Within-plate Tectonic Setting Research Articles

Geochemistry and zircon U–Pb dating of amphibolite in the Menderes Massif (western Türkiye): first record of Late Devonian extension

ABSTRACT The timing of extensional events in northern Gondwana remains elusive. In relations to this event, a new zircon U-Pb age and Hf isotopic compositions, and whole-rock geochemistry provided here for the amphibolites in the basement unit of the Menderes Massif (MM) within the Anatolide-Tauride Block (ATB). LA-ICP-MS U-Pb zircon dating of one sample yielded a crystallization age of 367.0 ± 2.5 Ma, providing the first record of Late Devonian (Famennian) mafic magmatism in the Menderes Massif. This sample contains inherited zircon grains with ages spanning from 588 Ma to 541 Ma, likely sourced from late Neoproterozoic country rocks. Chondrite-normalized rare earth element patterns exhibit enrichment in light rare earth elements and flat patterns for heavy rare earth elements, suggesting affinities with tholeiitic EMORB and ocean island basalt. The geochemical data, coupled with the positive εHf(t) values ranging from 0.5 to 7.8, indicate that the amphibolite likely originated from juvenile melts generated by partial melting of the asthenospheric mantle. This melt was subsequently modified by crustal contamination and fractional crystallization. Contemporaneous EMORB- and OIB-like mafic rocks are identified in the eastern Taurides and Iran, respectively. Both regional geological and geochemical evidence proposes that Late Devonian magmatism in the Anatolide-Tauride Block occurred in a within-plate tectonic setting. This magmatic activity might have been triggered by the northward subduction of the Paleotethys beneath the Pontides during the Devonian, a process that would produce a slab-pull force, creating an extensional tectonic regime along the northern margin of Gondwana.

Native Metals and Alloys in Trachytes and Shoshonite from the Continental United States and High-K Dacite from the Bolivian Andes: Magmatic Origins of Ore Metals in Convergent and Within-Plate Tectonic Settings

Native Metals and Alloys in Trachytes and Shoshonite from the Continental United States and High-K Dacite from the Bolivian Andes: Magmatic Origins of Ore Metals in Convergent and Within-Plate Tectonic Settings

Geochronology and Geochemistry of the Xianghualing Granitic Rocks: Insights into Multi-Stage Sn-Polymetallic Mineralization in South China

Multi-stage magmatic events associated with large tungsten-tin polymetallic deposits in the Nanling Range have been the subject of extensive research spanning many years. In this paper we report the results of a systematic study of the petrology, whole-rock geochemistry, zircon U-Pb chronology, and trace element geochemistry of granite bodies exposed in the Xianghualing ore field. They show that the granites in the study area are characterized by high SiO2 (63.83%–75.29%), Al2O3 (13.12%–18.87%), Rb (565–3260 ppm), Nd (67.3–113.5 ppm) and Ta (23.2–129.0 ppm) and by low MgO (0.02%–0.22%), TiO2 (0%–0.02%), Sr (5.3–80.5 ppm) and Ba (7.9–66.4 ppm). The rocks are highly differentiated A-type peraluminous granite, which originated in an extensional within-plate tectonic setting. Based on U-Pb dating and trace element analysis, the following multi-stage magma-hydrothermal events were identified: (1) Paleozoic (~347 Ma) and Triassic (~206 Ma) magmatic stages (initial enrichment epochs of ore-forming elements), (2) Jurassic (~161 Ma) magmatic-hydrothermal stage (mineralization epoch), and (3) Cretaceous hydrothermal overprinting stage (with peaks in the Early Cretaceous ~120 Ma and Late Cretaceous ~80 Ma). From an economic point of view, the Late Cretaceous appears to have great potential for tungsten-tin mineralization. Zircon trace element geochemistry indicates that the ore-forming fluids related to tin mineralization in the Cretaceous originated from the crust and underwent highly differentiated evolutionary processes under relatively reducing conditions. This paper emphasizes the Cretaceous tungsten-tin metallogenic events in the Nanling Range and provides an essential basis and new ideas for further tin-tungsten exploration.

