Significant Crustal Contribution Research Articles

Mantle-crust-interaction and subduction dynamics in Central Cameroon: Evidence from the Pan-African Linté microgranular magmatic enclaves (MMEs) and host syenite

The Central African Fold Belt (CAFB) in Cameroon is characterized by voluminous intrusive rocks hosting microgranular magmatic enclaves (MMEs) whose petrogenesis and tectonic implications remain unclear. Here, we present detailed geochronological and geochemical data of the host rocks and their MMEs from the Linté pluton in central Cameroon. The MMEs are finer-grained than their host syenite and both rocks are composed of alkali feldspar, plagioclase, biotite, amphibole and pyroxenes as main mineral phases. The enclaves show lower Mg#, total alkalis and transition elements contents, and are more HFSEs depleted than the host syenite. Both rocks are characterized by high-K, metaluminous I-type, alkaline to sub-alkaline and magnesian affinities. The Ti-in-zircon thermometer suggests crystallization temperatures of 615–1080 °C and 613–1008 °C for syenites and MMEs, respectively. Their zircon trace element distribution and patterns coupled with whole-rock geochemical features indicate that the Linté intrusive rocks were most likely originated from interaction of mantle-derived magma with crustal material. LA-ICP-MS UPb zircon analyses yield emplacement age of 599 ± 3 Ma and 597 ± 4 Ma for the host syenite and MMEs, respectively, indicating coeval crystallization date. Integrated whole rock geochemistry, UPb dating and Ti-in-zircon thermometry data indicate that the Linté syenite were probably formed from a mantle-derived ultrapotassic alkaline melt with significant crustal contribution, while MMEs are likely derived from a similar but separate melt with more fractionation and crustal contamination. The genesis of the host syenite and MMEs magmas started with break-off of the Congo plate after subduction and collision. This initiated an upwelling thermal pulse and underplating of mafic magma which led to partial melting of the Paleoproterozoic crust of the Central Cameroon.

Petrologic and geochronological constraints on the polymetamorphic evolution of the collisional granites, Araçuaí Orogen (SE Brazil).

Collisional granites of the Araçuaí Orogen, southeastern Brazil, record petrological and geochronological evidence for multiple crustal melting during the orogeny evolution. U-Pb zircon data indicate that these granites crystallized at 586 ± 2 M.y. High-grade metamorphism (M1) involved partial melting by fluid-absent reactions that produced the first generation of garnet in temperatures of approx. 750°C. Preservation of the mineral assemblage A1 (garnet-biotite-plagioclase-K-feldspar-quartz-ilmenite-melt) indicates that most of the generated melt was lost from these rocks at or near peak metamorphic conditions. A second metamorphic event (M2) is characterized by growth of a second generation of garnet in preserved A2 assemblage (garnet-sillimanite-biotite-plagioclase-K-feldspar-quartz-ilmenite-melt). Mineral equilibria modeling constrains conditions of M2 metamorphism to 713-729 °C and 6.2-7.3 kbar. Retrograde assemblage (A3) records equilibrium conditions at 610-660 °C. The Hf isotope composition indicates significant crustal contribution to the genesis of the collisional granites. The elevated geotherms in thickened crust provide enough heat for the M1 event at 562 ± 2 M.y. Subsequent heating probably associated to the transfer of mantle heat to the crust during the extensional thinning and gravitational collapse of the orogen lead to the M2 event at 526 ± 4 M.y. This event is concomitant to the emplacement of the post-collisional magmas in the orogen.

Rhyolites in continental mafic Large Igneous Provinces: Petrology, geochemistry and petrogenesis

We present a detailed review of the petrological and geochemical aspects of rhyolite and associated silicic volcanic rocks (up to 20 ​vol% of all rocks) reported to date from twelve well known Phanerozoic continental mafic Large Igneous Provinces (LIPs). These typically spread over ≤104 ​km2 (rarely 105 ​km2 for Paraná-Etendeka) area and comprise ≤104 ​km3 of extrusive silicic rocks, erupted either during or after the main basaltic eruption within <5 Myr, with some eruption(s) continuing for ≤30 Myr. These rhyolites and associated silicic volcanic rocks (60−81 ​wt.% of SiO2) are mostly metaluminous to peraluminous and are formed via (i) fractional crystallization of parental mafic magma with negligible crustal contamination, and (ii) melting of continental crust or assimilation and fractional crystallization (AFC) of mafic magma with significant crustal contribution. Rhyolites formed by extensive fractional crystallization are characterized by the presence of clinopyroxene phenocrysts, exhibit steep negative slopes in bivariate major oxides plots and weak to no Nb-Ta anomaly; these typically have temperature >900 ​°C. Rhyolites formed by significant crustal contribution are characterized by strong negative Nb-Ta anomalies, absence of clinopyroxene phenocrysts, and are likely to have a magma temperature <900 ​°C. Geochemical signatures suggest rhyolite melt generation in the plagioclase stability field with a minor fraction originating from lower crustal depths. A large part of the compositional variability in rhyolites, particularly the Sr-Nd-Pb-O isotope ratios, suggests a significant role of continental crust (upper crustal melting or AFC) in the evolution of these silicic rocks in the continental mafic LIPs.

