Granitoids of the Ufalei block (South Urals): Sr-Nd isotope systematics, geodynamic position and genetic reconstructions

Petrogeochemical and isotopic-geochronological signatures in granitoids developed in structures with complex geological history represent an important feature for reconstructing paleogeodynamic settings. Granitoids are widespread in the western slope of the Urals, where the Uralian Orogen contacts via a collage of different-age blocks of the east European Platform. The Ufalei block located in the Central Urals megazone at the junction between the South and Middle Urals’ segments represents one such boundary structure with multistage geological evolution. The isotopic ages obtained by different methods for acid igneous rocks range from 1290 to 245 Ma. We determined close Rb-Sr and Sm-Nd ages (317 Ma) for granites of the Nizhnii Ufalei Massif. By their petrochemical parameters, granitoids and host granite-gneisses differ principally from each other: the former are close to subduction-related, while the latter, to continental-riftogenic varieties. The primary ratio (⁸⁷Sr/8⁶Sr)₀ = 0.70428 and ?_Nd ≈ +4 values indicate significant contribution of oceanic (island-arc?) material to the substrate, which served as a source for granites of the Nizhnii Ufalei Massif. Model Nd ages of granites vary from 641 to 550 Ma. Distinct oceanic rocks and varieties with such ages are missing from the surrounding structures. New isotopic dates obtained for ultramafic and mafic rocks from different zones of the Urals related to the Cadomian cycle imply development of unexposed Upper Riphean-Vendian “oceanic” rocks in the central part of the Ufalei block, which played a substantial role in the formation of the Nizhnii Ufalei granitoids. Such rocks could be represented, for example, by fragments of the Precambrian Timanide-type ophiolite association. The analysis of original materials combined with published data point to the heterogeneous composition and structure of the Ufalei block and a significant part of the western segment of the Central Uralian Uplift and extremely complex geological history of the region coupling the Uralian Orogen with the East European Platform in the present-day structure.     

Archean U-Pb isotope age of zircon from dunite of the Nizhny Tagil massif (the Uralian Platinum Belt)

Doklady Earth Sciences - Material identity of dunite from zoned-type massifs of the Aldan shield (Kondyor, Chad and others) and dunite “cores” from the Uralian Platinum Belt (i.e., UPB)...     

Attribution of the Langshan Tectonic Belt: Evidence from zircon U-Pb ages and Hf isotope compositions

This study describes a previously unidentified Neoproterozoic mafic dyke emplaced in the northern flank of the Langshan Tectonic Belt. This dyke intruded into the micaquartz schist of the Zhaertaishan Group, and yielded an age of 908 ± 8 Ma. The youngest U-Pb ages of micaquartz schist from the Zhaertaishan Group in the Langshan area were 1118 ± 33 Ma,1187 ± 3 Ma and 1189 ± 39 Ma,suggesting that the depositional age of the protolith of the schist was between 908 ± 8 Ma and 1118 ± 33 Ma. In addition, 436 U-Pb age data and 155 Lu-Hf isotopic data from six samples in the Langshan Tectonic Belt and one Permian greywacke from the Wuhai area show distinct differences between the northern and southern flanks of the Main Langshan area. The U-Pb ages of the northern flank are primarily Meso-Neoproterozoic; similar ages have not been identified in the southern flank to date.Moreover, two-stage Hf model ages of the northern flank feature three age peaks at ～900 Ma,～1700 Ma and ～2600 Ma; this differs from Hf model ages of the southern flank, which feature one strong age peak at ～2700 Ma. These results suggest that the northern and southern flanks of the Main Langshan area have different geochronologic characteristics and should be divided further. Based on the U-Pb ages and Hf model ages, the northern and southern flanks of the Main Langshan area are named the North and South Langshan Tectonic Belts. Comparison of the U-Pb age and two-stage Hf model age distributions from the North Langshan Tectonic Belt, South Langshan Tectonic Belt, Alxa Block and the North China Craton(NCC) reveal that the North Langshan Tectonic Belt is similar to the Alxa Block and that the South Langshan Tectonic Belt is similar to the NCC. In addition, the zircon U-Pb age of 860 ±7 Ma commonly observed in the Alxa Block was detected in the Permian greywacke from the Wuhai area of the NCC, which suggests that the amalgamation of the North and South Langshan Tectonic belts(i.e.,the amalgamation of the Alxa Block and the NCC), occurred between Devonian and late Permian.     

