Geochemistry,petrogenesis and radioactive mineralization of two coeval Neoproterozoic post-collisional calc-alkaline and alkaline granitoid suites from Sinai,Arabian Nubian Shield

The Younger Granites of Yahmid-Um Adawi area, located in the southeastern part of Sinai Peninsula, comprise two coeval Late Neoproterozoic post-collisional alkaline (hypersolvous alkali-feldspar granites; 608–580?Ma) and calc-alkaline (transsolvous monzo- and syenogranites; 635–590?Ma) suites. The calc-alkaline suite granitoids are magnesian and peraluminous to metaluminous, whereas the alkaline ones are magnesian to ferroan alkaline to slightly metaluminous. Both granitoid suites exhibit many of the typical geochemical features of A-type granites such as enrichment in Nb (>20?ppm), Zr (>250?ppm), Zn (>100?ppm) and Ce (>100?ppm) and high 10000*Ga/Al₂O₃ ratios (>2.6) and Zr?+?Nb?+?Y?+?Ce (>350?ppm). Accessory mineral saturation thermometers demonstrated former crystallization of apatite at high temperatures prior to zircon and monazite separation from the magma for both granitoid suites. The mild zircon saturation temperatures of the studied Younger Granites (around 800?°C) imply low-temperature crustal fusion and incomplete melting of the largely refractory zircon. The two Younger Granite suites were semi-synchronously evolved during the post-collisional stage of the Arabian-Nubian Shield subsequent to the collision between the juvenile shield crust and the older pre-Neoproterozoic continental blocks of west Gondwana. Their parental magmas has been generated by melting of crustal source rocks with minor involvement from mantle, which might participated chiefly as a source of heat necessary for fusion of the crustal precursor. Extensive in-situ gamma-ray spectrometry revealed anomalously high radioactivity of some Younger Granite exposures along Wadi Um Adawi (eU; 388–746?ppm and eTh; 1857–2527?ppm) and pegmatitic pockets pertaining to the calc-alkaline suite (equivalent U and Th; 212–252?ppm and 750–1757?ppm, respectively). The radioactivity of the syngenetic pegmatites arises from the primary radioactive minerals uranothorite and thorite together with the U- and/or Th-bearing minerals zircon, columbite, samarskite and monazite. The anomalously high radioactivity of some Younger Granite exposures in Wadi Um Adawi stem from their appreciable enclosure of the epigenetic uranium minerals metatorbenite and uranophane.     

Mineralogical,geochemical and Sr-Nd isotopes characteristics of fluorite-bearing granites in the Northern Arabian-Nubian Shield,Egypt: Constraints on petrogenesis and evolution of their associated rare metal mineralization

The Central Eastern Desert (CED) of Egypt, a part of Neoproterozoic Arabian Nubian Shield (ANS), embraces a multiplicity of rare metal bearing granitoids. Gabal El-Ineigi represents one of these granitic plutons and is a good example of the fluorite-bearing rare metal granites in the ANS. It is a composite pluton consisting of a porphyritic syenogranite (SG; normal granite) and coarse- to medium-grained highly evolved alkali-feldspar granite (AFG; fluorite and rare metal bearing granite) intruded into older granodiorite and metagabbro-diorite rocks. The rock-forming minerals are quartz, K-feldspar (Or_94-99), plagioclase (An_0-6) and biotite (protolithonite-siderophyllite) in both granitic types, with subordinate muscovite (Li-phengite) and fluorite in the AFG. Columbite-(Fe), fergusonite-(Y), rutile, zircon and thorite are the main accessory phases in the AFG while allanite-(Ce) and epidote are exclusively encountered in the SG. Texture and chemistry of minerals, especially fluorite, columbite and fergusonite, support their magmatic origin. Both granitic types are metaluminous to weakly peraluminous (A/CNK = 0.95–1.01) and belong to the post-collisional A2-type granites, indicating melting of underplated mafic lower crust. The late phase AFG has distinctive geochemical features typical of rare metal bearing granites; it is highly fractionated calc-alkaline characterized by high Rb, Nb, Y, U and many other HFSE and HREE contents, and by extremely low Sr and Ba. Moreover, the REE patterns show pronounced negative Eu anomalies (Eu/Eu¹ = 0.03 and 0.06) and tetrad effect (TE_1,3 = 1.13 and 1.27), implying extensive open system fractionation via fluid–rock interactions that characterize the late magmatic stage differentiation. The SG is remarkably enriched in Sr, Ba and invariably shows a relative enrichment in light rare-earth elements (LREEs). The SG rocks (569 ± 15 Ma) are characterized by relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7034–0.7035) that suggest their derivation from the mantle, with little contamination from the older continental crust. By contrast, the AFG has very high ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios that reflect the disturbance of the Rb-Sr isotopic system and may give an indication for the high temperature magma-fluid interaction. The positive εNd(t) values of AFG (+7.40) and SG (+5.17), corresponding to young Nd-T_DM2 ages ranging from 707 to 893 Ma, clearly reflect the juvenile crustal nature of Gabal El-Ineigi granitoids and preclude the occurrence of pre-Neoproterozoic continental crust in the ANS. The field relationships, chemical, petrological and isotopic characteristics of El-Ineigi SG and AFG prove that they are genetically not associated to each other and indicate a complex origin involving two compositionally distinct parental magmas that were both modified during magmatic fractionation processes. We argue that the SG was formed by partial melting of a mid-crustal source with subsequent fractional crystallization. In contrast, the AFG was generated by partial melting and fractionation of Nb- and Ta-rich amphibole (or biotite) of the lower crust. The appreciable amounts of fluorine in the magma appears to be responsible for the formation of rare metal element complexes (e.g., Nb, Ta, Sn and REEs), and could account for the rare metal mineralization in the El-Ineigi AFG.     

