Subduction Of Neo-Tethys Oceanic Crust Research Articles

High temperature – low pressure metamorphism during subduction of Neo-Tethys beneath the Iranian plate: evidence for mafic migmatite formation in the Alvand complex (western Iran)

Subduction of Neo-Tethys oceanic lithosphere beneath the Iranian plate during the Mesozoic formed several igneous bodies of ultramafic to intermediate and felsic composition. Intrusion of these magmas into a regional metamorphic sequence (the Sanandaj-Sirjan Zone) caused partial melting and formation of migmatites with meta-pelitic protoliths. The Alvand complex (west Iran) is a unique area comprising migmatites of both mafic and pelitic protoliths. In this area, the gabbroic rocks contain veins of leucosome at their contact with pyroxenite and olivine gabbro. These leucosomes are geochemically and mineralogically different from leucosomes of the meta-pelitic migmatites and clearly show properties of I-type granites. Microscopic observations and whole rock compositions of the mafic migmatite leucosomes show that migmatization occurred through partial melting of biotite, hornblende and plagioclase. Thermobarometric calculations indicate 800 °C and 3.7 kbar for partial melting, although phase diagram modeling demonstrates that the presence of water could decrease the solidus temperature by about 40 °C. Our results suggest an asthenospheric magma upwelling as the source of heat for partial melting of the gabbroic rock during subduction of Neo-Tethys oceanic crust under the western edge of the Iranian plate. The present study also reveals relationships between migmatization and formation of S- and I -type granites in the area.

Petrogenesis of Dehsard felsic rocks in the southwest of Kerman, Iran: Inference for the evolution of Sanandaj-Sirjan zone

The Upper Triassic Dehsard felsic rocks (DFR), leucocratic to hololeucocratic granitic rocks, are exposed in the southernmost of the Phanerozoic Sanandaj-Sirjan zone in the Zagros orogenic belt of south-Central Iran. They intruded into the Paleozoic Khabr and Rutchun metamorphic complexes that consist of marble, greenschist, calcschist and micaschist rocks. The DFR are cut through by monzonitic and dioritic dikes. Comprehensive petrological and geochemical studies disclosed DFR as a plagiogranite. They are calc-alkaline, metaluminous to slightly peraluminous I-type granitoids and exhibit enrichment in large ion lithophile elements (LILEs; e.g. Na, K, Rb, Ba) in contrast to the depletion of high field strength elements (HFSEs; e.g. Nb, Ta, P, Ti). Their chondrite normalized REE patterns show LREE enrichment [(La/Yb)N = 4.41–23.77] and unfractionated HREE [(Gd/Yb)N = 0.60 to 2.23] that suggest the lack of garnet during partial melting processes. The positive Eu anomalies advocate plagioclase accumulation. The low Nb, Ta, P2O5, TiO2 content, isotopic data, LREEs enrichment, various tectono-magmatic discrimination diagrams may point out the formation of DFR in a mature island arc environment through partial melting of metabasic source with contribution of some greywacke sediments and subduction-related volatiles. The partial melting occurred above garnet stability field. The involvement of variable amounts of oceanic sedimentary rocks especially greywackes in the melting processes in a mature island arc setting, as characterised by εNdt results ranging from +0.93 to −5.19 at 220 Ma, could also cause the DFR inclination toward active continental margin environment. It seems that the development of an island arc in an intra-oceanic setting must be happened prior to the Late Jurassic–Early Cretaceous time, while northward subduction of Neotethys oceanic crust was in progress under southern margin of the Central Iranian Microcontinents. Subsequent collision of the island arc with SSZ led to the tectonic proximity of the DFR into SSZ constituents. This approach gives us new intuition for the renovation of the geodynamic history of Sanandaj–Sirjan zone.

Metallogenesis within continental collision zones: Comparisons of modern collisional orogens

Modern collisional orogens represent the natural laboratory for the study of metallogeny in continental collision zones. The Pyrenees, Alps, Zagros and Himalaya are all associated with Neo-Tethyan subduction and represent the youngest collisional orogens on Earth. Here, we compare these four orogens in terms of their composition, architecture, tectonic evolution, and metallogenic systems. The four orogens can be divided into simple and composite types. Simple orogens are represented by the Pyrenees and the Alps, and are characterized by narrow linear shapes in plain view and symmetric structures in cross-section, are free of arc magmatism, and are associated with the Mississippi Valley Valley-type Pb-Zn and orogenic gold deposits. The mineral deposits that form in these simple collisional orogens are generally related to processes that occur in the middle and upper crust. In contrast, composite orogens, as exemplified by the Zagros-Iranian and Himalayan-Tibetan Plateaus, are associated with broad orogenic plateaus in plain view and asymmetrical structures in cross-section, record extensive arc magmatism in continental margins, and are associated with a variety of deposit types including carbonatite-related rare earth element (REE), porphyry Cu-Mo, orogenic Au, Mississippi Valley type Pb-Zn, and detachment-fault-related polymetallic deposits. Although the subduction of Neo-Tethys oceanic crust occurred before the creation of simple collisional orogens in the Pyrenees and the Alps, these areas do not show the record of continental arc magmatism. In contrast, the composite collisional orogens are associated with the development of huge continental margin arcs prior to continental subduction, and the subduction was followed by reactivation of the subduction-modified arc lithospheric material, generating the ore-forming systems in these regions.