胶东三佛山花岗岩的成因:成岩物理化学条件、锆石U-Pb年代学及Sr-Nd同位素约束

三佛山花岗岩出露于华北克拉通东缘,其成岩物理化学条件、矿物学特征及准确的侵位年龄仍缺乏系统研究,暗色微粒包体与寄主岩石关系尚不明确,而这些问题的解决对于约束其成因至关重要。本文利用角闪石全Al压力计、黑云母Ti温度计,角闪石-黑云母矿物对温压计及全岩主、微量和锆石U-Pb年龄数据,限定了三佛山花岗岩岩石类型、侵位时代及成岩物理化学条件。此外,测试并收集了大量胶东地区早白垩世侵入体的Sr-Nd及锆石Hf-O同位素数据,综合此次矿物化学研究的新认识,探讨了三佛山花岗岩的岩浆源区和岩石成因。通过角闪石全Al压力计及角闪石-黑云母压力计得到其就位压力2.20kbar;采用角闪石全Al温度计、黑云母Ti温度计,得到的结晶温度为730~680℃。LA-ICP-MS U-Pb测试结果表明三佛山花岗岩侵位于119.6~114.2Ma。根据其地球化学特征,结合区域上年代学、Sr-Nd-Hf同位素研究成果,判定其为I型花岗岩,发育的暗色微粒包体及基性脉岩也应该是同时期的产物,并且局部可能存在岩浆混合作用。花岗岩属富钾-钙碱性花岗岩类(KCG),为挤压造山转变成伸展构造背景下的产物;其较高的初始Sr同位素比值和比较低的ε_(Nd)(t)值,全岩Nd两阶段模式年龄t_(2DM)(2447~2143Ma)等均指示源区主体为重熔的古元古代华南板块中下地壳。     

The middle Eocene high-K magmatism in Eastern Iran Magmatic Belt: constraints from U-Pb zircon geochronology and Sr-Nd isotopic ratios

ABSTRACT

Intrusive rocks are well-exposed in the south Birjand around the Koudakan is herein compared to previously studied outcrops along the middle Eocene to late Oligocene Eastern Iran Magmatic Belt. This pluton is composed mainly of monzonite, quartz-monzonite, and granite with high-K calc-alkaline to shoshonitic affinities. The U-Pb zircon geochronology from monzonite and quartz-monzonite reveals the crystallization ages of 40.96 ± 0.48 to 38.78 ± 0.78 Ma (Bartonian). The monzonite, quartz-monzonite, and granite rocks show similar REEs and trace element patterns, as well as limited variations in εNd_(i) and ⁸⁷Sr/⁸⁶Sr_(i) ratio, suggesting that they are a comagmatic intrusive suite. The chondrite and primitive mantle normalized rare earth and trace element patterns show enrichment in the light rare earth elements, K, Rb, Cs, Pb, Th, and U and depletion in heavy rare earth elements, Nb, Zr, and Ti. The εNd_(i) and ⁸⁷Sr/⁸⁶Sr_(i) values range from +1.32 to +1.68 and 0.7044 to 0.7047, respectively, identical to island-arc basalt composition. The whole-rock Nd model age (T_DM) for the intrusive rocks range between 0.69 and 0.73 Ga. These geochemical and isotopic signatures indicate a subduction-related sub-continental lithospheric mantle source for these rocks. Our new geochemical, isotopic, and geochronological studies integrated with previously published data indicate that the middle Eocene to late Oligocene magmatism in eastern Iran was formed in a post-collisional tectonic environment. We suggest the northeastward subduction of the Neo-Tethys ocean beneath the Lut block and the eastward subduction of the Sistan ocean beneath the Afghan block caused mantle wedge to be metasomatized by slab components. At a later stage, a collision between the Lut and Afghan blocks was accompanied by the lithospheric delamination, and the subsequent asthenospheric upwelling led to the melting of the metasomatized sub-continental lithospheric mantle and the generation of middle Eocene to late Oligocene magmatism in the Eastern Iran Magmatic Belt.     

Detrital zircon U-Pb ages in the Rif Belt (northern Morocco): Paleogeographic implications

Detrital zircon U-Pb age distributions in Mesozoic and Cenozoic rocks from the External Rif and Maghrebian Flysch Complex (including the so-called Mauretanian internal flysch units) are very similar, strongly suggesting that the External Rif and the entire Maghrebian Flysch Complex were part of the same NW African paleomargin. These patterns include scarce Paleozoic zircon grains that show influence from the Sehoul Block. Neoproterozoic and Paleoproterozoic grains are abundant with a dominant Ediacaran zircon population at ca. 590 Ma, which could have been sourced from the Variscan Moroccan Mesetas, the northern components of the West African Craton, or from Triassic sediments from the Central High Atlas and Argana basins. Mesoproterozoic zircon ages between 1.1 and 1.6 Ga were also observed (15% in the combined age spectra), the nearest sources for these being in the central part of the West African Craton. Transport of the Mesoproterozoic grains to the NW African paleomargin requires northward-directed fluvial systems parallel to the Central Atlantic continental margin of Africa. In contrast, samples from the Internal Rif or Alborán Domain are different to those from the External Rif and Maghrebian Flysch Complex, especially in the scarcity of Mesoproterozoic zircons, suggesting that the Alborán Domain was not a source area for zircons found in the NW African paleomargin.     