Post-collisional magmatism in the northern Arabian-Nubian Shield: The geotectonic evolution of the alkaline suite at Gebel Tarbush area, south Sinai, Egypt

Post-collisional alkaline magmatism (∼610–580 Ma) is widely distributed in the northern part of the Neoproterozoic Arabian-Nubian Shield (ANS), i.e. the northern part of the Egyptian Eastern Desert and Sinai. Alkaline rocks of G. Tarbush constitute the western limb of the Katharina ring complex (∼593 ± 16 Ma) in southern Sinai. This suite commenced with the extrusion of peralkaline volcanics and quartz syenite subvolcanics intruded by syenogranite and alkali feldspar granite. The mineralogy and geochemistry of these rocks indicate an alkaline/peralkaline within-plate affinity. Quartz syenite is relatively enriched in TiO₂, Fe₂O₃, MgO, CaO, Sr, Ba and depleted in SiO₂, Nb, Y, and Rb. The G. Tarbush alkaline suite most likely evolved via fractionation of mainly feldspar and minor mafic phases (hornblende, aegirine) from a common quartz syenite parental magma, which formed via partial melting of middle crustal rocks of ANS juvenile crust. Mantle melts could have provided the heat required for the middle crustal melting. The upper mantle melting was likely promoted by erosional decompression subsequent to lithospheric delamination and crustal uplift during the late-collisional stage of the ANS. Such an explanation could explain the absence or scarce occurrence of mafic and intermediate lithologies in the abundant late- to post-collisional calc-alkaline and alkaline suites in the northern ANS. Moreover, erosion related to crustal uplift during the late-collision stage could account for the lack or infrequent occurrence of older lithologies, i.e. island arc metavolcanics and marginal basin ophiolites, from the northern part of the ANS.     

Geochemistry and Petrogenesis of Late Ediacaran Rare-metalAlbite Granites of the Arabian-Nubian Shield

The Abu Dabbab albite granite(ADAG), in the central Eastern Desert of Egypt, hosts the most significant rare metal ore deposit in the northern part of the Neoproterozoic Arabian-Nubian Shield. Here, we report detailed field,petrographic, mineralogical and geochemical investigation of the ADAG, an isolated stock-like granitic body with sharp intrusive contacts against metamorphic country rocks, probably emplaced at about 600 Ma. The fine-grained porphyritic upper unit is a preserved remnant of the shallowly-emplaced apex of the magma chamber, whereas the medium-grained lower unit crystallized at deeper levels under subvolcanic conditions. The peraluminous leucocratic ADAG shares common geochemical characteristics with post-collisional intraplate A-type magmas. In addition to the conspicuous enrichment in Na2 O, the ADAG is remarkable for its anomalous concentrations of Ta, Nb, Li, Hf, Ga, Sn, Zn and heavy rare-earth elements. Nb-Ta minerals in the ADAG are mixed with Fe-Mn oxides, forming black patches that increase in abundance toward of the base of the intrusion. Columbite-tantalite, cassiterite and wolframite are the most important ore minerals.Pronounced negative Eu anomalies(Eu/Eu* = 0.10–0.24) reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interaction. The ADAG was most likely generated by partial melting of the juvenile middle crust of the ANS as the geotherm was elevated by erosional uplift following lithospheric delamination and it was emplaced at the intersection of lineations of structural weakness. Although formation of the ADAG and its primary enrichment in rare metals are essentially due to magmatic processes, late-stage metasomatism caused limited redistribution of rare metals. Fluid-driven subsolidus modification was limited to the apex of the magma chamber and drove development of greisen, amazonite, and quartz veins along fracture systems.     

