Neoproterozoic SHRIMP U–Pb zircon ages of silica-rich Dokhan Volcanics in the North Eastern Desert,Egypt

Chronology of Neoproterozoic volcanosedimentary successions remains controversial for many regions of the Arabian–Nubian Shield, including the Dokhan Volcanics of NE Egypt. New U–Pb zircon SHRIMP ages have been obtained for 10 silica-rich ignimbrites and two subvolcanic dacitic bodies, mapped as Dokhan Volcanics, from the North Eastern Desert of Egypt. Crystallization ages range between 592 ± 5 and 630 ± 6 Ma (Early Ediacaran). Apparently, the late consolidation of the Arabian–Nubian Shield was accompanied by the evolution of isolated volcanic centres and basin systems which developed during a period of approx. 40 Ma, independently in space and time and probably under changing tectonic regimes. The obtained age data together with other previously published reliable ages for Dokhan Volcanics suggest two main pulses of volcanic activity: 630–623 Ma and 618–592 Ma. Five samples contain inherited zircons, with ages of 669, 715–746, 847 and 1530 Ma, supporting models that North Eastern Desert crust is mainly juvenile Neoproterozoic crust.     

U–Pb ID-TIMS dating of igneous and metaigneous rocks from the El-Sibai area: time constraints on the tectonic evolution of the Central Eastern Desert,Egypt

This paper presents new ID-TIMS U–Pb zircon and titanite ages from the El-Sibai gneiss complex in the Eastern Desert of Egypt. The zircon data support previous studies, indicating that the protoliths of the gneissic (oldest) units in the area were emplaced during the East African orogeny, and do not represent an older pre-Neoproterozoic, reworked cratonic basement. The crystallization ages of three compositionally distinct orthogneiss protoliths are c. 685, 682 and 679 Ma, respectively. A U–Pb titanite age from one orthogneiss overlaps with the protolith age, indicating that the gneisses did not undergo post-magmatic high-temperature metamorphism. The gneissic textures of the rocks are therefore interpreted to reflect syn-emplacement deformation. This, and evidence for static amphibolite facies metamorphism in country-rock metavolcanics, lead us to conclude that the gneisses of El-Sibai do not represent an exhumed middle crustal gneiss dome, but are part of the island arc affined allochthon into which they were emplaced synchronously with NW-ward nappe translation. We also report ages from rocks cross-cutting the gneisses and the surrounding island arc affined assemblages that yield the hitherto youngest robust pre-Cretaceous intrusive ages in the Eastern Desert. The dated rocks are an anorthosite and a cross-cutting syenogranite giving ages of c. 541 and 540 Ma, respectively. We consider this late magmatic pulse to be anorogenic, most likely reflecting a separate extensional event involving asthenospheric upwelling and decompression melting of the mantle.     

Age constraints on the formation and emplacement of Neoproterozoic ophiolites along the Allaqi–Heiani Suture,South Eastern Desert of Egypt

Ophiolites are key components of the Neoproterozoic Arabian–Nubian Shield (ANS). Understanding when they formed and were emplaced is crucial for understanding the evolution of the ANS because their ages tell when seafloor spreading and terrane accretion occurred. The Yanbu–Onib–Sol Hamed–Gerf–Allaqi–Heiani (YOSHGAH) suture and ophiolite belt can be traced ∼ 600 km across the Nubian and Arabian shields. We report five new SHRIMP U–Pb zircon ages from igneous rocks along the Allaqi segment of the YOSHGAH suture in southernmost Egypt and use these data in conjunction with other age constraints to evaluate YOSHGAH suture evolution. Ophiolitic layered gabbro gave a concordia age of 730 ± 6 Ma, and a metadacite from overlying arc-type metavolcanic rocks yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 733 ± 7 Ma, indicating ophiolite formation at ∼ 730 Ma. Ophiolite emplacement is also constrained by intrusive bodies: a gabbro yielded a concordia age of 697 ± 5 Ma, and a quartz-diorite yielded a concordia age of 709 ± 4 Ma. Cessation of deformation is constrained by syn- to post-tectonic granite with a concordia age of 629 ± 5 Ma. These new data, combined with published zircon ages for ophiolites and stitching plutons from the YOSHGAH suture zone, suggest a 2-stage evolution for the YOSHGAH ophiolite belt (∼ 810–780 Ma and ∼ 730–750 Ma) and indicate that accretion between the Gabgaba–Gebeit–Hijaz terranes to the south and the SE Desert–Midyan terranes to the north occurred as early as 730 Ma and no later than 709 ± 4 Ma.     

