL) textures in intermediate rocks of the Garmsar area, respectively (Cpx: clinopyroxene, Ol: olivine, Pl: plagioclase, Opaq: opaque, and Chl: chlorite (Kretz, 1983)).

Clinopyroxene is the most abundant mineral in the

Garmsar area rocks after plagioclase Clinopyroxene

phenocrysts are euhedral to subhedral, sometimes reveal

zonation, and are rarely converted to actinolitic hornblende

Clinopyroxenes formed in glomeroporphyritic aggregates

are embayed in some cases and rarely exhibit hourglass

twinning

The crystals of olivine minerals are largely anhedral

and are seen to be embayed Olivine phenocrysts are

mainly replaced by iddingsite and show a skeletal texture

in some cases

Primary opaque minerals are seen to be euhedral to

subhedral in groundmass Secondary opaque minerals

are produced by clinopyroxene, olivine, and biotite

hydrothermal alteration

Amphiboles are anhedral to subhedral In most cases,

these minerals are completely altered to chlorite and are

indistinguishable

Biotite minerals are coarse to fine in size, are often

replaced by chlorite, and are embayed in some cases

Quartz minerals are seen in both primary and secondary forms Most quartz minerals in the intermediate rocks of the Garmsar area are secondary and together with calcite have filled the cavities

Apatite minerals with needle shapes and prismatic forms are the most important accessory minerals in the Garmsar rocks and are seen as inclusions in clinopyroxene, plagioclase, and biotite minerals These minerals are also found between minerals derived from alteration, such as chlorite, epidote, and calcite

Microprobe analyses of minerals (Tables 1–4) showed the presence of plagioclase with composition in the range

43-45En39-45Fs9-15 (diopside; Figure 7b), olivine classified as

mainly sanidine) in rock samples of the Shurab area

6 Whole-rock geochemistry 6.1 Major elements

Whole-rock major geochemical analyses of all representative samples of the Shurab and Garmsar

concentrations (from 16.51 to 17.6 wt.% in group 1 samples of the Shurab area, from 18.14 to 18.57 wt.% in group 2 samples of the Shurab area, from 17.07 to 18 wt.%

in group 3 samples of the Shurab area, and from 15.9 to 19.92 wt.% in the Garmsar mafic samples), high amounts

of CaO (from 6.98 to 10.39 wt.% in group 1 samples of the Shurab area, from 4.22 to 9.17 wt.% in group 2 samples of the Shurab area, from 3.83 to 8.42 wt.% in group 3 samples

of the Shurab area, and from 6.56 to 11.47 wt.% in the

to 1.19 wt.% in group 1 samples of Shurab, from 0.98 to 1 wt.% in group 2 samples of Shurab, from 0.95 to 1.08 wt.%

in group 3 samples of Shurab, and in the Garmsar mafic samples from 0.9 to 1.2 wt.%), supporting a calc-alkaline

and Garmsar areas (e.g., <1.5) are typical for subduction zone-generated magmas (Rajesh and Rao, 2007)

In terms of whole-rock major geochemical analyses and in agreement with the petrographic features, all representative samples of the Shurab area are classified into three groups: group 1 comprises basalt/trachybasalt, and groups 2 and 3 have a basaltic trachyandesite composition (Figure 8a) (Le Maitre, 1989)

The mafic and intermediate rocks of the Garmsar area are plotted in basalt and trachybasalt/basaltic trachyandesite fields, respectively (Figure 8a)

Rocks of the Shurab and Garmsar areas are also

8b) (Winchester and Floyd, 1977)

Table 1 Microprobe analyses of plagioclase minerals in volcanic

rocks of the Shurab area (Central Basin).