Magmatic and post-magmatic evolution of post-collisional rare-metal bearing granite: The Neoproterozoic Homrit Akarem Granitic Intrusion,south Eastern Desert of Egypt,Arabian-Nubian Shield

The Homrit Akarem granitic intrusion (HAGI) outcrops near the western edge of the south Eastern Desert basement exposure in Egypt. It is a composite of two cogenetic intrusive bodies: an early albite granite phase shallowly emplaced at the apex of a magmatic cupola, and a later subjacent pink granite phase with a marginal zone of muscovite granite and better preservation of magmatic features. Mineral chemistry of primary biotite and garnet, together with whole-rock chemistry, identify the HAGI as a highly fractionated A-type peraluminous intrusion. The chemistry of F-dominant, Li-bearing, Fe³⁺-rich primary magmatic mica in the pink granite resembles that typically found in highly evolved Nb-Y-F pegmatites. The HAGI is the evolved product of a primary magma generated by partial melting of juvenile crust of the Arabian-Nubian Shield (ANS), emplaced along a regional strike-slip fault system that promoted its ascent. The main emplacement mechanism and evolutionary sequence of the HAGI was magmatic, although secondary minerals and textures resulting from hydrothermal fluid interactions are observed, especially at its margins. Primary columbite-(Mn) crystallized from melt and was partly replaced by secondary fluorcalciomicrolite. The high fluorine content of magmatic fluids exsolved from the intrusion is indicated by quartz-fluorite veins, greisenization, albitization, and F-bearing secondary oxide minerals. The magmatic derivation of this fluid is demonstrated by the F-dominant primary mica, a siderophyllite-polylithionite solid solution commonly known as zinnwaldite. The chemistry of zinnwaldite constrains the F/OH activity ratio and oxygen fugacity of its parental melt and thereby resolves the ambiguity between pressure and the effects of F in controlling the normative quartz content of rare-metal granites. The HAGI is less mineralized than the post-collisional rare-metal granites found further east in the south Eastern Desert, replicating a trend observed previously in the central Eastern Desert and suggesting that east-west zoning in rare metal enrichment is a persistent feature across latitudes at the western edge of the ANS.     

Geochemistry and Geotectonic Setting of Neoproterozoic Rocks from Northern Ethiopia (Arabian-Nubian Shield)

Whole-rock geochemical analysis of metavolcanic and plutonic rocks from Mai Kenetal-Negash area, Tigrai, northern Ethiopia was carried out to evaluate their magma type and original tectonic environment. Their major element content has been modified by secondary alteration, and trace and rare-earth elements have been used to investigate their petrogenesis. The low content of compatible elements of the metavolcanic rocks and their relatively high content of incompatible elements and light REE point to a depleted source region, whereas the plutonic rocks show a more pronounced REE fractionation (La_N/Lu_N >4). Metavolcanic and plutonic rocks seem to be cogenetic. Discriminant diagrams suggest that the majority of the metavolcanic and all the plutonic rocks are members of a calc-alkaline suite developed in a volcanic arc setting. The overall geochemical characteristics of both sets of rocks are consistent with the arc accretion models postulated in Sudan, Egypt and Saudi Arabia for the Neoproterozoic evolution of the Arabian-Nubian Shield.     

