Physiographic-Tectonic Zoning of Iran’s Sedimentary Basins

Base on geological history evaluation using regional stratigraphy, sedimentary environments, magmatic activities, metamorphism and structural trend in Iran, physiographic-tectonic zoning map of Iran’s sedimentary basins has prepared. This map has prepared to point out the basement tectonics role in Iran. It contains twenty-four different provinces. Iran has composed from different plates: Arabian plate in south and west, Cimmerian manipulated in north and east, Eurasian plate in northeast margin. Cimmerian manipulated at least can be divided to the smaller part, East-Central Iran and North-Central Iran microcontinents. There are evidences for thick-skinned tectonics in the border zones of these plates and microcontinents, especially in Sanandaj-Sirjan overthrust belts that it formed by crustal stacking wedges. Also, Neoproterozoic-Phanerozoic Tectonic column in the Arabian, Cimmerian and Eurasian plates under Iran Country area have introduced.

Fission Track Thermochronology Evidence for the Cretaceous and Paleogene Tectonic Event of Nyainrong Microcontinent, Tibet

Fission track dating was applied to analyze the 20 samples from Nyainrong microcontinent, and we obtained 20 apatite and 15 zircon fission track ages. The results show single population grain ages with a single mean age and associated central ages mainly ranging from 108±7Ma to 35±4Ma.Their mean track lengths are 12.2-13.9 μm with a single peak. Zircon fission track age range from 78±3 Ma to 117±4 Ma. The results represented the two tectonic uplift events in the study area, namely the Cretaceous and Paleogene periods. According to thermal history modeling results, uplifting rates of two tectonic events is 0.31-0.1 mm/a and 0.07-0.04 mm/a respectively. Combined with field condition and study results, it is suggested that the Cretaceous tectonic uplift event was related to the closure ocean basin caused by Qaingtang-Lhasa collision, and the Paleogene tectonic uplift event was related to the south to thrust system caused by Indo-Asian collision.

Petrogenesis of Volcanic Rocks in the Khabr-Marvast Tectonized Ophiolite:Evidence for Subduction Processes in the South-Western Margin of Central Iranian Microcontinent

The Late Cretaceous Khabr-Marvast tectonized ophiolite is located in the middle part of the Nain-Baft ophiolite belt, at the south-western edge of the central Iranian microcontinent. Although all the volcanic rocks in the study area indicate subduction-related magmatism （e.g. high LILE （large ion lithophile elements） / HFSE （high field strenght elements） ratios and negative anomalies in Nb and Ta）, geological and geochemical data clearly distinguish two distinct groups of volcanic rocks in the tectonized association： （1） group 1 is comprised of hyaloclastic breccias, basaltic pillow lavas, and andesite sheet flows. These rocks represent the Nain-Baft oceanic crust; and （2） group 2 is alkaline lavas from the top section of the ophiolite suite. These lavas show shoshonite affinity, but do not support the propensity of ophiolite.

Constructing the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western Bainaimiao arc terrane,southeastern Central Asian Orogenic Belt

Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt(CAOB).The Bainaimiao arc terrane(BAT)is one of the most important Precambrian microcontinents in southeastern CAOB,however,few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton.This study presents an integrated study of geochronology,zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT.The latest Neoproterozoic(ca.546 Ma)diabases display low SiO2(46.52-49.24 wt.%)with high MgO(8.23-14.41 wt.%),Cr(66-542 ppm)and Ni(50-129 ppm),consisting with mantle origin.Their highly negative zirconεHf(t)(-12.0 to-24.7)and high Fe/Mn ratios(62.1-81.7)further indicate a significantly enriched mantle source.Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root,we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust.The Early Paleozoic(ca.475-417 Ma)arc intrusions in western BAT can be divided into PeriodsⅠandⅡat approximately 450 Ma.The PeriodⅠ(>450 Ma)intrusions contain abundant mafic minerals like hornblende and pyroxene,and show positive zirconεHf(t)(+1.5 to+10.9).They are predominantly medium-K calc-alkaline with broad correlations of SiO2 versus various major and trace elements,which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7 kbar.We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts.By contrast,the PeriodⅡ(≤450 Ma)intrusions are characterized by variable zircon eHf(t)(-15.0 to+11.5)with irregular variations in most major and trace elements,which are more akin to the arc magmas generated in an open system.The general occurrence of elder inherited zircons,along with the relatively high Mg#(>45)of some samples,call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components.In combination with the Early Paleozoic tectonic melanges flanking western BAT,we infer that the compositional transition from PeriodⅠtoⅡcan be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB.The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT,but also on the associated crustal construction processes before the final arc-continent accretion.