SIMS zircon U–Pb and mica K–Ar geochronology,and Sr–Nd isotope geochemistry of Neoproterozoic granitoids and their bearing on the evolution of the north Eastern Desert,Egypt

Granitic rocks are commonly used as means to study chemical evolution of continental crust, particularly, their isotopic compositions, which reflect the relative contributions of mantle and crustal components in their genesis. New SIMS and K–Ar geochronology, isotope, geochemical, and mineral chemistry data are presented for the granitoid rocks located in and around Gabal Dara in the Northern Eastern Desert of Egypt. The granitoid suite comprises quartz diorites, Muscovite (Mus) trondhjemites, and granodiorites intruded by biotite-hornblende (BH) granites and alkali feldspar (AF) granites. Mus trondhjemite, granodiorite and BH granite exhibit I-type calc alkaline affinities. Mus trondhjemite and granodiorite show medium-K calc-alkaline and metaluminous/mildy peraluminous affinities, whereas BH granites have high-K calc-alkaline and metaluminous character. Concordant ²⁰⁶Pb/²³⁸U weighted mean ages together with geochemical peculiarities suggest that Mus trondhjemites (741 Ma) followed by granodiorites (720 Ma) are genetically unrelated, and formed in subduction-related regime by partial melting of lower oceanic crust together with a significant proportion of mantle melt. The genesis of Mus trondhjemites is correlated with the main event in the evolution of the Eastern Desert, called “~750 Ma crust forming event”.The field and geochemical criteria together with age data assign the high-K calc-alkaline BH granites (608–590 Ma) and alkaline AF granites (600–592 Ma) as post-collisional granites. The differences in geochemical traits, e.g. high-K calc-alkaline versus alkaline/peralkaline affinities respectively, suggest that BH granites and AF granites are genetically unrelated. The age overlap indicating coeval generation of calc-alkaline and alkaline melts, which in turn suggests that magma genesis was controlled by local composition of the source. The high-K calc-alkaline BH granites are most likely generated from lithospheric mantle melt which have been hybridized by crustal melts produced by underplating process. AF granites exhibit enrichment in K₂O, Rb, Nb, Y, and Th, and depletion in Al₂O₃, TiO₂, MgO, CaO, FeO, P₂O₅, Sr, and Ba as well as alkaline/peralkaline affinity. These geochemical criteria combined with the moderately fractionated rare earth elements pattern (La_N/Yb_N = 9–14) suggest that AF granite magma might have been generated by partial melting of Arabian–Nubian Shield (ANS) arc crust in response of upwelling of hot asthenospheric mantle melts, which became in direct contact with lower ANS continental crust material due to delamination. Furthermore, a minor role of crystal fractionation of plagioclase, amphibole, biotite, zircon, and titanomagnetite in the evolution of AF granites is also suggested. The low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7033–0.7037) and positive ε_Nd(T) values (+ 2.32 to + 4.71) clearly reflect a significant involvement of depleted mantle source in the generation of the post-collision granites and a juvenile nature for the ANS.     