Constraints of Mantle and Crustal Sources Interaction during Orogenesis of Pre- and Post-collision Granitoids from the Northern Arabian-Nubian Shield: A Case Study from Wadi El-Akhder Granitoids,Southern Sinai,Egypt

The Egyptian older and younger granitic rocks emplaced during pre- and post-collision stages of Neoproterozoic Pan-African orogeny, respectively, are widely distributed in the southern Sinai Peninsula, constituting 70% of the basement outcrops. The Wadi El-Akhder, southwestern Sinai, is a mountainous terrain exposing two granitoid suites, namely the Wadi El-Akhder Older Granites (AOG) and the Homra Younger Granites (HYG). The AOG (granodiorites with subordinate tonalite compositions) have geochemical characteristics of medium-K calc-alkaline, metaluminous to mildly peraluminous granitoids formed in an island-arc environment, which are conformable with well-known Egyptian older granitoids rocks, whereas the HYG display calc-alkaline to slightly alkaline nature, peraluminous syeno-, monzogranites and alkali feldspar granites matching well those of the Egyptian younger granites. With respect to the AOG granitoids, the HYG granites contain lower Al2O3, FeO*, MgO, MnO, CaO, TiO2, Sr, Ba, and V, but higher Na2O, K2O, Nb, Zr, Th, and Rb. The AOG are generally characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values (e.g., negative Nb and Ta anomalies). The geochemical features of the AOG follow assimilation-fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source. The chemical characteristics of the AOG are remarkably similar to those of subduction-related granitoids from the Arabian-Nubian Shield (ANS). The compositional variations from monzogranites through syenogranites to alkali feldspar granite within HYG could not be explained by fractional crystallization solely. Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments, which is similar to the most of younger granitic suites in the ANS.     

新疆东天山哈尔里克二叠纪奥莫尔塔格碱性花岗岩特征、成因及构造意义

新疆哈尔里克山东段发育的奥莫尔塔格碱性花岗岩岩体侵位于小铺东花岗岩中,岩体岩性较为均一,呈紫红色,含有典型的碱性镁铁矿物镁钠铁闪石和霓石,副矿物有锆石、钛铁矿以及萤石等。化学成分上,该岩体富硅、碱、铁,贫钙、镁,低铝,富Rb、Th、Cs、U、Zr,贫Ba、Sr、P、Ti等,Nb、Ta、Hf和Ga等元素的含量也较高,104Ga/Al值变化于3.41~3.65之间。稀土元素总量较高,配分模式右倾,并具明显的Eu负异常(δEu=0.30~0.33);具有较高的锆饱和温度(940~952℃,平均为947℃)。锆石LA-ICP-MS定年测得代表性样品的U-Pb年龄为288.9±1.6 Ma,表明该岩体属于早二叠世岩浆活动的产物。奥莫尔塔格碱性花岗岩具有偏高的εHf(t)值(+8.7~+12.5)和较年轻的单阶段模式年龄(600~430 Ma),并以具较高的Y/Nb值(2.92~3.17)特征类似于A2型花岗岩。结合区域构造背景和岩体的地球化学特征分析,认为奥莫尔塔格A2型碱性花岗岩是新生地壳发生部分熔融形成相当于英云闪长岩、花岗闪长岩等钙碱性的岩石后,它们作为源岩发生再次部分熔融的产物。     

Geochemistry and petrogenesis of the Ediacaran post-collisional Jabal Al-Hassir ring complex,Southern Arabian Shield,Saudi Arabia

The Jabal Al-Hassir ring complex is located between latitudes 19°21′ and 19°42′ N,  and longitudes 42°55′ and 4312′ E, Southern Arabian Shield. It is an alkaline to highly fractionated calc-alkaline granite complex consisting of an inner core of biotite granite followed outward by porphyritic sodic-calcic amphibole (ferrobarroisite) granite. U–Pb zircon geochronology indicates that the Jabal Al-Hassir ring complex was emplaced at ca. 620 Ma. The granites display highly fractionated geochemical features (i.e., Eu/Eu* = 0.05–0.35; enrichment of K, Rb, Th, U, Zr, Hf, Y and REE; depletion of Ta, Nb, Ba, Sr, P, Eu, and Ti). Jabal Al-Hassir granites are post-collisional plutonic rocks and contain abundant microcline perthite and sodic-calcic amphibole, sharing the petrological and chemical features of A₂-type granites. Sr_i values range from 0.70241 to 0.70424, are similar to those expected for magmas extracted from a Neoproterozoic depleted source and much lower than what would be expected, if there was minor involvement of pre-Neoproterozoic continental crust. The geochemical characteristics indicate that their magma was most plausibly represented by partial melting of juvenile lower crust following the collision between East and West Gondwana at the final stage of the Arabian Shield evolution. The data presented in this study are therefore consistent with an intraplate, post-collisional magmatism formed at the beginning of a transition from convergent to extensional tectonics.     