6.2 Trace elements

To minimize the effects of fractional crystallization and

alteration, petrogenesis estimations of the two areas’

magmatisms are inferred based on the HFSE and HREE

abundances of mafic rocks As a consequence, the

Garmsar area samples with intermediate composition

were excluded

In all the geochemical diagrams presented, filled

diamonds, opened triangles, and squares have been used

to distinguish the three rock groups of the Shurab area,

respectively The mafic rocks of the Garmsar area are

marked with a star

In terms of trace element compositions, all representative samples of the Shurab area reveal medium- and high-K calc-alkaline natures on Ta/Yb vs Th/Yb (Pearce, 1982) and Th vs La (Gill, 1981) diagrams (Figure 9)

The mafic rocks of the Garmsar area show high-K alkaline natures (Figure 9)

calc-Chondrite-normalized (after Sun and McDonough, 1989) REE patterns of the analyzed samples are illustrated

in Figure 10a The mafic rocks of both areas display REE patterns characterized by LREE-enriched and HREE-depleted segments typical of arc lavas (e.g., McCulloch and

Table 2 Microprobe analyses of clinopyroxene phenocrysts in volcanic rocks of the Shurab area (Central Basin).

Analysis Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx SiO2 49.65 50.38 50.86 51.4 50.57 49.9 50.95 50.06 50.13 50.02 50.19 50.82 51.48 TiO2 0.99 0.8 0.63 0.56 0.67 0.94 0.89 0.95 0.85 0.86 0.85 0.87 0.83

Al2O3 4.97 3.78 4.51 4.76 5.74 4.96 3.07 4.78 3.69 3.71 3.88 3.59 2.64

Cr2O3 0.12 0.04 0.4 0.08 0.06 0 0 0.00 0.00 0.00 0.06 0.00 0.00 FeO* 8.86 8.86 5.82 5.72 5.78 8.25 9.11 8.25 8.56 8.63 8.67 8.75 9.48 MnO 0.2 0.21 0.12 0.14 0.11 0.14 0.22 0.17 0.19 0.22 0.22 0.21 0.29 MgO 13.71 13.91 15.26 15.66 15.06 14.01 13.88 14.05 14.07 14.08 14.05 13.95 13.95 CaO 21.43 21.85 21.97 21.47 21.43 21.38 21.54 21.54 21.87 21.87 21.77 21.70 21.70

Na2O 0.28 0.33 0.31 0.29 0.32 0.37 0.35 0.37 0.35 0.35 0.32 0.35 0.35

K2O 0.00 0.00 0.00 0.00 0.01 0.00 0.03 0.00 0.02 0.02 0.01 0.02 0.02 Total 100.2 100.16 99.88 100.09 99.75 99.96 100.046 100.16 99.72 99.75 99.98 100.26 100.75

Wo 44.6 44.73 45.41 44.39 45.06 44.49 44.19 44.6 44.76 44.67 44.63 44.56 43.98

En 39.7 39.64 43.89 45.07 44.06 40.56 39.63 40.48 40.08 40.03 40.1 39.85 39.33

Fs 14.64 14.4 9.55 9.44 9.66 13.56 14.89 13.53 13.88 14.01 14.08 14.29 15.39

Ac 1.05 1.24 1.15 1.1 1.23 1.4 1.28 1.39 1.29 1.29 1.19 1.31 1.29 Cpx: Clinopyroxene, Wo: wollastonite, En: enstatite, Fs: ferrosillite, Ac: acmite.

Gamble, 1991; Hawkesworth et al., 1993; Kelemen et al.,

the Shurab area, from 1.84 to 2.29 ppm in group 2, from

2.86 to 3.26 ppm in group 3, and from 2.66 to 3.52 ppm in

in group 1 of the Shurab area, from 1.82 to 1.91 ppm in

group 2, from 2.22 to 2.34 ppm in group 3, and from 2.2 to

3.37 ppm in the Garmsar rocks

The slightly positive Eu anomaly in normalized

patterns of all three groups from the Shurab area (Figure

10a) suggests the abundant presence of accumulated

plagioclase (Zhang et al., 2010)

Because the Eu anomaly is controlled by plagioclase

and pyroxene, the absence of a Eu anomaly in the

Garmsar samples can be attributed to the simultaneous

crystallization and the lack of fractional removal of

plagioclase and pyroxene (Martin, 1999)