Pan-African adakitic rocks of the north Arabian–Nubian Shield: petrological and geochemical constraints on the evolution of the Dokhan volcanics in the north Eastern Desert of Egypt

The Precambrian basement of Egypt is part of the Red Sea Mountains and represents the north-western part of the Arabian–Nubian Shield (ANS). Five volcanic sections are exposed in the Egyptian basement complex, namely El Kharaza, Monqul, Abu Had, Mellaha and Abu Marwa. They are located in the north Eastern Desert (ED) of Egypt and were selected for petrological and geochemical studies as they represent the Dokhan volcanics. The volcanics divide into two main pulses, and each pulse was frequently accompanied by deposition of immature molasse type sediments, which represent a thick sequence of the Hammamat group in the north ED. Compositionally, the rocks form a continuum from basaltic andesite, andesite, dacite (lower succession) to rhyodacite and rhyolite (upper succession), with no apparent compositional gaps. These high-K calc-alkaline rocks have strong affinities to subduction-related rocks with enriched LILEs (Rb, Ba, K, Th, Ce) relative to high field strength elements (Nb, Zr, P, Ti) and negative Nb anomalies relative to NMORB. The lower succession displays geochemical characteristics of adakitic rocks with SiO₂ >53 wt%, Al₂O₃ >15 wt%, MgO >2.5 wt%, Mg# >49, Sr >650 ppm, Y <17 ppm, Yb <2 ppm, Ni >25 ppm, Cr >50 ppm and Sr/Y >42.4. They also have low Nb, Rb and Zr compared to the coexisting calc-alkaline rhyodacites and rhyolites. The highly fractionated rhyolitic rocks have strong negative Eu anomalies and possess the geochemical characteristics of A-type suites. Trace element geochemical signatures indicate a magma source consistent with post-collisional suites that retain destructive plate signatures associated with subduction zones. The adakitic rocks in the northern ANS are generated through partial melting of delaminated mafic lower crust interacting with overlying mantle-derived magma. The Dokhan volcanics were likely generated by a combination of processes, including partial melting, crystal fractionation and assimilation.     

Neoproterozoic arc–back-arc system in the Central Eastern Desert of Egypt: Evidence from supra-subduction zone ophiolites

Ophiolites are widely distributed in the Central Eastern Desert (CED) of Egypt, occurring as clusters in the northern (NCEDO) and southern (SCEDO) segments. Mineralogical and geochemical data on the volcanic sections of Wizer (WZO) and Abu Meriewa (AMO) ophiolites as representatives of the NCEDO and SCEDO, respectively, are presented.The WZO volcanic sequence comprises massive metavolcanics of MORB-like compositions intruded by minor boninitic dykes and thrust over island-arc metavolcanic blocks in the mélange matrix. Such transitional MORB-IAT-boninitic magmatic affinities for the WZO metavolcanics suggest that they most likely formed in a protoarc–forearc setting. Chemical compositions of primary clinopyroxene and Cr-spinel relicts from the WZO volcanic section further confirm this interpretation. The compositional variability in the WZO volcanic sequence is comparable with the associated mantle rocks that vary from slightly depleted harzburgites to highly depleted harzburgites containing small dunite bodies, which are residues after MORB, IAT and boninite melt formation, respectively. Source characteristics of the different lava groups from the WZO indicate generation via partial melting of a MORB source which was progressively depleted by melt extraction and variably enriched by subduction zone fluids. MORB-like magma may have been derived from ~ 20% partial melting of an undepleted lherzolite source, leaving slightly depleted harzburgite as a residuum. The generation of island-arc magma can be accounted for by partial melting (~ 15%) of the latter harzburgitic mantle source, whereas boninites may have been derived from partial melting (~ 20%) of a more refractory mantle source previously depleted by melt extraction of MORB and IAT melts, leaving ultra-refractory dunite bodies as residuum.The AMO volcanic unit occurs as highly deformed pillowed metavolcanic rocks in a mélange matrix. They can be categorized geochemically into LREE-depleted (La/Yb_CN = 0.41–0.50) and LREE-enriched (La/Yb_CN = 4.7–4.9) lava types that show an island arc to MORB geochemical signature, respectively, signifying a back-arc basin setting. This is consistent, as well, with their mantle section. Source characteristics indicate depleted to slightly enriched mantle sources with overall slight subduction zone geochemical affinities as compared to the WZO.Generally, CED ophiolites show supra-subduction zone geochemical signature with prevalent island arc tholeiitic and minor boninitic affinities in the NCEDO and MORB/island-arc association in the SCEDO. Such differences in geochemical characteristics of the NCEDO and SCEDO, along with the abundance of mature island arc metavolcanics which are close in age (~ 750 Ma) to the ophiolitic rocks, general enrichment in HFSE of ophiolites from north to south, and lack of a crustal break and major shear zones, is best explained by a geotectonic model whereby the CED represents an arc–back-arc system above a southeast-dipping subduction zone.     