The Neoproterozoic Kolet Um Kharit bimodal metavolcanic rocks, south Eastern Desert, Egypt: a case of enrichment from plume interaction?

Neoproterozoic metavolcanic rocks of Kolet Um Kharit (KUKh) in the southern Eastern Desert of Egypt have been traditionally regarded as a bimodal island-arc sequence. However, geological and geochemical arguments presented here make this interpretation doubtful. Geochemically, these rocks are classified into mafic (tholeiitic basalts) and felsic (high-K rhyodacites to rhyolites) groups. Both the KUKh mafic and felsic metavolcanic rocks show similar geochemical characteristics, implying a genetic link. They have comparable trace element ratios, such as Zr/Nb (27–30 vs. 20–36), Y/Nb (5.44–6.25 vs. 5.05–5.9), K/Rb (577–1164 vs. 573–937), Ba/La (4.29–25–9 vs. 11.4–16.2), Nb/Yb (1.82–2.03 vs. 1.76–1.99). Similarly both groups have parallel LREE-enriched patterns (La/Yb_CN=2.37–2.81 vs. 2.55–3.17); and negative Nb and Ta anomalies (Nb/La_pm=0.51–0.58 vs. 0.45–0.52 and Ta/La_pm=0.51–0.62 vs. 0.49–0.55). The observed negative Nb and Ta anomalies in the KUKh metavolcanic rocks cannot be attributed to crustal contamination or fractional crystallization. These rocks could represent either a remnant of break-up LIP or were derived from an enriched mantle source containing subduction components beneath an intraoceanic back-arc basin. The recognition of the KUKh rocks as derived from an enriched mantle source revives interest in models that involve enrichment from “plume” interaction during the evolution of the Arabian-Nubian Shield.     

Late Triassic granites from the northwestern margin of the Tibetan Plateau,the Dahongliutan example: petrogenesis and tectonic implications for the evolution of the Kangxiwa Palaeo-Tethys

The Dahongliutan granitic pluton consists of two-mica granites and is located in the eastern part of the Western Kunlun Orogen, northwestern Tibetan Plateau. Zircon separates from the pluton yield a SIMS U–Pb age of 217.5 ± 2.8 Ma. Rocks from the pluton contain relatively high and uniform SiO₂ (72.32–73.48 wt%) and total alkalis (Na₂O + K₂O = 8.07–8.67 wt%) and are peraluminous and high-K calc-alkaline to shoshonitic in composition. The Dahongliutan granites are relatively depleted in the high-field-strength elements and the heavy rare earth elements (HREEs) and have relatively high Rb, and low Ba and Sr concentrations. They contain low total rare earth element (REE) concentrations. The light REEs are strongly enriched relative to the HREEs, with (La/Yb)_N values of 28.56–37.01. The ε_Nd(t) values range from ?10.6 to ?8.8, and (⁸⁷Sr/⁸⁶Sr)_i = 0.7142–0.7210. Zircons from the pluton yield ε_Hf(t) values of ?13.8 to ?1.6, and δ¹⁸O = 10.5–11.6‰. Petrographic and geochemical features of the pluton indicate that the granites are S-type and were derived from parting melting of a mixture of metasedimentary and minor metaigneous sources in the middle–lower crust. Magmatic differentiation was dominated by the fractional crystallization of plagioclase, K-feldspar, muscovite, biotite, and accessory monazite, allanite, and Fe–Ti oxides. Regional granitoids were emplaced in the Early-to-Middle Triassic. Other younger granitoids, with ages of 240–200 Ma, are mostly I-type in character and were likely derived from multiple types of source rock, suggesting the source was heterogeneous Triassic crust. Such a scenario is consistent with their formation in a post-collisional setting. Our new data, combined with other geological evidence, suggest that the collision between the Tianshuihai and southern Kunlun terranes occurred between ca. 250 and 240 Ma, resulting in the closure of the Palaeo-Tethys. Post-collisional tectono-magmatic events may have occurred between 240 and 200 Ma.     