新疆皮山镁质矽卡岩矿床(含糖玉)成因及锆石SHRIMP U-Pb定年

新疆和田透闪石集合体(软玉)矿带约1 300 km,是世界上最大的软玉矿带。除传统上认识的白玉、青玉和墨玉外,近年来在皮山县发现的一种软玉呈红棕色(糖玉),较为少见。该糖玉矿体产于镁质大理岩与石英闪长岩之间的镁质矽卡岩中,显示矿床为接触交代变质成因。通过镜下观察,发现该地区的糖玉主要由纤维状透闪石和极少量的杂质矿物组成。糖色杂质呈浸染状、叶片状、细脉状或沿裂隙分布,并形成于玉石成矿期或构造活动间歇。糖玉颜色与镁质大理岩中大量的红褐色氧化物有关。通过电子探针和X光粉晶衍射测试均表明糖玉主要由透闪石组成,并含少量伊利石、镁橄榄石、透辉石、磁铁矿等杂质矿物。软玉(细粒透闪石集合体)主要通过透闪石交代大理岩和透辉石形成。样品全岩的化学成分与透闪石晶体化学组成类似,全岩稀土配分模式显示Eu负异常(δEu=0.09~0.28)、LREE亏损、HREE平坦、整体稀土含量(1.94×10~(-6)~26.52×10~(-6))、Cr_2O_3(0.00~0.03×10~(-6))和Ni(0.00~0.01×10~(-6))含量低。成矿流体中氢同位素δD为-81.0‰~-84.0‰,均值为-82.25‰;δ~(18)O在330℃时为3.16‰~5.48‰。这些氢和氧同位素的数值显示形成软玉的成矿流体主要由岩浆热液、大气降水和大理岩脱出的CO2组成。这些糖玉的地球化学和成矿流体组成与已报道的典型的镁质矽卡岩矿床中软玉的组成类似。从透闪石集合体中选出的岩浆锆石年龄约456±7 Ma,这个年龄可以认为是形成该糖玉矿床年龄的上限。     

Character of zircon distribution in dunites of the South Urals (Sakharin and East Khabarnin massifs)

Zircons in dunites of the Sakharin and East Khararnin massifs, situated in the South Urals and part of the platinum-bearing Uralian belt, were investigated for the first time. Several types were identified in the polygene-polychronous zircon assemblage of both massifs. The first is represented by Proterozoic (from 1517 ± 12 to 2693 ± 9 Ma) crystals similar to those widely spread in the Riphean and more ancient Uralian deposits. The second type includes dunite varieties of typical magmatic habit and of ages from 377 ± 3.6 Ma to 402 ± 3 Ma. The third contains crystals and crystal fragments of a high degree of crystallinity. The concordant Archean dating (2808 ± 26 Ma) for zircons of this type determines the minimum age of the dunite substrate. Zircons of Type 4 are heterogeneous, consist of the relict (nuclear) part represented by crystals of Types I and III and the newly formed mantle, and are consistent with zircons of Type II.     

U-Pb zircon dating,Sr-Nd isotope and petrogenesis of Sarduiyeh granitoid in SE of the UDMA,Iran: implication for the source origin and magmatic evolution

ABSTRACT

The Sarduiyeh granitoid (SG) is intruded in the southeastern part of the Dehaj-Sarduiyeh volcano-sedimentary belt in the southeastern end of the Urumieh-Dokhtar Magmatic Arc (UDMA) in Iran. The medium-to-coarse-grained granitoid unit, with granular texture consists mainly of diorite, tonalite, granodiorite and monzogranitic rocks. Mineralogically, these rocks consist mainly of plagioclase, K-feldspar, quartz, biotite and hornblende. The whole rock geochemical analyses indicates that the SG is calc-alkaline, I-type, metaluminous, enriched in large ion lithophile elements (LILE; such as K, Cs, Pb) and depleted in high field strength elements (HFSE; such as Ti, Nb, Ta, Zr). Chondrite normalized plot of SG rare earth elements (REE) show light rare earth element enrichments with (La_N/Yb_N = 2.44–8.68) and flat heavy rare earth element patterns with (Gd_N/Yb_N = 1.02–1.36). The rather high Y (av. 19.35 ppm), low Sr content (av. 293.76 ppm) and low Cr and Ni contents (av. 20.1 and 4.69 ppm, respectively) of the SG demonstrate its normal calc-alkaline and non-adakitic nature, the features of Jebal Barez-type granitoids. The geochemical characteristics and isotopic composition, low I_Sr (0.7046–0.7049) and positive ?^tNd (+3.4 to +4.03) values, of the SG suggest that its parental magma formed as a result of partial melting from metabasic rocks of lower crust in a subduction-related arc setting. Fractionation of an assemblage dominated by plagioclase, K-feldspar, amphibole and magnetite may have been responsible for the evolution of the SG magma. U-Pb zircon geochronology gives an age of 27.95 ± 0.27 Ma for the SG, suggesting that the final collision between the Arabian plate and Central Iranian microcontinent may have happened in the Late Oligocene.