Zircon U–Pb age constraints on the Paleoproterozoic sedimentary basement of the Ediacaran Porongos Group,Sul-Riograndense Shield,southern Brazil

Thick quartzites record significant information on cratonic environments during long geological periods. The capacity to resist weathering and deformation turn the quartzite covers especially useful in the provenance studies of Precambrian basins. Provenance of 194 detrital zircon grains from two samples of thick quartzite cover on the Paleoproterozoic Encantadas Complex displays mostly Paleoproterozoic (95%) and minor Archean (5%) sources. The results indicate that sediments were derived from the La Plata Craton with the maximum depositional age at 2.03 Ga possibly up to 1.7 Ga. In comparison, the adjacent Porongos Group has provenance data of 61 detrital zircon grains indicating mostly Mesoproterozoic (69%), subordinately Paleoproterozoic (26%) and minor Archean ages (5%). Considering previous published data, the Porongos Group is Ediacaran in age and probably chronocorrelated with sedimentary basins from the Tandilia Belt (Argentina). Therefore, the quartzite cover and the Porongos Group require distinct evolution in time and in tectonic environment.     

Granitoid evolution in Sinai,Egypt, based on precise SHRIMP U–Pb zircon geochronology

Late-stage Pan-African granitoids, including monzogranite, syenogranite and alkali granite, were collected from four separate localities in Sinai. They were selected to represent both the calc-alkaline and alkaline suites that have been viewed as forming separate magmatic episodes in the Eastern Desert of Egypt, with the transition to alkali granite at ~ 610 Ma taken to mark the onset of crustal extension. Although intrusive relations were observed in the field, the emplacement ages of the granitoids cannot be distinguished within analytical uncertainty and they all formed within a restricted time span from 579 to 594 Ma. This indicates that the two suites are coeval and that some calc-alkaline rocks were also likely generated during the late extensional phase. These ages are identical to those recently obtained from similar rocks in the North-Eastern Desert, confirming that Sinai is the northern extension of the Eastern Desert Pan-African terrane of Egypt. Rare inherited zircons with ages of ~ 1790 and ~ 740 Ma are present in syenogranite from northeastern Sinai and indicate that older material is present within the basement. A few zircons record younger ages and, although some may reflect later disturbance of the main zircon population, those with ages of ~ 570 and 535 Ma probably reflect thermal events associated with the extensive emplacement of mafic and felsic dykes in both northeastern and southern Sinai.     

Listvenite–lode association at the Barramiya gold mine,Eastern Desert,Egypt

Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses.The mineralized quartz veins are characterized by abundant carbonic (CO₂ ± CH₄ ± H₂O) and aqueous-carbonic (H₂O–NaCl–CO₂ ± CH₄) inclusions along intragranular trails, whereas aqueous inclusions (H₂O–NaCl ± CO₂) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ³⁴S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ³⁴S_Σs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO₂ via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites.     

LA-ICPMS U–Pb zircon age constraints on the provenance of Cretaceous sediments in the Yichang area of the Jianghan Basin,central China

Detrital zircon U–Pb ages have been shown to be a powerful tool for provenance analysis and determining the exhumation of sediment source areas. This paper presents the results of detrital zircon LA-ICPMS U–Pb ages for Cretaceous sediments from the Yichang area of the Jianghan Basin, central China. The results provide new information on the provenance of these sediments and the detailed exhumation process of the Huangling Dome. Zircons with different age populations have been derived from the strata of the Huangling Dome. The Liantuo, Gucheng and Nantuo formations and the Kongling complex were exhumed, leading to deposition of the early Cretaceous Wulong Formation, which provides the sources of zircons with age peaks at 3.1–3.0, 2.5 and 1.8 Ga. Exhumation of the Huangling granitoid and contemporary volcanics provided the source of the late Cretaceous Luojingtan Formation, which contains zircons with age peaks at 1.1–0.95 and 0.83–0.74 Ga. The Qinling-Dabie orogeny supplied zircons with an age cluster of 0.27 to 0.18 Ga. These results indicate the timing of initial exhumation for the Huangling granite. They also show how overlying strata was first uplifted and eroded, followed by exposure of underlying strata at the surface during continued exhumation.     