Timing,petrogenesis, and setting of granites from the southern Beishan late Palaeozoic granitic belt,Northwest China and implications for their tectonic evolution

Late Palaeozoic granites are widely distributed in the southeastern Beishan area, which is located in the central part of the southern Central Asian Orogenic Belt (CAOB). U–Pb zircon dates of five late Palaeozoic granitic plutons from the southeastern Beishan area yield Permian ages: 285 ± 2 Ma (Shuwojing and Western Shuwojing plutons), 269 ± 3 Ma (Jianquanzi and Jiuquandihongshan plutons), and 260 ± 1 Ma (Jiujing pluton). The early Permian Shuwojing pluton, an unfractionated calc-alkaline biotite monzogranite with slightly positive εNd(t) (+0.7 and +0.6) and low initial ⁸⁷Sr/⁸⁶Sr (0.70722 and 0.70712), and the early Permian Western Shuwojing pluton, a high-K calc-alkaline biotite granite with slightly negative εNd(t) (?0.2 and ?0.1) and low initial ⁸⁷Sr/⁸⁶Sr (0.70390 and 0.70919), are likely derived from a mixture of depleted (juvenile) mantle and upper continental crustal (or sedimentary) material. The mid-Permian Jianquanzi and Jiuquandihongshan monzogranites have highly fractionated potassium-rich calc-alkaline characteristics with negative εNd(t) (?4.3) and very high initial ⁸⁷Sr/⁸⁶Sr (0.71949), reflecting a continental crustal component. The compositionally diverse Jiujing pluton and the granodiorite and high-Sr monzogranite phases display adakite-like compositions with relatively low εNd(t) (?0.1 and ?2.2) and high initial ⁸⁷Sr/⁸⁶Sr (0.70822 and 0.70913). The Jiujing low-Sr monzogranite has higher initial ⁸⁷Sr/⁸⁶Sr (0.73464) and lower εNd(t) (?2.8), indicating a significant continental crustal component in its genesis. This work, combined with the regional geology and previous studies, suggest that the early to middle Permian southern Beishan plutons formed in a post-collisional environment, probably an intracontinental rift environment linked to asthenospheric upwelling in response to the break-off of a subducted slab. In the late Permian, the southern Beishan area was in a compressive tectonic regime and thickening of the continental crust resulted in the formation of the Jiujing adakite-like granite.     

Transpressive Structures in the Ghadir Shear Belt,Eastern Desert,Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian‐Nubian Shield during Gondwana Agglutination

Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric(S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation(L2), and defines ～NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ～NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW-to NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.     

Geochemistry and petrogenesis of a peralkaline granite complex from the Midian Mountains, Saudi Arabia

A zoned intrusion with a biotite granodiorite core and arfvedsonite granite rim represents the source magma for an albitised granite plug near its eastern margin and radioactive siliceous veins along its western margin. A study of selected REE and trace elements of samples from this complex reveals that the albitised granite plug has at least a tenfold enrichment in Zr, Hf, Nb, Ta, Y, Th, U and Sr, and a greatly enhanced heavy/light REE ratio compared with the peralkaline granite. The siliceous veins have even stronger enrichment of these trace elements, but a heavy/light REE ratio and negative eu anomaly similar to the peralkaline granite. It is suggested that the veins were formed from acidic volatile activity and the plug from a combination of highly fractionated magma and co-existing alkaline volatile phase. The granodiorite core intrudes the peralkaline granite and has similar trace element geochemistry. The peralkaline granite is probably derived from the partial melting of the lower crust in the presence of halide-rich volatiles, and the granodiorite from further partial melting under volatile-free conditions.     