Sources of K-bentonites across the Ordovician-Silurian transition in South China: Implications for tectonic activities on the northern and southern margins of the South China Block

During the Ordovician-Silurian transition, continuous large-scale explosive volcanism occurred around the world, including in South China, which reflected intense global plate movement in this transition. The tectonic activities leading to volcanism in the South China Block during this transition, including the northern and southern margins, remain controversial. K-bentonite altered from volcanic ash maintains reliable records of local and global tectonic events. Accordingly, research on the sources of K-bentonite layers in South China across this transition can provide implications for disputes about tectonic activities in the South China Block. This study presents a detailed geochemical analysis of the sources of K-bentonite layers in the Wufeng Formation (Upper Ordovician Katian stage) and Longmaxi Formation (Lower Silurian Rhuddanian stage) in three sections and one well in South China. Both bulk and zircon geochemical features indicate that the K-bentonite layers in South China during this transition had two sources, implying that volcanism originated from two different locations with two different geodynamic settings in or near the South China Block. K-bentonite from a within-plate (anorogenic) tectonic setting implies that within-plate extension on the northern margin of the South China Block was initiated in the Late Ordovician. K-bentonite from an arc-related (orogenic) tectonic setting implies that subduction-collision process occurred on the southern margin of the South China Block during this transition.

Subduction to post-collisional volcanism in the Northern Arabian-Nubian Shield: Genesis of Cryogenian/Ediacaran intermediate-felsic magmas and the lifespan of a Neoproterozoic mature island arc

The Neoproterozoic intermediate-felsic volcanic rocks in the Wadi Ambagy area, Eastern Desert of Egypt, comprise three units: Dokhan intermediate-felsic volcanics, post-Dokhan felsic volcanics and felsites. The Dokhan lavas are generally porphyritic and composed of andesite and dacite with subordinate basalt, whereas the post-Dokhan felsic volcanics are represented by phyric and aphyric rhyolites. The medium-K calc-alkaline nature, LILE enrichment, Ta-Nb anomalies and LREE-enriched patterns of the Dokhan volcanic rocks are consistent with development in a subduction setting. Arc crustal thickness estimation using average (La/Yb)n is ~ 32 km, indicating that these rocks were generated in a Neoproterozoic island arc with thick crust. The subduction magmas were generated by hydrous mantle melting and they evolved by fractional crystallization. In contrast, the post-Dokhan felsic volcanics and felsites have medium- to high-K and shoshonitic affinities, respectively. They are highly silicic with weak to absent Ta-Nb anomalies and significant Nb, Zr, Hf and REE enrichment, which increases from the post-Dokhan felsic volcanics to the felsites. These geochemical features suggest generation in a post-collisional to within-plate tectonic setting by partial melting of mafic or intermediate arc crustal sources. SHRIMP U-Pb zircon dating indicates a crystallization age of ~ 660 Ma for the subduction-related Dokhan andesite and dacite, reflecting a magmatic arc lifespan longer than previously considered. The change of setting from subduction to post-collisional was accompanied by an increased role of crustal sources in the generation of the felsic magmas, resulting in differentiation of the accreted arc crust.

The Neoproterozoic to Triassic tectonic evolution of Jangbong Island in the northwestern Gyeonggi Massif on the Korean Peninsula

Jangbong Island is located in the coastal area of the northwestern Gyeonggi Massif, and its basement is composed of Paleoproterozoic gneiss. The basement is covered by Neoproterozoic metasedimentary rocks such as pelitic schist, calc-schist, and quartz schist. These rocks are intruded by mafic dikes metamorphosed under amphibolite facies conditions. In the study area, two different metamorphic conditions are confirmed: the first was intermediate-P/T peak metamorphism (680-630 °C/8.6-7.3 kbar), and the second was low-P/T metamorphism (600-560 °C/5.7-3.1 kbar) showing a clockwise P-T path. All rocks in the studied area are intruded by Triassic gabbro and granite. Age measurement using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields intrusion ages of 917 ± 33 Ma and 873 ± 32 Ma for metabasites. Two different metamorphic ages are also obtained from them: 255 ± 12 ~ 254.3 ± 1.3 Ma and 231.5 ± 1.9 ~ 229.8 ± 1.4 Ma for the first and second metamorphic episodes, respectively. The Triassic gabbro yields an intrusion age of 229.1 ± 0.57 Ma and formed in a post-collisional tectonic setting with Triassic granite. The geochemical analysis confirms that Jangbong Island metabasites formed in a within-plate tectonic setting and are all alkaline basalts and mostly tholeiitic. The 917-873 Ma rift-related mafic intrusion is well matched with the Neoproterozoic rift-related mafic intrusions in the northern Gyeonggi Massif and along the southeastern margin of the North China Craton (NCC). The 255-254 Ma intermediate-P/T peak metamorphism is well matched with the collisional event in the northern Gyeonggi Massif, and the 231-230 Ma low-P/T metamorphism and 229 Ma post-collisional igneous activity are well correlated with post-collisional events in the northern Gyeonggi Massif. Together with previous data, these data support the correlation between the northern Gyeonggi Massif and the NCC.

