Accessory chrome spinels are scattered throughout the serpentinite masses in two allochthonous thrust sheets belonging to the Penjween–Walash sub-zone of the northwestern Zagros Suture Zone in ...Kurdistan. Based on field evidence, the serpentinites are divided into two groups: (1) highly sheared serpentinites (110–80 Ma), which occupy the lower contact of the ophiolitic massifs of the Upper Allochthonous sheet (Albian–Cenomanian age), and (2) ophiolitic mélange serpentinites of mixed ages (150 and 200 Ma) occurring along thrust faults on the base of the volcano-sedimentary segment (42–32 Ma) of the Lower Allochthonous sheet. The Cr-spinels of both groups show a wide range of YCr (Cr/(Cr + Al) atomic ratio) from 0.37 to 1.0, while the XMg (Mg/(Mg + Fe2+) atomic ratio) ranges from 0.0 to 0.75. Based on the Cr-spinel compositions of the entire dataset and in conjunction with back-scattered electron imaging, from core to rim, three spinel stages have been recognized: the residual mantle stage, a Cr-rich stage and a third stage showing a very narrow magnetite rim. These three stages are represented by primary Cr-spinel, pre-serpentinization metamorphosed spinel and syn- or post-serpentinization spinel, respectively. The chemical characteristics of primary (first-stage) Cr-spinels of both serpentinite groups indicate a tectonic affinity within a fore-arc setting of peridotite protoliths. The second stage indicates that Cr-spinels have undergone subsolidus re-equilibration as a result of solid–solid reaction during pre-serpentinization cooling of the host rock. Here the primary Cr-spinel compositions have been partly or completely obscured by metamorphism. During the third stage, the Cr-spinels have undergone solid–fluid re-equilibration during syn- or post-serpentinization processes. Both the second and third stages point to diachronous metamorphic paths resulting from continuous tectonic evolution influenced by either slow or fast uplift of mantle protoliths. In the fast metamorphic paths, the primary chrome spinels are flanked by a very narrow magnetite rim. The presence of two groups of distally separated serpentinites with different emplacement ages and fore-arc tectonic affinity could indicate that the closure of the Tethys Ocean culminated in two fortuitous subduction processes.
Protrusions and lenses of serpentinite–matrix mélanges occur at several places along the thrust faults of the Zagros Suture Zone. They separate the lower allochthonous thrust sheet, the ‘Lower ...Allochthon’ (i.e. Walash–Naopurdan nappe), of Paleocene–Eocene age from sediments of the Arabian platform and the upper thrust sheet of Mesozoic, ophiolite-bearing terranes termed the ‘Upper Allochthon’ (i.e. Gemo–Qandil nappe). The serpentinite–matrix mélanges occur mostly as stretched bodies (slices) on both sides of the Lower Allochthon (Hero, Halsho and Pushtashan (HHP) and Galalah, Qalander and Rayat (GQR)). Their overall chondrite-normalized rare earth element (REE) patterns form two main groups. Group One exhibits enrichment in the total REEs (> 1 × chondrite) whereas the Group Two pattern shows depletion (i.e. < 1 × chondrite). Bulk-rock MORB-normalized profiles of Group Two are almost flat in the MREE–HREE region with flattening profiles in the Gd–Lu range (> 3 times the MORB composition). In comparison with Group One, Group Two has extremely high REE content and displays variable depletions in the moderately incompatible high-field-strength elements (HFSEs) (Zr, Hf, Y) relative to their adjacent REEs. The REEs in the GQR serpentinite–matrix mélanges have a noticeably high LREE content, and a positive Eu anomaly, and their HREE content never reaches more than 1 × chondrite (i.e. < 0.01 to 1 × chondrite). The latter indicates that the hemipelagic sedimentary, melt-like components (i.e. high LREE, U/La, La/Sm and low Ba/Th) control the geochemical peculiarities of this type of serpentinite. The HHP serpentinite–matrix mélanges, however, are either equally divided between the two REE pattern groups (e.g. Hero, Halsho) or inclined towards Group One (e.g. Pushtashan). Contrary to GQR serpentinites, the variation in HHP serpentinite–matrix mélanges spans a compositional spectrum from U/La-rich to more Ba/Th-rich. Such ratio variations reflect the large variation in these two subducted sedimentary components (i.e. carbonate and hemipelagic sediment mix). The obvious differences in the trace element signatures of the GQR and HHP serpentinite–matrix mélanges might be related to plate tectonic parameters such as convergence rate, subduction age and thickness and type of subducted slab. It is more likely that the influx of subducted components to the mantle wedge relied heavily on the composition of the sedimentary inputs. These vary considerably with time from the relatively deepwater hemipelagic sediments (Qulqula Radiolarite Formation) to platform carbonate sediments (Balambo limestone). The trace element signatures of the GQR and HHP serpentinite–matrix mélanges might suggest multi-staging of the allochthonous sheet emplacement on the Arabian platform sediments.