阿尔泰南缘克朗盆地铁木尔特花岗岩体年龄、地球化学特征及成因

铁木尔特黑云母花岗岩出露于阿尔泰造山带南缘克朗盆地北西侧,岩体主要为黑云母花岗岩。锆石的SHRIMPU-Pb定年结果表明该岩体结晶年龄为459±4.9Ma。岩石具有高的SiO_2(71.92%～76.58%)和Al_2O_3(12.92%～14.55%)含量,富碱(K_2O+Na_2O=7.09%～8.11%)、高钾(K_2O/Na_2O=1.1～1.62)、低P_2O_5(0.14%)和MgO+FeO(5%)含量,强过铝质(ACNK=1.13～1.34);富集LREE和Th、U、Pb,贫Ba、P、Sr、Ti、Nb,呈现明显的Eu负异常(δEu=0.33～0.53);并具有低的Al_2O_3/TiO_2(100)比值和高的CaO/Na_2O(0.3)比值。以上特征表明该岩体属高钾钙碱性高温型强过铝花岗岩。同时岩石具有低的Sr初始值(~(87)Sr/~(86)Sr=0.701727～0.709951)和相对高的ε_(Nd)(459Ma)(-1.43～-0.98)值,两阶段Nd模式年龄较为均一(t_(2DM)=1.2～1.3Ga)。岩体的原始岩浆可能是富含白云母和黑云母的变泥质岩在大于875℃的条件下脱水熔融的产物,在部分熔融过程中有磷灰石、斜长石、钛铁矿等矿物的残留,并混入少量的幔源物质。结合阿尔泰南缘的区域地质背景综合分析,推测岩体形成于俯冲作用的初期,可能是在挤压背景下由于局部伸展减压以及异常地幔和深部热流不均匀上升提供的热影响发生的部分熔融产生。     

Geochronology and petrogenesis of Cretaceous A-type granites from the NE Jiangnan Orogen,SE China

The Jiangnan Orogen is located at a key tectonic position along the junction between the Yangtze and Cathaysia blocks. We obtained detailed major and trace elements, whole-rock Nd + zircon Hf isotope data, and U–Pb age data from several Mesozoic granites, including the Fuling (FL), Taiping–Huangshan (TH), Lingshan (LS), Sanqingshan (SQS), and Baijuhuajian intrusions in order to investigate their sources and petrogeneses related to extension in South China. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of zircon from the FL, TH, SQS, and LS bodies yield Early Cretaceous ages of 124–135 Ma. These plutons are alkali-feldspar granites to syenogranites–monzogranites, and show A-type affinities. They have high K₂O and total alkali contents, and are enriched in rare earth elements (except for Eu), Zr, and other high-field-strength elements as well as high Ga/Al ratios, and are depleted in Ba and Sr. These granites are metaluminous to weakly peraluminous (ACNK from 0.81 to 1.27). The whole-rock ?Nd(T) values of??5.34 to??0.96 are coupled with zircon ?Hf(T) values (from??5.3 to +4.24), and all samples plot along the mantle array. Field observations, geochronology, geochemistry, Nd isotopic, and zircon Hf isotopic compositions suggest that they formed by the partial melting of Mesoproterozoic metamorphic basement, with input from juvenile, mantle-derived materials in the shallow (<30 km) crust at high temperatures (756–965°C). These melts underwent crystal fractionation of biotite, plagioclase, and K-feldspar. The upwelling of asthenosphere triggered partial melting of the metamorphic protolith in a back-arc or intra-arc rift setting, reflecting rollback of the Pacific plate. Our research adds new geochronologic constraints on Cretaceous (135–120 Ma) A-type granites from the NE sector of the Jiangnan Orogen. Combined with previous research, we suggest that three main episodes of late Mesozoic extensional tectonism took place in South China: (1) 190–170 Ma (mainly inland), (2) 165–120 Ma (including 165–150 Ma in SE Shi-Hang, 135–120 Ma in NE Shi-Hang, and ～125 Ma in the Lower Yangtze River Belt), and (3) 100–90 Ma (coastal area), showing an oceanwards younging trend due to the subduction of the Palaeo-Pacific plate.     

南秦岭早古生代碱性岩地球化学特征及其成因机制：研究进展与展望

南秦岭地区在早古生代经历了大规模且持续性的碱性岩浆活动,其丰富的碱性岩石记录为探索深部地幔物质组成、壳内岩浆演化过程以及地球动力学演化机制提供了珍贵的地质载体。文章基于团队近期对于区域上典型碱性岩的地球化学分析结果和成因机制探讨,旨在对南秦岭早古生代碱性岩浆的源区和演化过程进行全面地约束。南秦岭早古生代碱性岩石类型主要包括一套成分从碱性玄武质向粗面质变化,呈双峰式分布的碱性火山- 侵入岩组合,以及少量与碱性硅酸岩（角闪辉石岩、正长岩）- 碳酸岩共生杂岩体。岩相学、年代学和地球化学证据显示这些碱性岩具有相同的地幔源区,其中演化程度较低的镁铁质端元记录了南秦岭早古生代交代岩石圈地幔的部分熔融事件,交代介质主要为硅酸盐熔体。演化程度较高的碱性岩端元（粗面- 正长岩、碳酸岩）来源于初始镁铁质组分的岩浆分异过程,其中粗面- 正长岩类主要受到以长石和单斜辉石为主的分离结晶作用控制。中生代热液交代过程主要记录在北大巴山东部和武当地块西南缘的早古生代碱性岩体中,热液交代作用促进了碳酸岩杂岩体中稀土元素的富集成矿。副矿物年代学和独居石Nd同位素特征反映了热液可能形成于岩体本身的再活化事件,晚三叠世秦岭地区的造山运动可能对此过程具有促进作用。     