Documentation of the Sirjan Orocline in the southeast Sanandaj-Sirjan Zone, Iran

At the southeastern part of the SanandajSirjan Zone of Iran, a group of structural elements outline a large-scale arc curvature around a vertical axis. This curvature comprises several elongated structural elements and their dividing faults, axialfold traces, layering, and foliation. The most frequent lithological units include Paleozoic metamorphic rocks, Mesozoic-Paleogene sedimentary rocks, and Mesozoic magmatic-ophiolitic complex disposed in several anticlines and synclines, forming a horseshoeshaped structure with a 240-km arc length and a 90-km wavelength. We name this structure the Sirjan Orocline, and characterize this structure here through field observations and satellite image analyses. The Sirjan Orocline formed during the late EoceneOligocene related to the most significant deformation event after regional metamorphism. The final form of this structural arc is affected by a younger tectonic event that compressed and transected this structure.

Terrane Tectonics in the Northeast Part of Northeast of China

As the members of Chinese Group of the international cooperative project of “Mineral Resources, Metallogenesis, and Tectonics of Northeast Asia”, the authors had the opportunity to review the recent achievement of regional geology in this area. This paper is confined to a brief discussion of the nature, composition and evolution of terranes in a part of Northeast China. Nine terranes were recognized. A splicing pattern of when and how the amalgamation of 9 terranes into one microcontient is proposed here.

Geochronological evidence for existence of South Mongolian microcontinent——A zircon U-Pb age of grantoid gneisses from the Yagan-Onch Hayr-han metamorphic core complex

A zircon U-Pb age of (916±16) Ma is measured for grantoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex and represents the crystallization age of the grantoid magma. This age provides evidence for the existence of the South Mongolian microcontinent, which is consistent with the analysis of the regional geology.

Pan-African and early Paleozoic tectonothermal events in the Nyainrong microcontinent:Constraints from geochronology and geochemistry

To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the con- straints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in north- ern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 2~6pbF38U concordant age of zircon is 505-517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have char- acteristics of high silicon, alkali-rich, alkalic rate AR =1.73-3.7, the differentiation index DI = 70.78-90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 x Ga/AI ranges from 2.12 to 2.41, and P205 and A1203 content decreased with SiO2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with re- gional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The prelimi- nary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean- type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.

Geochemical evidence for the formation and evolution of North Qinling microcontinent

Nd-Pb isotope and incompatible trace element compositions of the Precambrian basements, mantle-derived mafic rocks and feldspar of granite from East Qinling and its adjacent tectonic units are studied. It is concluded that the North Qinling was not part of the North China Craton, but an independent microcontinent in geological history. This microcontinent was built upon a newly formed continental crust derived from strong depleted mantle. It is also suggested that the microcontinent formed initially in an oceanic island setting as the margin of the North China Craton.

^(40)Ar/^(39)Ar Thermochronology Constraints on Jurassic Tectonothermal Event of Nyainrong Microcontinent

To reveal the Jurassic tectonothermal event occurring to the Nyainrong microcontinent which is gripped among the Bangong-Nujiang suture zone,^40Ar/^39Ar dating was carried out on the basement orthogneiss and Jurassic granitc gneiss in the microcontinent. In the heating stage, four sam- pies exhibited a flat plateau age, with the value Tp concentrated in the range of 166-176 Ma; isochron age Ti was concentrated in the range of 165-175 Ma, and their corresponding ages were the consistent within allowable range. The ages should be representative of the era of the final deformation of the Amdo gneiss and cooling emplacement of the magmatic rock in the Jurassic. The geochronological studies have shown that the final deformation of microcontinent crystalline basement and the cooling of the Mesozoic large-scale tectonothermal events occurred in late Middle Jurassic. In Middle Jurassic, Nyainrong microcontinent experienced strong tectonic movement. Combining with the geochronologi- cal with isotope geochemistry for the microcontinent, the cause of the tectonothermal event should be attributed to the collision between the Nyainrong microcontinent and South Qiangtang Block following the northward subduction of Bangong-Nujiang oceanic crust.