The Walash and Naopurdan groups are incorporated into the lower allochthonous thrust sheet in the Iraqi Zagros Suture Zone (IZSZ). 40Ar–39Ar dates on magmatic feldspar separates from both Walash and ...Naopurdan volcanic rocks indicate an Eocene–Oligocene age (43.01 ± 0.15 to 24.31 ± 0.60 Ma). The Walash and Naopurdan groups form a thrust sheet that is structurally overlain by an upper allochthon of Cretaceous arc‐related rocks (106–92 Ma) now known as the Hasanbag igneous complex (formerly known as the Gemo–Qandil Group). The Walash and Naopurdan lower allochthon is thrust over the foreland basin Red Beds series. Volcanic and subvolcanic units in the Walash and Naopurdan groups were studied from the Mawat, Galalah–Choman, Leren, and Qalander–Sheikhan provinces. Most of these rocks are basaltic to andesitic for both the Naopurdan and Walash suites. The petrographic study shows that these rocks are affected by metamorphic alteration under greenschist facies conditions, but preserve primary porphyritic textures with some relict igneous plagioclase, pyroxene, and hornblende. The enrichments in LREE/HREE and high Th/Nb and Nb/Zr show that the Walash and Naopurdan rocks have distinct subduction‐related signatures: specifically island‐arc tholeiite for the Naopurdan and calc‐alkaline to alkaline for the Walash suites. Hence the Walash and Naopurdan suites are back‐arc and arc systems, respectively, that developed 43–24 Ma. Accordingly, the IZSZ contains a full record of Neo‐Tethys pre‐collision‐related volcanism in dual subduction settings, from the Early Cretaceous (Hasanbag igneous complex) to the Eocene–Oligocene (Walash–Naopurdan suites). Final continent–continent collision started when the last of the Neo‐Tethys Ocean was subducted beneath the Iranian continent, resulting in its collision with the Arabian Plate, probably during the Middle Miocene. This reinforces a continuity of events along the entire edge of the Arabian Plate from Turkey, through Iraq and Iran, and into Oman.
A combined petrographic and geochemical study of the Jurassic Formations, Western Desert of Iraq was carried out to infer their tectonic setting, provenance history, and weathering in the source ...area. Texturally, these sandstones are immature, poorly sorted, and grain supported. Abundance of feldspars, especially plagioclase indicates rapid deposition of sediments from nearby source rocks. Using the geochemical classification diagram, the Jurassic sandstones are classified mainly as Fe-sand, quartzarenite, and sublitharenite, which is also supported by the petrographic study. The transition trace elements like Co, Ni, Cr, and Cu are lower in the Jurassic sandstones than upper continental crust (UCC) values. Plotting the present data on the provenance discriminating diagrams, most of Hussainiyat and Najmah sandstones fall within felsic (acidic) igneous provenance, and Amij and Muhaiwir sandstones mostly fall in the field of intermediate igneous provenance. The poor correlation between Cr and Ni (
r
= −0.110, number of samples
n
= 17) implies that these sandstones were derived from felsic source rocks. Tectonic setting discrimination diagrams based on major elements suggest passive margin and active continental margin. As indicated by chemical index of alteration (CIA) values for the Jurassic sandstones (averages 50 to 65), their source area underwent low to moderate degree of chemical weathering. The petrography and geochemistry results are consistent with a semihumid to semiarid climate for the deposition of these sandstones.