Zircon U–Pb age and Sr–Nd–Hf isotopic constraints on the age and origin of Triassic mafic dikes,Dalian area,Northeast China

Post-orogenic mafic rocks from Northeast China consist of swarms of dolerite dikes. We report a new U–Pb zircon age, as well as whole-rock geochemical and Sr–Nd–Hf isotopic data. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon analysis yielded an age of 210.3 ± 1.5 million years (i.e. Triassic) for these mafic dikes. Most Dalian mafic rocks exhibit low K₂O + Na₂O contents, and span the border between alkaline and calc-alkaline rock associations in the total alkali–silica diagram. The investigated dikes are also characterized by relatively high (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7061–0.7067) and negative ?_Nd (t) (?4.7 to??4.3) and ?_Hf (t) values (?4.1 to??1.1), implying that they were derived from an enriched lithospheric mantle source. The mafic dikes are characterized by relatively low MgO (4.65–5.44 wt.%), Mg^# (41–44), and compatible element content [such as Cr (89.9–125 ppm) and Ni (56.7–72.2 ppm)], which are the features of an evolved mafic magma. No evidence supports the idea that the mafic rocks were affected by significant assimilation or crustal contamination during emplacement. We conclude that the dolerites formed in a post-orogenic extensional setting, related to lithospheric delamination or ‘collapse’ of the Central Asian Orogenic Belt (CAOB), also termed the Xingmeng Orogenic Belt in China.     

Contribution of SHRIMP U–Pb zircon geochronology to unravelling the evolution of Brazilian Neoproterozoic fold belts

The South-American continent is constituted of three major geologic–geotectonic entities: the homonym platform (consolidated at the end of the Cambrian), the Andean chain (essentially Meso-Cenozoic) and the Patagonian terrains, affected by tectonism and magmatism through almost all of the Phanerozoic. The platform is constituted by a series of cratonic nuclei (pre-Tonian, fragments of the Rodinia fission) surrounded by a complex fabric of Neoproterozoic structural provinces.     

Zircon U–Pb and pyrite Re–Os age constraints on pyrite mineralization in the Yinjiagou deposit,China

We report new zircon U–Pb and pyrite Re–Os geochronological studies of the Yinjiagou poly-metallic deposit, sited along the southern margin of the North China Craton (SMNCC). In this deposit, pyrite, the most important economic mineral, is intergrown/associated with Mo, Cu, Au, Pb, Zn, and Ag. Prior to our new work, the age of chalcopyrite–pyrite mineralization was known only from its spatial relationship with molybdenite mineralization and with intrusions of known ages. The U–Pb and Re–Os isotope systems provide an excellent means of dating the mineralization itself and additionally place constraints on the ore genesis and metal source. Zircons separated from the quartz–chalcopyrite–pyrite veins include both detrital and magmatic groups. The magmatic zircons confine the maximum age of chalcopyrite–pyrite mineralization to 142.0 ± 1.5 Ma. The Re–Os results yield an age of 141.1 ± 1.1 Ma, which represents the age of the chalcopyrite–pyrite mineralization quite well. The common Os contents are notably low (0.5–20.1 ppt) in all samples. In contrast, the Re contents vary considerably (3.0–199.2 ppb), most likely depending on intensive boiling, which resulted in an increase of Re within the pyrite. This study demonstrates that the main chalcopyrite–pyrite mineralization occurred late in the magmatic history and was linked to a deeper intrusion involving dominant mantle-derived materials. This mineralization event might be related to the Early Cretaceous lithospheric destruction and thinning of the SMNCC.     