Petrogenesis of the Cenozoic volcanic rocks in Baekdu volcano in northeastern Asia and the expected depth of the magma chamber based on geochemistry, mineral chemistry, and Sr-Nd-Pb isotope chemistry

Baekdu volcano is a Cenozoic composite volcano in northeastern Asia that consists of a basaltic plateau, a basaltic shield, and a trachytic to comenditic stratovolcano. In this study, the petrogenesis and tectonic setting of Baekdu volcano are studied based on the whole-rock geochemistry and Sr-Nd-Pb isotope compositions of the Baekdu volcanic rocks. The K–Ar age and mineral chemistry are also investigated to estimate the evolution of the magma chamber depth beneath Baekdu volcano. The Toudao and Baishan basalts formed ca. 2.5 Ma and 1.1 Ma and erupted from magma chambers located at depths of 11.5 ± 2.2 km and 24.9 ± 3.8 km, respectively. The Xiaobaishan and Middle trachytes and the comendites formed during the Pleistocene from magma chambers located at depths of 2.9 ± 0.2 km, 5.4 ± 0.45 km, and 4.4 ± 0.3 km, respectively. These data suggest that the trachytes and comendites erupted from magma chambers located at depths of approximately 3–5 km. The Baishan basalts have geochemical characteristics comparable with those of ocean island basalt (OIB), whereas the Toudao basalts show enriched mid-ocean ridge basalt (E-MORB) or OIB characteristics. The trends of several components, such as the CaO/Al2O3 ratio and the TiO2 and Sc concentrations, are different between older basalts (>2 Ma, including the Toudao basalts) and younger basalts (<1.5 Ma, including the Baishan basalts), indicating that the magma-forming process changed ca. 2–1.5 Ma. The enrichment in Th/Yb ratios in the Toudao basalts indicates that they experienced crustal contamination. Trachytes and comendites are considered to be products of the fractional crystallization of the Baishan basalt based on the similar geochemical trends of the two rocks. Crustal assimilation (up to 20%) also occurred with fractional crystallization (more than 70%) during the formation of peralkaline silicic volcanic rocks. All volcanic rocks in Baekdu volcano have Sr-Nd-Pb isotope compositions comparable with the enriched mantle (EM1)-like signature. This EM1-like signature may have originated from the contamination of asthenospheric mantle with Paleoproterozoic lithosphere that sank into the asthenospheric mantle due to delamination caused by regional extensional events in northeastern Asia between the Cretaceous and the Cenozoic. The characteristics of trace element compositions indicate a within-plate tectonic setting for Baekdu volcano. These data imply that Baekdu volcano origin may have been related to a plume of EM1-type upper mantle under the North China Craton.

Zircon U-Pb Dating of Magmatism and Mineralizing Hydrothermal Activity in the Variscan Karkonosze Massif and Its Eastern Metamorphic Cover (SW Poland)

SHRIMP (Sensitive high resolution ion microprobe) zircon U-Pb dating of the two main igneous rocks types in the Karkonosze Pluton, porphyritic and equigranular monzogranite, yield 206Pb/238U ages between 312.0 ± 2.9 and 306.9 ± 3.0 Ma. These coincide, within uncertainty, with the majority of previous dates from the pluton, which indicate development of the main magmatic processes between ca. 315 and 303 Ma. They also coincide with molybdenite and sulfide Re-Os ages from ore deposits developed during magmatic and pneumatolitic-hydrothermal (e.g., Szklarska Poręba Huta and Michałowice) or/and metasomatic and hydrothermal (e.g., Kowary, Czarnów and Miedzianka) processes forming Mo-W-Sn-Fe-Cu-As-REE-Y-Nb-Th-U mineralization. The 206Pb/238U zircon age of 300.7 ± 2.4 Ma from a rhyolite porphyry dyke (with disseminated base metal sulfide mineralization) in the Miedzianka Cu-(U) deposit coincides with the development of regional tectonic processes along the Intra-Sudetic Fault. Moreover, at the end-Carboniferous, transition from a collisional to within-plate tectonic setting in the central part of the European Variscides introduced volcanism in the Intra-Sudetic Basin. Together, these processes produced brecciation of older ore mineralization, as well as metal remobilization and deposition of younger medium- and low-temperature hydrothermal mineralization (mainly Cu-Fe-Zn-Pb-Ag-Au-Bi-Se, and Th-U), which became superimposed on earlier high-temperature Mo-W-Sn- Fe-As-Cu-REE mineralization. A few 206Pb/238U ages &gt; 320 Ma remain to be reconciled, but might be due to the high U and Th contents of the zircon and the strong influence of overprinting pneumatolitic-hydrothermal processes.