Geological, geochemical, and geochronological investigations of the upper volcanics (UV) of Lower allochthon (LA) of Mawat nappes (MN) were used to elucidate the geologic relationships between the ...LA, the Qulqula Rise (i.e., accretionary complex terrane (ACT)) and peripheral Foreland Basin Assemblage (FBA), (Maastrichtian–Paleogene). The
40
Ar-
39
Ar geochronological data suggest that the Middle to Late Paleogene magmatic history of Late Walash arc volcanic activity of UV can be divided into an early phase of andesitic volcanism around 43.1 ± 0.3 Ma and subsequent extensional arc phase magmatism of a time lapse between 40.1 ± 0.3and 32.3 ± 0.4 Ma. The geochemical data establish the existence of gradients in trace element ratios (i.e., Ba/La, Th/Yb, U/La, Ba/Th, Rb/Y, Ba/Y, and Nb/Y) throughout the studied samples and reflect differences in the degree of mantle wedge depletion influenced by addition of a considerable proportion of sedimentary derived input of the subducting Arabian slab. The trace, rare earth elements and isotopes of the studied samples have emphasized differences in the geochemical characteristics of andesites in comparison with other members of Walash. The andesites have subduction-related signature with the isotope (low Nd and high Sr isotope ratios) characteristic of a melt component input from cratonically-derived sediments. The diabases intruding in the basalts and andesites have a transitional composition from andesitic to N-type mid-ocean ridge basalts geochemistry with a noticeable subduction-related signature, and are interpreted to form in an extensional arc evolving into back arc basin setting. As evidenced from infant arc magmatism (Albian–Cenomenian), MN recorded protracted kinematic at convergent plate margins with a most notable was the development of the ACT (i.e., Qulqula Rise) adhered to the Arabian platform carbonates (Albian–Cenomenian) leading to the inception of peripheral FBA occurred by the erosion and unroofing of advancing ACT. Notably, the volcano flysch of Walash and Nummulite carbonate of Naopurdan were still accumulating on the remaining oceanic lithosphere further to the northeast and in part coeval with Paleogene molasses sedimentation on flexural FBA further to the southwest of the ACT.
The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two ...allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or “Ophiolite-bearing terrane” of Albian-Cenomenion age (97–105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from ~45 to ~ 40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (~ 80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event.
40
Ar/
39
Ar dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78 ± 0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as ~ 400 °C/Ma and ~ 3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78 ± 0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).
The world’s largest ongoing collisional orogeny is the Europe Alps–Himalayan–SE Asian belt and is a natural laboratory to understand many processes that have shaped the continents. Due to political ...instability and conflict throughout this millennium, the Iraq (Kurdish) sector of the Zagros mountain chain is the least studied part of this orogenic system. In Iraq, the Zagros contains the suture between the Arabian subcontinent to the south and west and the Iranian edge of the Eurasian continent to the north and east. The suture zone is marked by several allochthons of Neotethyan ophiolitic and volcanic arc assemblages that were obducted onto the Arabian margin. New geochronological data, including SHRIMP U-Pb zircon, integrated with whole rock geochemistry, indicates that both Cretaceous (˜96 Ma) and Cenozoic (˜40 Ma) assemblages are present. The relationships between these units are complicated, thus some Cretaceous arc rocks were intruded by Cenozoic arc rocks, and out-of-sequence thrusting has interleaved and juxtaposed assemblages of different ages. Ongoing wrench faulting since continental collision at ˜14 Ma has further complicated the pattern of lithotectonic units, particularly those that were obducted out of the Neotethyan realm. The new data indicate that the Iraqi sector of Neotethys was not ‘quiet’ in the Cretaceous, but contains fragments of arcs of that age, contiguous with those along strike in Turkey, Iran and the Himalayas.