Geochemical and zircon U–Pb age constraints on the origin of the Mesozoic Xigaze ophiolite,Yarlung Zangbo suture zone,SW China

The Mesozoic Xigaze ophiolite is a key to understanding the tectonic evolution of the Yarlung Zangbo suture zone. Although many studies have been reported, the formation age and petrogenesis of the Xigaze ophiolite remain controversial. In this paper, new geochronological and geochemical data for mafic dikes (diabase, dolerite), lavas, and gabbros of the Xigaze ophiolite are provided to constrain the origin of the Xigaze ophiolite. Combined with previous studies, three new zircon U–Pb ages of samples from two gabbro and one dolerite samples show that the Xigaze ophiolite was produced at two distinct stages of 174–149 Ma and 137–123 Ma. Whole-rock geochemical data indicate that these rocks exhibit N-MORB-like features, but the gabbros are more depleted in trace elements and belong to cumulates. Geochemical characters, combined with their positive ε_Nd(t) values (+3.2 to +9.6), suggest that these samples originated from depleted mantle sources with minor influence of slab-derived fluids. Considering the previous studies on the Yarlung Zangbo suture zone, the Xigaze ophiolite was likely generated in an active continental margin fore-arc basin with a multistage model associated with the northward subduction of the Yarlung Zangbo Neo-Tethys Ocean beneath the Lhasa terrane. The Middle–Late Jurassic ophiolitic massifs (174–149 Ma) were produced as the result of slab rollback and were followed by subsequent slab break-off at ~ 150 Ma. The fore-arc lithosphere may be frozen at ~150–137 Ma, consistent with the termination of the Gangdese arc magmatism during this period. The Early Cretaceous ophiolitic massifs (137–123 Ma) were developed in relation to the reinitiation of the Neo-Tethyan oceanic lithosphere subduction, the retreat of the subduction zone, and the creation of a fore-arc basin with strong hyperextension in a new cycle.     

Structural controls,temperature–pressure conditions and fluid evolution of orogenic gold mineralisation at the Betam mine,south Eastern Desert,Egypt

The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (L_CO2 + L_aq ± V_CO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (L_CO2 ± V_CO2) and aqueous H₂O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H₂O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm³, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f _S2 to the range of 10⁻¹⁰ to 10^−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f _O2 through pressure fluctuation and CO₂ effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.     

U–Pb (zircon) and geochemical constraints on the age,origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu–Au district,southern Mongolia

Uranium–Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu–Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (ε_Nd(t) range from + 3.1 to + 7.5 and ²⁰⁶Pb/²⁰⁴Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have < 1.5% TiO₂) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu–Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium–Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu–Au deposits are ~ 372 Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu–Au mineralization are ~ 366 Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu–Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu–Au deposits from younger magmatic suites in the district.     

Late Neoproterozoic strongly peraluminous leucogranites, South Eastern Desert, Egypt – petrogenesis and geodynamic significance

Summary Late Neoproterozoic garnet-bearing leucogranites are developed locally along thrust faults in the South Eastern Desert, Egypt. This work presents field observations, whole rock major and trace element abundances, Rb–Sr isotope data and mineral chemistry for three occurrences in the Sikait-Nugrus area. Field observations show that the leucogranites cut the faults and their contact with the country rocks is sharp with no indication of contact metamorphism. They were intruded into a low-grade metamorphosed ophiolitic melange and a high-grade metamorphosed metasedimentary succession of biotite schist composition. Numerous biotite schist enclaves, having irregular and diffuse contacts, are recorded within the leucogranites. Whole rock Rb–Sr ages of the leucogranites from two different localities are 610±20 and 594±12Ma respectively; they are interpreted as emplacement ages. The leucogranites contain more than 70% SiO₂, and they are strongly peraluminous (A/CNK>1.1) with low TiO₂, Fe₂O₃*, MgO, CaO, Ba, Sr, LREE, Eu/Eu* and Sr/Ba and high Rb, Rb/Zr, Rb/Sr and Rb/Ba. These geochemical parameters and the low initial ⁸⁷Sr/⁸⁶Sr ratios (0.703) indicate crustal derivation by dehydration partial melting from a juvenile protolith similar to the exposed biotite-rich metasediments. Models for the tectonic setting of these leucogranites suggest their emplacement during an extensional tectonic stage that follows continental collision. It is proposed that crustal heating, caused by decompression along shear zones, is responsible for the production of these granitic melts. The results support previous hypotheses and further document a regional late Neoproterozoic extensional tectonic event, which is probably related to the initial break-up of Gondwana.     