Geochemical evidence for deep mantle melting and lithospheric delamination as the origin of the inland Damavand volcanic rocks of northern Iran

The intraplate Damavand volcanic field that overlies the Alborz Mountains in northern Iran is dominantly comprised of trachyandesite and alkali olivine basalt. Alkali olivine basalt is the most primitive rock type among Damavand lavas and exhibits a number of geochemical characteristics (e.g., high MgO, Ni and Cr abundances) that preclude substantial differentiation and crustal contamination. The lavas are light rare earth element enriched and have small negative Nb–Ta–Ti and positive Ba and Pb anomalies. The initial Nd and Sr isotope compositions of these lavas plot within the mantle array and cluster near bulk silicate Earth-values, while Pb isotope ratios are typical of modern upper crust. Oxygen isotope ratios range from + 6.5 to + 6.9‰. Although previous studies suggest that the mantle source of the Damavand volcanic rocks was metasomatized by the release of fluids during Tertiary subduction of Neo-Tethys oceanic crust under the Iranian Plate, the geochemistry of the alkali olivine basalts (e.g., BSE-like Sr and Nd isotopic compositions; the position of samples on Ti/V vs. Zr/Nb, Ce/Pb vs. Ce, and Th–Ta–Hf diagrams) is not reconcilable with derivation of the parental magma from such a mantle source. In addition, the Mid-Ocean Ridge Basalt (MORB) normalized trace element patterns of the Damavand alkaline olivine basalts most resemble those of Ocean Island Basalt (OIB) that implies the role of deep mantle source in their genesis. It seems that local upwelling of deep mantle materials underneath the Alborz Mountains resulted in the eruption of intraplate Damavand lavas. Compressional stress exerted on the Iranian plate following Neo-Thetys closure was the likely cause of sub-continental lithosphere delamination which led to such local mantle upwelling.

The cretaceous Ladakh arc of NW himalaya—slab melting and melt–mantle interaction during fast northward drift of Indian Plate

The Kohistan–Ladakh Terrane in the NW Himalaya is a remnant of a Cretaceous arc sequence obducted onto the Indian margin. This paper presents a geochemical study (major and trace elements and Sr, Nd, Pb isotopes) of the Mid-Cretaceous lavas of the Ladakh side of the arc sequence, which were erupted in response to northward subduction of Neo-Tethys oceanic crust. Lavas from the western Ladakh in Pakistan can be divided into three groups which, from north to south, are: (1) the Northern Group of back-arc tholeiites [0.5<(La/Yb) N<1.4; 0.3<(Nb/La) N<1.4; 4<εNd<8; 38.66< 208Pb/ 204Pb<38.80], (2) the Southern Group of arc tholeiites [1.8<(La/Yb) N<3.9; 0.1<(Nb/La) N<0.6; 5<εNd<6; 38.40< 208Pb/ 204Pb<38.66], and (3) the Katzarah Formation of tholeiitic Nb-rich lavas [3.4<(La/Yb) N<9.8; 1.4<(Nb/La) N<2.1; 3<εNd<5], including radiogenic Pb lavas [39.31< 208Pb/ 204Pb<39.51] and less radiogenic lavas [38.31< 208Pb/ 204Pb<38.55]. Magmas from the eastern Ladakh in India show a simple series of more evolved arc volcanics from basalts to rhyolites [basalts and basaltic andesites: 2.5<(La/Yb) N<5.7; 0.4<(Nb/La) N<0.5; 1.8<εNd<5.5; 38.70< 208Pb/ 204Pb<38.80]. Isotope and trace element data of western Ladakh lavas are compatible with high-degree melting (14–21%) of a fertile MORB-mantle source. An adakitic lava [(La/Yb) N=55.8; (Nb/La) N=0.3; εNd=1.7; 208Pb/ 204Pb=39.00] and a Mg-poor Nb-rich basalt [(La/Yb) N=4.6; (Nb/La) N=1.3; εNd=−2; 208Pb/ 204Pb=39.07] are spatially associated with the tholeiitic arc lavas. Isotope compositions of all the lavas, and in particular the radiogenic Nb-rich and adakitic lavas suggest three-component mixing between depleted mantle similar to the Indian MORB mantle, and enriched components similar to the volcanogenic or pelagic sediments. The geochemical diversity of magma types is attributed to contribution of melts from the subducted crust and associated sediments, and their subsequent interaction with the mantle. Such melt–mantle interactions can also be inferred from relicts of sub-arc mantle found in Indian Ladakh. These results lead to a geodynamic reconstruction of the Kohistan–Ladakh arc as a single entity in the Mid-Cretaceous, emplaced south of the Asian margin. Slab melting imply subduction of young oceanic crust, as already proposed for the Oman ophiolite farther west. The fast northward drift of the Indian Plate could have triggered wide-scale inversion of the divergent tectonic regime responsible for the opening of the Neo-Tethys Ocean. Our results suggest breaking of the young oceanic crust initiated at the ridge rather than at passive plate boundaries.