Ages and tectonic settings of the Neoproterozoic igneous rocks in the Gyeonggi Massif of the southern Korean Peninsula and the correlation with the Neoproterozoic igneous rocks in China

The Neoproterozoic igneous rocks in the northern and southern Gyeonggi Massifs in the southern Korean Peninsula are important for the interpretation of the Neoproterozoic tectonic evolution of the Korean Peninsula and northeastern Asia. The Gonamsan and Chuncheon metabasites in the northern Gyeonggi Massif intruded in a within-plate tectonic setting at ca. 851–873 Ma and 888 Ma, respectively. The Nb/Yb ratios (7–23) of these metabasites represent enriched mid-ocean ridge basalt (E-MORB) to ocean island basalt (OIB) characteristics. Most zircons found within the Gonamsan metabasites have positive εHf(t) values and Paleoproterozoic to Mesoproterozoic isotopic model ages (TDM2). These metabasites have initial 87Sr/86Sr ratios that range between 0.7033 and 0.7058 and εNd(t) values of −0.68 to 5.02. These geochemical characteristics of these metabasites indicate that these rocks formed in a rift-related environment.The Neoproterozoic metabasites from the Dangjin area in the southwestern Gyeonggi Massif can be subdivided into arc and rift types. The Dangjin arc-type metabasites intruded at ca. 820–833 Ma. These metabasites have Nb/Yb ratios of 4–7, Nb/U ratios of 3.58–27.5 and show an increase in Th/Nb ratios with increasing La/Sm ratios. Most zircons from these metabasites have negative εHf(t) values and Paleoproterozoic-Archean TDM2 ages, except one that has positive εHf(t) values and a Mesoproterozoic TDM2 age. Most these metabasites have initial 87Sr/86Sr ratios of 0.7048–0.7105 and negative εNd(t) values from −11.67 to −5.25. These geochemical characteristics indicate that the arc type Dangjin metabasites experienced crustal contamination during differentiation in an arc tectonic environment.The rift-related Dangjin metabasites intruded at ca. 793 Ma in a similar manner to the Gonamsan and Chuncheon metabasites. The Nb/Yb ratios (8–11) of these metabasites are higher than those of the arc-type metabasites. These metabasites have lower initial 87Sr/86Sr ratios of 0.704 and higher εNd(t) values of 1.96 than the Dangjin arc-type metabasites. Together with the results of previous work, this study indicates that the tectonic setting of the Dangjin-Hongseong area was an arc during the period 820–900 Ma and then changed into a rift during the period 703–793 Ma.The Neoproterozoic igneous rocks in the northern Gyeonggi Massif can be correlated with those formed in the within-plate tectonic setting in the southern and southeastern margins of the North China Craton during the period 830–930 Ma. On the other hand, the Neoproterozoic igneous rocks in the southwestern Gyeonggi Massif can be correlated to the Neoproterozoic arc- and rift-related igneous rocks in the northern margin of South China Craton, which intruded during 850–871 and 637–820 Ma, respectively.

A Gondwana pre-break up related magmatism in Bafoussam area, west Cameroon: source characteristics and 40Ar/39Ar geochronology

The Gondwana pre-break up related formations in Bafoussam area are transitional calc-alkaline doleritic dykes exhibiting high Alumina and low Ti-Mg contents. Their REE compositions are similar to those of E-MORB (Nd/Nb≈1, Zr/Nb≤20). A high partial melting of about 20% of Garnet peridotites source having primitive mantle composition, is inferred to the studied rocks. They are slightly evolved 50&lt;Mg#&lt;54, and its clinopyroxenes are augite of high temperature (600°C-1100°C). The studied dykes were emplaced in a within-plate tectonic setting and yield a 40Ar-39Ar plateau ages of 229±7Ma. These ages are slightly different from those of dolerites from the Oban-Obudu massif (Nigeria), from the basaltic dyke of the Cameroon Volcanic Line and from the Karoo-Ferrar mafic magmatism, all interpreted as magmatism that preceded the break-up of Gondwana. The emplacement age of 229±7Ma of dolerites from Bafoussam area relates them to the very early stage of Gondwana pre-break up magmatism in central Africa.