All rocks of the Shurab and Garmsar areas have an evident Sm inflection Enrichment of Nd relative to Sm is characteristically found for the Em1 (enriched mantle 1) mantle source (Litasov et al., 2001)

Figure 10b shows primitive mantle-normalized trace element patterns for mafic samples of both areas In this diagram, the samples of both areas exhibit enrichment

in large-ion lithophile elements (LILEs: Cs, Ba, Rb, Th) relative to the high field-strength elements (HFSEs: Ti, Hf,

Zr, and REEs), as well as troughs for Nb, Ta, Hf, and Zr and peaks for U, Ba, and La, all of which are indicators for arc-related magmatism (Zanetti et al., 1999)

There are many similarities in incompatible trace element patterns for all rocks of the three groups from the Shurab area and only slight differences can be seen

In comparison with group 1 rocks, group 2 and 3 rocks, which show many similarities in their incompatible trace

Table 3 Microprobe analyses of olivine phenocrysts in volcanic rocks of the Shurab area (Central Basin).

element patterns, are less depleted in Nb and Ta, have

negative or no Sr anomalies, tend to have U variation and

variable LILEs, and seem to be enriched in Ba relative to

other LILEs (Figure 10b)

The Sr positive anomaly in group 1 rocks is related

to the abundant presence of accumulated plagioclase as

noted in petrographic studies (Rollinson, 1993) and the

presence of amphibole in the melting source (Karmalker

et al., 2005)

The Sr negative anomaly in group 2 and 3 rocks is

related to the alteration of primary calcic plagioclase (Hui

et al., 2011)

The Sr positive anomaly in the Garmsar area rocks,

similar to the group 1 rocks from the Shurab area, is related

to the abundant presence of accumulated plagioclase

Trace element patterns of rocks of both the Shurab and

Garmsar areas are similar to the Patagonia mafic rocks in

the continental back-arc setting from Argentina (Figure

10b)

low (from 44.79 to 47.71 wt.% in the group 1 samples,

from 51.82 to 53.36 wt.% in the group 2 samples, and

from 49.81 to 52.47 wt.% in the group 3 samples) and

on Harker variation diagrams for the three rock groups,

elements (i.e Th, Rb, and Nb) do not show a regular

fractional crystallization process had little effect on the evolution of the three magma groups

Harker diagrams for the Garmsar area rocks show that

is typically characteristic of magmatic processes by either fractionation or partial melting (Rollinson, 1993)

7 Discussion

According to several authors (e.g., Pearce, 1983), the source

of subduction-related magmas has been enriched in Sr, K,

Rb, Ba, and Th by metasomatizing agents Also, Zanetti et

al (1999) showed that LREE and LILE enrichments and negative HFSE anomalies in mantle rocks can be attributed

to subduction-related agents These agents are depleted in

Nb and Ta and enriched in Pb, Sr, Ba, and alkalis

Table 4 Microprobe analyses of K-feldspar minerals in volcanic rocks of the Shurab area (Central Basin).

In the following sections we study two geochemical

reservoirs (i.e the unmodified source mantle and the

subduction-related components) involved in the genesis

of the magmatism associated with salt diapirs in both the

Shurab and Garmsar areas

7.1 Composition of the unmodified source mantle

McCulloch and Gamble (1991) used HSFEs to examine the

composition of unmodified subduction-related mantle

This approach has the advantage that metasomatic agents

are generally less enriched in HSFEs than LREEs (i.e

have a negative Nb anomaly), and so HSFE ratios of the subduction-related magmas (e.g., Ta/Yb) will be relatively less affected by the process of slab component addition than ratios involving LILEs

According to the diagram of Ta/Yb vs Th/Yb (Figure 12a) (Pearce and Peate, 1995), the compositions of the unmodified source mantle of magmatisms in the Shurab and Garmsar areas are located between the enriched mantles (Em) and the enriched-DMM (E-DMM) end-members

Figure 7 Composition of: (a) feldspar, based on the classification of Deer et al (1992); (b) clinopyroxene (Morimoto, 1989); and (c)

olivine (Morimoto, 1989) in the Shurab area.