U–Pb zircon,zircon Hf and whole-rock Sm–Nd isotopic constraints on the evolution of Paleoproterozoic rocks in the northern Gawler Craton

U–Pb zircon analyses from a series of orthogneisses sampled in drill core in the northern Gawler Craton provide crystallisation ages at ca 1775–1750 Ma, which is an uncommon age in the Gawler Craton. Metamorphic zircon and monazite give ages of ca 1730–1710 Ma indicating that the igneous protoliths underwent metamorphism during the craton-wide Kimban Orogeny. Isotopic Hf zircon data show that 1780–1750 Ma zircons are somewhat evolved with initial ε_Hf values –4 to +0.9, and model ages of ca 2.3 to 2.2 Ga. Isotopic whole rock Sm–Nd values from most samples have relatively evolved initial ε_Nd values of –3.7 to –1.4. In contrast, a mafic unit from drill hole Middle Bore 1 has a juvenile isotopic signature with initial ε_Hf zircon values of ca +5.2 to +8.2, and initial ε_Nd values of ～+3.5 to +3.8. The presence of 1775–1750 Ma zircon forming magmatic rocks in the northern Gawler Craton provides a possible source for similarly aged detrital zircons in Paleoproterozoic basin systems of the Gawler Craton and adjacent Curnamona Province. Previous provenance studies on these Paleoproterozoic basins have appealed to the Arunta Region of the North Australian Craton to provide 1780–1750 Ma detrital zircons, and isotopically and geochemically similar basin fill. The orthogneisses in the northern Gawler Craton also match the source criteria and display geochemical similarities between coeval magmatism in the Arunta Region of the North Australian Craton, providing further support for paleogeographic reconstructions that link the Gawler Craton and North Australian Craton during the Paleoproterozoic.     

Lu–Hf and O isotopic compositions on single zircons from the North Eastern Desert of Egypt,Arabian–Nubian Shield: Implications for crustal evolution

Neoproterozoic juvenile crust is exposed in the Eastern Desert of Egypt, between the Nile and the Red Sea, forming the basement to Cambrian and younger sedimentary strata in the northernmost part of the Arabian–Nubian Shield (ANS). In order to reveal how the crust of this vast region was formed, four examples of widespread Neoproterozoic (653–595 Ma) calc-alkaline and alkaline intrusive rocks in the northwestern most exposures, in the NE Desert of Egypt (NED) were studied. Single zircon Hf–O isotopic compositions of these intrusives were used to characterize the Neoproterozoic syn- and post-collisional granitoids in the NED. The ~ 653 Ma Um Taghir syn-tectonic granodiorite (I-type) displays isotopic characteristics of a depleted mantle source, such as high εHf(t) (+ 9.1 to + 11.2) and mantle δ¹⁸O (mean = + 5.12‰). In contrast, the ca. ~ 600 Ma post-collision A-type granites (Al-Missikat, Abu Harba, and Gattar) show slightly higher δ¹⁸O values (+ 5.15 to 6.70) and slightly lower εHf(t) values (+ 6.3 to + 10.6, mean = + 8.6). We interpret these isotopic data to reflect melting of a juvenile Neoproterozoic mantle source that assimilated slightly older Neoproterozoic crustal material during magma mixing. The involvement of crustal component is also supported by Hf-crustal model ages (0.67–0.96 Ga) and by the occurrence of xenocrystic zircons with U–Pb ages older than the crystallization ages, indicating melting of predominantly Late Neoproterozoic crustal protoliths.     