Mineralogical and Geochemical Constraints on Magma Evolution and Late-Stage Crystallization History of the Breivikbotn Silicocarbonatite, Seiland Igneous Province in Northern Norway: Prerequisites for Zeolite Deposits in Carbonatite Complexes

The present work reports on new mineralogical and whole-rock geochemical data from the Breivikbotn silicocarbonatite (Seiland igneous province, North Norway), allowing conclusions to be drawn concerning its origin and the role of late fluid alteration. The rock shows a rare mineral association: calcite + pyroxene + amphibole + zeolite group minerals + garnet + titanite, with apatite, allanite, magnetite and zircon as minor and accessory minerals, and it is classified as silicocarbonatite. Calcite, titanite and pyroxene (Di36–46 Acm22–37 Hd14–21) are primarily magmatic minerals. Amphibole of mainly hastingsitic composition has formed after pyroxene at a late-magmatic stage. Zeolite group minerals (natrolite, gonnardite, Sr-rich thomsonite-(Ca)) were formed during hydrothermal alteration of primary nepheline by fluids/solutions with high Si-Al-Ca activities. Poikilitic garnet (Ti-bearing andradite) has inclusions of all primary minerals, amphibole and zeolites, and presumably crystallized metasomatically during a late metamorphic event (Caledonian orogeny). Whole-rock chemical compositions of the silicocarbonatite differs from the global average of calciocarbonatites by elevated silica, aluminium, sodium and iron, but show comparable contents of trace elements (REE, Sr, Ba). Trace element distributions and abundances indicate within-plate tectonic setting of the carbonatite. The spatial proximity of carbonatite and alkaline ultramafic rock (melteigite), the presence of “primary nepheline” in carbonatite together with the trace element distributions indicate that the carbonatite was derived by crystal fractionation of a parental carbonated foidite magma. The main prerequisites for the extensive formation of zeolite group minerals in silicocarbonatite are revealed.

The rare metal deposits of the El Garra El Hamra syenites, South Western Desert, Egypt

The El Garra El Hamra complex is a small high-level plug (with associated volcanics) that was emplaced along an ENE-WSW trending dextral strike-slip fault corresponding to the Guinean-Nubian lineament. Owing to continued movement along this fault, the intrusion split into two parts that were displaced horizontally by approximately 580 m. The intrusion consists mainly of syenites (fresh and altered) and granites (altered) that are locally overlain by alkaline trachyte and rhyolite and are hosted by Nubian sandstones. The fresh syenites are composed of perthitic K-feldspars, albite, alkali pyroxenes (aegirine-augite and aegirine) and secondary alkali amphiboles (arfvedsonite and riebeckite). Accessory minerals include opaques, apatite, zircon, xenotime-(Y) and bastnäsite-(Ce). Calcite and hematite are alteration products. Geochemically, the syenites are peralkaline; their CIPW norms classify them as ranging between syenite and quartz syenite in compositions. They were emplaced in a within-plate tectonic setting along the Guinean-Nubian lineament (Kalabsha), close to the extension of the Chephren-Kom Ombo trend (a branch of the Trans-African Shear Zone). They were evolved from a magma compositionally similar to Ocean island basalt (OIB). Numerous dextral and sinistral strike-slip faults cut the intrusion and may have facilitated the circulation of fluids that altered them hydrothermally. These faults developed during or shortly after emplacement of the intrusion or perhaps during subsequent reactivation of the Guinean-Nubian and Trans-African shear zones and were accompanied by dykes that range in compositions from intermediate to acidic composition; most of which strike ENE-WSW. Hydrothermal alteration of the syenites locally led to extreme enrichment in rare metals (e.g. LREE, HREE, Zr, Hf, Th, U, Nb and Be) as the minerals zircon, gittinsite, bastnäsite-(Ce), parisite-(Ce), monazite–(Ce), REE-rich fluorapatite, chevkinite-(Ce), xenotime-(Y), pyrochlore, columbite-(Fe), Nb-rutile and thorite. Fluorite and barite are important gangue minerals.

Geochronological and geochemical studies of the OIB-type Baiyanghe dolerites: implications for the existence of a mantle plume in northern West Junggar (NW China)

AbstractIn this paper, zircon U–Pb geochronology, major and trace elements, and Sr–Nd isotope geochemistry of the Baiyanghe dolerites in northern West Junggar of NW China are presented. The U–Pb dating of zircons from the dolerites yielded ages of 272.2±4 Ma and 276.7±6.2 Ma, which indicate the emplacement times. The dolerites are characterized by minor variations in SiO2(46.89 to 49.07 wt%), high contents of Al2O3(13.60 to 13.92 wt%) and total Fe2O3(11.14 to 11.70 wt%), and low contents of MgO (2.67 to 3.64 wt%) and total alkalis (Na2O+K2O, 5.1 to 5.97 wt%, K2O/Na2O = 0.37–0.94), which indicate affinities to metaluminous tholeiite basalt. The REE pattern ((La/Sm)N= 2.25–2.34, (La/Yb)N= 7.42–8.36), V–Ti/1000 and 50*Zr–Ti/50–Sm discrimination diagrams show that these rocks are OIB-type. The high contents of Zr and Ti indicate a within-plate tectonic setting, and samples plot in the ‘plume source’ field shown on the Dy/Yb(N)versus Ce/Yb(N)diagram. The positive εNd(t) values (+7.09 to +7.48), high initial87Sr/86Sr ratios (0.70442 to 0.70682) and depletions of Nb and Ta elements in the samples can be explained by the involvement of subducted sediments. In summary, it is possible that the Baiyanghe dolerites were derived from an OIB-like mantle source and associated with a mantle plume tectonic setting. Therefore, our samples provide the youngest evidence for the existence of a mantle plume, which may provide new insights into the Late Palaeozoic tectonic setting of West Junggar.