In NE Iraq, the eastern edge of the Arabian plate is overlain by arc rock allochthons whose genesis and tectonic emplacement were related to the consumption and closure of the Neotethys Ocean. This ...paper demonstrates the occurrence of unrelated Paleogene arc rocks in two adjacent allochthons. The Bulfat Igneous Complex at Wadi Rashid (NE Iraq) is an intrusion within the Upper Allochthon Albian–Cenomanian Gimo–Qandil sequence suprasubduction zone assemblage. A thrust separates this allochthon from the underlying Lower Allochthon of the Eocene-Oligocene Walash–Naopurdan volcanic-sedimentary arc rocks. The Bulfat Igneous Complex at Wadi Rashid consists of gabbro and granitic composite intrusions in which components mingle down to a small scale. Textural relationships in the Bulfat Igneous Complex rocks indicate emplacement at high crustal levels with rapid cooling, which is consistent with amphibole geobarometry indicating crystallisation pressures between ~250 and 300Mpa. Ti-rich igneous pargasite and Ti-rich igneous Fe-biotite from gabbroic and granitic components yielded 40Ar/39Ar ages of 39.23±0.21 and 38.87±0.24Ma respectively. These ages agree within analytical error and suggest coeval emplacement and rapid cooling of mafic and felsic magmas in the Eocene, in an event that was distinct and much younger than the host Albian–Cenomanian rocks. This igneous event was unrelated to formation of Cenozoic rocks in the underlying, tectonically separate, lower allochthon. The trace element signatures of the Wadi Rashi volcanic rocks show volcanic-arc characteristics for the granites and the gabbroic rocks resemble E type MORB. The presence of Eocene arc-related rocks in two allochthons suggests complexity in Paleogene subduction systems, with possibly two subduction zones operating at that time.
Display omitted
•The Bulfat Igneous Complex (Zagros Suture Zone) intrudes Cretaceous intra-oceanic rocks.•Bulfat 40Ar/39Ar igneous hornblende and biotite ages are 39.23±0.21 and 38.87±0.24Ma.•The Eocene arc-related Bulfat rocks are unrelated to their Cretaceous intra-oceanic host.•A model is proposed for dual Eocene subduction late in the closure of Neotethys in Iraq.
Abstract
The
W
alash and
N
aopurdan groups are incorporated into the lower allochthonous thrust sheet in the
I
raqi
Z
agros
S
uture
Z
one (
IZSZ
).
40
A
r–
39
A
r dates on magmatic feldspar separates ...from both
W
alash and
N
aopurdan volcanic rocks indicate an
E
ocene–
O
ligocene age (43.01 ± 0.15 to 24.31 ± 0.60
M
a). The
W
alash and
N
aopurdan groups form a thrust sheet that is structurally overlain by an upper allochthon of
C
retaceous arc‐related rocks (106–92
M
a) now known as the
H
asanbag igneous complex (formerly known as the
G
emo–
Q
andil
G
roup). The
W
alash and
N
aopurdan lower allochthon is thrust over the foreland basin
R
ed
B
eds series. Volcanic and subvolcanic units in the
W
alash and
N
aopurdan groups were studied from the
M
awat,
G
alalah–
C
homan,
L
eren, and
Q
alander–
S
heikhan provinces. Most of these rocks are basaltic to andesitic for both the
N
aopurdan and
W
alash suites. The petrographic study shows that these rocks are affected by metamorphic alteration under greenschist facies conditions, but preserve primary porphyritic textures with some relict igneous plagioclase, pyroxene, and hornblende. The enrichments in
LREE
/
HREE
and high
T
h/
N
b and
N
b/
Z
r show that the
W
alash and
N
aopurdan rocks have distinct subduction‐related signatures: specifically island‐arc tholeiite for the
N
aopurdan and calc‐alkaline to alkaline for the
W
alash suites. Hence the
W
alash and
N
aopurdan suites are back‐arc and arc systems, respectively, that developed 43–24
M
a. Accordingly, the
IZSZ
contains a full record of
N
eo‐
T
ethys pre‐collision‐related volcanism in dual subduction settings, from the
E
arly
C
retaceous (
H
asanbag igneous complex) to the
E
ocene–
O
ligocene (
W
alash–
N
aopurdan suites). Final continent–continent collision started when the last of the
N
eo‐
T
ethys
O
cean was subducted beneath the
I
ranian continent, resulting in its collision with the
A
rabian
P
late, probably during the
M
iddle
M
iocene. This reinforces a continuity of events along the entire edge of the
A
rabian
P
late from
T
urkey, through
I
raq and
I
ran, and into
O
man.
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