南阿尔金茫崖地区花岗岩类锆石SHRIMPU-Pb定年、Lu-Hf同位素特征及岩石成因

南阿尔金茫崖地区早古生代花岗岩锆石SHRIMP U-Pb定年结果表明,阿克提山花岗岩为264±1Ma,柴水沟花岗岩分别为404±5Ma、406±4Ma,其中的辉绿岩为454±4Ma,常春沟花岗岩分别为411±5Ma、406±3Ma,茫崖镇北石英闪长岩为466±5Ma,阿卡龙山花岗岩为469±6Ma。锆石Lu-Hf同位素分析表明,εHf(t)值大多数为正值,少数继承性锆石为负值,反映了它们的源岩以新生地壳为主,同时,也混有少量的古大陆壳的成分。结合区域地质特征和各岩体的岩石地球化学特征,将南阿尔金茫崖地区早古生代花岗质岩浆活动划分为3期,第一期(465~469Ma)岩石组合为石英闪长岩+花岗闪长岩+花岗岩,具有岛弧火成岩的地球化学属性,其形成可能与洋壳的俯冲作用有关;第二期(404~411Ma)岩石组合为花岗闪长岩+二长花岗岩+正长花岗岩,具有A型花岗岩的地球化学特征,可能与板块碰撞后造山带块体均衡调整有关,第三期(264Ma)岩石组合为石英闪长岩+二长花岗岩+正长花岗岩,也具有I型花岗岩的特征,可能与阿尔金断裂的活动有关。     

胶东地区郭家岭花岗闪长岩的地球化学特征及成因

本文系统地报道了郭家岭花岗闪长岩的主量元素、微量元素和Sr-Nd同位素组成,重点讨论了郭家岭花岗闪长岩的岩石成因、成岩物质来源及其地质意义.研究结果表明,郭家岭花岗闪长岩的SiO2含量从62%至72%,Na2O/K2O=0.64-1.79,多数样品大于1.0;K2O+Na2O值为7.34-8.49%,铝指数A/CNK=0.82～1.1,属于准铝质或过铝质Ⅰ型花岗岩;高Sr含量(Sr＞800μg/g)、低Y和Yb含量(Y＜10μg/g;Yb＜1.0μg/g)、轻重稀土分馏强烈(LaN/YbN=17.4～47.8);这些地球化学特征类似于adakite、太古代TTG岩系、Na质花岗岩及HiSrBa花岗岩而不同于岛弧环境的英安岩.高的初始87Sr/86Sr比值(0.7094～0.7114),负的δNd(t)值(-11.2～-17.5),表明花岗闪长岩与adakite、HiSrBa花岗岩的成因不同而类似于Na质花岗岩和TTG岩系,即郭家岭花岗闪长岩是由下部地壳镁铁质岩石脱水部分熔融作用形成的.Nd同位素与胶东地区基性脉岩的Nd同位素组成相近,表明二者有可能具有相似或相同的源区.但基性脉岩出露面积明显小于花岗岩,二者之间存在着SiO2成分间隔,并且基性脉岩的不相容元素明显高于花岗闪长岩,从而表明花岗闪长岩不是基性脉岩结晶分异的产物,这与由幔源岩浆直接分异形成的高Sr、Ba花岗岩(HiSrBa)的成因不同.因此,花岗闪长岩有可能来源于早先与脉岩源区相似的基性岩浆底侵作用而形成的下地壳镁铁质岩石.其地球化学特征表明母岩浆在岩浆演化过程中经历了少量地壳混染作用.Mg#＞50的岩石具有高Sr、Sr/Y、La/Yb等地球化学特征,表明岩浆形成时源区残留相中含有密度较大的石榴石,从而可能引起下地壳物质及岩石圈地幔的拆沉.这对研究胶东地区地区乃至中国东部在中生代期间岩石圈强烈减薄作用的机制和过程具有重要的地质意义.