Rare Earth Element Deposits of Alkaline Igneous Rocks

Alkaline igneous complexes host deposits of rare earth elements (REE), which represent one of the most economically important resources of heavy REE and Yttrium (Y). The hosts are differentiated rocks ranging from nepheline syenites and trachytes to peralkaline granites. These complexes usually occur in continental within-plate tectonic settings associated with rifts, faults, or hotspot magmatism. The REE mineralization is found in layered alkaline complexes, granitic stocks, and late-stages dikes and rarely trachytic volcanic and volcaniclastic deposits. The bulk of REE is present in accessory minerals, which can reach percentage levels in mineralized zones. The mineralization contains various REE-bearing minerals that can display complex replacement textures. Main REE minerals present in these deposits are bastnäsite, eudialyte, loparite, gittinsite, xenotime, monazite, zircon, and fergusonite. The parent magmas of alkaline igneous complexes are derived from partial melts of mantle sources. Protracted fractional crystallization of the magma led to an enrichment in REE, particularly in the late stages of magma evolution. The primary magmatic mineralization is commonly overprinted (remobilized and enriched) by late magmatic to hydrothermal fluids. Elevated abundances of U and Th in the deposits make a gamma-ray (radiometric) survey an important exploration tool, but also represent a significant environmental challenge for exploitation.

Neoproterozoic Rosetta Gabbro from northernmost Arabian–Nubian Shield, south Jordan: Geochemistry and petrogenesis

An Ediacaran mafic intrusion of south Jordan is a distinctive appinitic igneous rock with a possibly unique texture, characterized by spherical clots up to 40mm in diameter composed of amphibole cores from which plagioclase euhedra radiate; we call it the Rosetta Gabbro. It is exposed as a small (ca. 750m2) outcrop in the Neoproterozoic basement of south Jordan. A second outcrop of otherwise similar gabbro is located about 400m to the north of the Rosetta Gabbro, but it lacks the distinctive texture. The Rosetta Gabbro could represent a magma pipe. It intrudes the Aqaba Complex (~600Ma) granitoids and metasediments of the Janub Metamorphic Complex (633–617Ma). The gabbro is an Ol- to QZ tholeiite with the following chemical characteristics: SiO2=46.2–47.8wt.%; Al2O3=16.4–17.7wt.%, TiO2=1.70–2.82wt.%, Na2O=1.27–2.83wt.%. K2O=0.82–1.63wt.%; Mg# 58–63; Σ REE=70–117ppm; La/Yb ~6 to 8; and Eu/Eu*=1.05–1.2. The investigated gabbro has the geochemical features of a continental flood tholeiitic basalt emplaced in a within-plate tectonic setting. Two varieties of amphiboles are found: 1) large, 3–5mm, brown ferri-titanian-tschermakite (K0.09Na0.28)(Na0.20Ca1.80)(Mn0.04Fe3+1.1Mg2.34Fe2+0.90Ti0.29Al0.22)(Al1.85Si6.15)O22(OH)1.95 of the calcic amphibole group which is riddled with opaques; and 2) acicular yellowish-light green ferrian-magnesiohornblende (K0.04Na0.153)(Ca1.755Na0.245) (Fe3+0.66Mn0.01Fe2+1.01Mg3.03Ti0.06Al0.22)(Al1.03Si6.97)O22(OH)1.95. Scattered flakes of phlogopite also occur. Tabular radiating plagioclase (An64–79) are complexly twinned, with broad lamellae that show no zoning. Laser-ablation ICP-MS analyses of amphibole and plagioclase reveal considerable variation in trace element abundance, in spite of more subtle major element variations except for TiO2 in amphibole. The REE in the amphibole shows an order of magnitude variation with a concave-downward pattern and a positive Eu anomaly Eu/Eu*=0.6–2, though far less pronounced compared to the Eu/Eu*=5–45 of plagioclase. The 3D dandelion-like texture of the rosettas is broadly similar to “Chrysanthemum Stone”, which is a diagenetic growth in sedimentary rock, but we can find no description of similar textures in igneous rocks. The formation of the rosettas is thought to reflect loss of magmatic water resulting in supersaturation of plagioclase, which grew rapidly around amphibole and may have floated in the magma. This implies magmatic evolution in shallow (10 to 12km deep) crust where temperatures were nevertheless in the range of ca. 750 to 900°C.