Implications of U–Pb SHRIMP zircon data on the age and evolution of the Felbertal tungsten deposit (Tauern Window, Austria)

U–Pb SHRIMP analyses of zircons from various lithologies and ore bodies of the Felbertal scheelite deposit (western and eastern ore field) and neighbouring areas allow the reconstruction of the pre-Alpine magmatic and metamorphic processes responsible for the tungsten mineralization. The ore deposit belongs to the Magmatic Rock Formation, which is tectonically squeezed between the Habach Phyllite Formation and the Basal Schist Formation (all members of the Habach Group). In both the eastern and western ore field, the pre-mineralization geological processes are marked by the emplacement of basalts (547±27?Ma). Ensialic back-arc extension provided pathways for gabbroic and pyroxenitic melts as well as normal "I-type" granitoids (minimum crystallization age of 529±18?Ma). The rock assemblage forms a magmatic arc on an approximately 2?Ga continental Gondwana (?) margin. Post-emplacement tectonism and metamorphism have converted the basalts to fine-grained amphibolites, the gabbroic and pyroxenitic rocks to coarse-grained amphibolites and hornblendites, and the granitoids to leucocratic orthogneisses, respectively. Tungsten mineralization is intimately related to small patches and dikes of differentiated granitoids in the eastern ore field and the K2 ore body in the western ore field. The granitic melts have supposedly been generated by ongoing differentiation of calcalkaline magmas. They cut the older lithologies and intruded along the same pathways as the earlier melts. Fluids have been carried up along a major line in the eastern ore field. They caused the formation of an elongate ore body with a scheelite-quartz stockwork zone (scheelite-bearing quartz veinlets and veins) and an overlying, likewise elongate, 900-m-long, scheelite-rich quartzite lens. In the western ore field, accompanying fluids produced the K2 ore body. In this ore body, an eruption breccia occurs above a mineralized quartzite. The breccia (younger than 529±18?Ma) contains mineralized quartzite clasts as well as barren fine-grained amphibolite clasts and leucocratic orthogneiss-clasts that are similar to the surrounding host rock equivalents. The quartzite, which represents the main mineralization stage of the K2 ore body, is unsuitable for dating. However, the scheelite-rich quartzite lens of the eastern ore field is probably coeval. This lens locally lies on top of a differentiated and strongly mineralized gneiss. The crystallization age of this gneiss is 529±17?Ma, and marks the peak of tungsten input in the eastern ore field. Small, differentiated granitic dikes, which cut both the K2 eruption breccia and the K2 quartzite in the western ore field, contain only minor scheelite and mark a decrease in mineralization at 519±14?Ma. Thus, a period between 530 and 520?Ma and a setting between magmatic arc and (ensialic) back-arc may properly explain the likely scenario for the primary tungsten input (stage-1 scheelite) by differentiated granitic melts of calcalkaline character. Surprisingly, a second stage-2 scheelite formation was induced in the western ore field by a Variscan granite intrusion (K1–K3 gneiss; 336±19?Ma), the emplacement time of which is pre-dated by a cross-cutting dacitic dike of 340±5?Ma. This mineralization, which occurs in small quartz veins and within a quartz aureole atop the intrusion as well as an even younger mineralization in shear zones (yellowish-fluorescent stage-2 scheelite porphyroblasts), is bracketed between 355?Ma (the upper age limit of the K1–K3 gneiss precursor) and 335?Ma (the lower age limit of the dacitic dike, which is stage-2 scheelite free). Supposedly, long-lasting Variscan (amphibolite facies) metamorphic conditions till 282±2?Ma extended the scheelite remobilization. They caused a further dispersion of scheelite and induced the growth of individual grains and of rims around older grains (bluish-fluorescent stage-3 scheelite). The Alpine metamorphism of lower amphibolite to upper greenschist facies conditions caused a further, minor scheelite remobilization, especially along some faults and quartz veins, including sparse, but large, whitish-bluish-fluorescent crystals (stage-4 scheelite).