Remnants of Early Mesozoic basalt of the Central Atlantic Magmatic Province in Cape Breton Island, Nova Scotia, Canada

Amygdaloidal basaltic flows of the Ashfield Formation were encountered in two drill holes in areas of positive aeromagnetic anomalies in the Carboniferous River Denys Basin in southwestern Cape Breton Island, Nova Scotia. One sample of medium-grained basalt yielded a plateau age of 201.8 ± 2.0 Ma, similar to the U–Pb and 40Ar/39Ar crystallization ages from basaltic flows and dykes in the Newark Supergroup. A second sample of zeolite-bearing basalt yielded a discordant age spectrum and a younger age of ca. 190 Ma, which is interpreted to date a widespread hydrothermal event related to zeolite formation. Whole-rock chemical data show that the Ashfield Formation basalt is low-Ti continental tholeiite, consistent with its within-plate tectonic setting. Chemically, it resembles basaltic flows in the Mesozoic Fundy and Grand Manan basins exposed in southern Nova Scotia and eastern New Brunswick and elsewhere in Central Atlantic Magmatic Province (CAMP). The age and geochemical data from the Ashfield Formation provide the first evidence for early Mesozoic CAMP volcanism in Cape Breton Island and demonstrate that the event was more widespread in Nova Scotia than previously thought, which has implications for its continuity and extent elsewhere within CAMP.

Late Carboniferous bimodal volcanic rocks and coeval A-type granite in the Suman Khad area, Southwest Mongolia: Implications for the tectonic evolution

The volcanic rocks in the Suman Khad area in Southwest Mongolia form a bimodal suite consisting mainly of peralkaline rhyolites with subordinate basalts. The rhyolite sample collected from the bimodal suite yielded a SHRIMP zircon U-Pb age of 314±5Ma (MSWD=1.41, n=12), which was interpreted to represent formation time of the bimodal volcanic suite. The basalts were characterized by enrichment in LILE and LREE, and depletion in HFSE, indicating their formation was related to subduction processes. These features, together with their positive εNd (t) values (6.3–6.7), suggest that the basalts were likely derived from a depleted mantle source metasomatized by subduction-related fluids. In various tectonic discrimination diagrams, the basalts exhibited a transition from true arc basalts to intraplate basalts and thus were suggested to from in a back-arc tectonic setting. The rhyolites show a close affinity to A-type granites with enrichment in LILE and LREE, depletion in Nb, Ta and Ti and positive εNd (t) values (6.0–6.4). Considering the observed distinct compositional gap between the endmembers of the bimodal suite, the rhyolites are proposed to originate from partial melting of juvenile basaltic crustal rocks rather than fractional crystallization of basaltic melt. The granite associated with the bimodal volcanic rocks yielded a SHRIMP zircon U–Pb ages of 312±5Ma (MSWD=0.75, n=13), indicating that the granite is contemporaneous with the bimodal volcanic suite. The granite samples showed typical A-type granitic geochemical affinities and are considered to have been formed by partial melting of crustal rocks in a within-plate tectonic setting. Based on a combination of the available data, we suggest that the Late Carboniferous bimodal volcanic suite together with the coeval A-type granites in the Suman Khad region probably document a back-arc basin extensional environment, which probably related to the roll-back of the Paleo-Asian oceanic plate during the northward subduction under the Central Mongolia microcontinent.

Composite dikes in four successive granitoid suites from Transbaikalia, Russia: The effect of silicic and mafic magma interaction on the chemical features of granitoids

In the study area of 60 by 15km, located south of Lake Baikal, coeval mafic rocks form small intrusive bodies, synplutonic dikes and composite mafic-felsic dikes in four successive Late Paleozoic and Early Mesozoic syenite and granite suites. Mafic rocks in all suites are monzogabbro with similar mineral and chemical composition. This attests that mantle-derived mafic magmas produced at post-collisional and within-plate tectonic settings were not noticeably distinguished in chemical composition within a time interval of about 60–65m.y. Detailed study of the interaction zones between synplutonic mafic magma and host syenite at two key localities allowed recognition of syenite remelting stage at about 900–950°C. Remelting was accompanied by Sr and Ba enrichment by diffusion from the adjacent mafic magma. Diffusive transport of Ba also caused enhanced BaO content, 0.37–0.54wt.%, in alkali feldspars of the newly-formed syenite. Remelting of the host syenite was followed by mingling, chemical interaction and mixing of mafic and silicic magmas within the contact zone. The results of petrographic and geochemical study of the composite and synplutonic dikes are used to interpret the chemical features of the large, about 1000 km2, essentially syenitic Ust-Khilok pluton. Two varieties of syenite are recognized in the pluton, one with Ba<2000ppm and another with Ba=2000–4000ppm. The data suggest the combined effects of two processes: (1) Ba enrichment of silicic melt by diffusion from the comingling mafic magma enclaves and synplutonic dikes and (2) accumulation of alkali feldspar crystals in the course of fractional crystallization evidenced by positive Eu anomalies in cumulate-enriched syenites. It is assumed that an excess of accumulated feldspar crystals caused a decrease in the proportion of melt per unit volume; this resulted in a reduced amount of zircon and, consequently, Zr from 400–700ppm in cumulate-free to 80–150ppm in cumulate-rich syenite.

Late Ediacaran post-collisional A-type syenites with shoshonitic affinities, northern Arabian-Nubian Shield: a possible mantle-derived A-type magma

The Abu Rumeil syenitic rocks represent the inner ring dyke of the Katherina Ring complex, southern Sinai, Egypt. They are divided petrologically into two types, alkali feldspar syenite and quartz syenite. The mineralogy and geochemistry of the syenites indicate an alkaline nature with a shoshonitic affinity. Although rare mafic xenocrysts overgrown by primary K-feldspars and overlapping rare earth element (REE) patterns indicate some role for crustal contamination, the trace element chemistry shows a dominant mantle contribution. The geochronology and field relations imply that the Abu Rumeil syenites were emplaced in a post-collisional, within-plate tectonic setting, yet they express the enrichments in large-ion lithophile elements relative to high field strength elements generally characteristic of subduction influence. We suggest that this signature is inherited from partial melting of a lithospheric mantle source previously affected by subduction during assembly of the Arabian-Nubian Shield. Little evidence of the early evolution of the suite is preserved; there are no associated mafic rocks. We therefore restrict our attention to a petrogenetic model that can explain the relations among the observed felsic composition. The REE patterns of all samples are enriched in light REE and fractionated, but it is notable that there are small positive Eu anomalies in the alkali-feldspar syenites contrasting with small negative Eu anomalies in the quartz syenites. Positive Eu anomalies suggest a cumulate nature for the alkali-feldspar syenites; there are also breaks in the slopes of most variation trends between the alkali-feldspar syenites and the quartz syenites. The general trends in all major oxides and trace elements within the suite can be modeled by fractional crystallization of feldspars—with smaller roles for pyroxene, biotite, apatite, and Fe-Ti oxides—from an intermediate liquid to form the quartz syenites and by assimilation of the near-liquidus phases into the same starting liquid to form the alkali feldspar syenites. The geothermobarometry of pyroxenes and amphiboles suggests shallow emplacement (<10 km depth) and crystallization temperatures ranging from 1100 °C down to 800 °C.

Petrogenesis of Middle Triassic volcaniclastic rocks from Balochistan, Pakistan: Implications for the break-up of Gondwanaland

Basaltic volcanic conglomerates near the Wulgai village in Balochistan occur in the undivided sedimentary rock unit of the Bagh complex which is the melange zone beneath the Muslim Bagh ophiolite. The presence of Middle Triassic grey radiolarian chert within the upper and lower horizon of the conglomerates suggests that the lavas, from which these conglomerates were principally derived, were eroded and re-deposited in the Middle Triassic. The Wulgai conglomerate contains several textural and mineralogical varieties of volcanic rocks, such as porphyritic, glomerophyric, intersertal and vitrophyric basalts. The main minerals identified in these samples are augite, olivine, plagioclase (An35-78) leucite and nosean, with apatite ilmenite, magnetite and hematite occurring as accessory minerals. These rocks are mildly to strongly-alkaline with low Mg# and low Cr, Ni and Co contents suggesting that their parent magma had undergone considerable fractionation prior to eruption. Trace element-enriched mantle-normalized patterns with marked positive Nb anomalies are consistent with 10%-15% melting of an enriched mantle source in a within-plate tectonic setting. It is proposed that this Middle Triassic intra-plate volcanism may represent mantle plume-derived melts related to the Late Triassic rifting of micro-continental blocks (including Afghan, Iran, Karakorum and Lhasa) from the northern margin of Gondwana.