1860-Ma I-Shaped Mafic Sills in the Murmansk Craton, Eastern Fennoscandia: Petrology and Tectonic Setting of Within-plate Mafic Events during Nuna Assembly

1860-Ma within-plate mafic rocks of the Eastern Murman Sill Province were studied in the northern part of the Murmansk Craton, where they compose sheet bodies (sills) up to 100 m thick in Archean granitoids and gneisses. The sills have a simple inner structure, show no macroscopic signs of in situ differentiation even in large bodies, and consist of homogenous medium-grained poikilophitic dolerites weakly varying in composition. Minor variations in mineral and chemical composition of the rocks allow us to regard the mafic sills as I-shaped sills. Their homogeneity is likely caused by the rapid crystallization of aphyric magmas. The studied sills are close to N-MORB in major-element composition, but significantly differ in trace element signatures. The low Cr and Ni concentrations in the dolerites indicate the high degrees of fractional crystallization, while the low HREE and HFSE contents suggest that primary komatiite melts were formed through the high-degree melting of a depleted mantle source. The absence of phenocrysts in chilled dolerites prevented the use mineralogic and petrographic criteria for estimating the crystallization conditions in intermediate magma chambers. AlphaMELTS 1.9 and PRIMELT3 MEGA modelling results indicate that the primary melts could be formed through a high-degree melting of ascending high-temperature mantle plume, with subsequent contamination and fractional crystallization in mid-crustal intermediate magma chambers. The Nd and Sr isotopic composition of the dolerites and constituent minerals suggests that the crustal contamination included several stages. The first, most significant crustal contribution (up to 10%) was related to the contamination by Archean tonalites in the intermediate mid-crustal chambers. This provided negative eNd values and radiogenic Sr isotopic composition (Sri = 0.702–0.704) in the dolerites. The second stage included the upper-crustal contamination during lateral propagation of melts along gentle weakened zones, which resulted in the wide variations of Sr-Nd isotopic composition in rocks and minerals. The third stage was related to the contamination by 87Sr-enriched fluid that is supported by the sharp increase of Sri (up to 0.708) at constant Nd isotopic composition in gabbro-pegmatites. Such fluid could be formed via biotite dehydration caused by heating of host granites by large volume of emplaced mafic melts. The age, volume, and composition of the studied dolerites and similarity with coeval mafic rocks in the Superior Craton indicate that the I-shaped mafic sills in the Murmansk Craton could be formed on the periphery of the Circum Superior large igneous province. It suggests that the Murmansk Craton of the Fennoscandian Shield and the Superior Craton of the Canadian Shield at 1860 Ma were joined into a single consolidated lithospheric block that host within-plate mafic rocks and likely represented an early core of the Paleoproterozoic Nuna supercontinent.

The link between an anorthosite complex and underlying olivine–Ti-magnetite-rich layered intrusion in Damiao, China: insights into magma chamber processes in the formation of Proterozoic massif-type anorthosites

Mafic–ultramafic intrusions comagmatic with Proterozoic massif-type anorthosites can provide insights into the parental magma from which large volumes of hyper-feldspathic rocks are produced. Recent deep drilling has unveiled a large olivine–Ti-magnetite-rich layered intrusion (named Dawusunangou) beneath the Damiao massif-type anorthosite complex in the North China Craton. The layered intrusion is composed of alternating olivine–Ti-magnetite-rich dark layers and plagioclase-rich light layers (ca. 35–80% plagioclase), with the latter also containing pod- or lens-shaped pyroxene–Ti-magnetite-rich aggregates. This layered intrusion shows low Mg# and REE patterns similar to the overlying Damiao anorthosite complex. Baddeleyite Pb–Pb geochronology yielded indistinguishable crystallization ages of ca. 1735 Ma for both the Dawusunangou layered intrusion and the Damiao anorthosite complex, suggesting coeval emplacement. Using the average bulk compositions of the two intrusions, mass balance calculations assuming 30–40% Dawusunangou and 70–60% Damiao would give a composition similar to high-Al basaltic magma. Collectively, these features indicate that the Dawusunangou layered intrusion represents the mafic residues after the segregation of the Damiao anorthosites from high-Al basaltic parental magma. A short-lived magma chamber is thought to have supplied the two intrusions. In situ crystallization with variable nucleation rates for plagioclase combined with the mafic minerals crystallizing in equilibrium proportions resulted in the formation of repeated dark and light layers of the Dawusunangou layered intrusion. The two intrusions are interpreted to have formed by multiple magma injections, instead of continuous differentiation of one melt. The parental magma was derived from a depleted mantle source with significant crustal contribution during magma evolution. The large Nd–Hf isotopic variations suggest contamination by Paleoarchean to Neoarchean crust.

Geology, Mineralogy, Fluid Inclusion, and H–O–S–Pb Isotope Constraints on Ore Genesis of the Keyue Sb–Pb–Zn–Ag Deposit in Southern Tibet

The Keyue deposit is a medium-sized deposit similar to the Zhaxikang deposit within the North Himalayan Metallogenic Belt (NHMB). The ore formation can be divided into Pb–Zn mineralization (stages 1 and 2), Sb–Ag mineralization (stages 3 and 4), and Sb–Hg mineralization (stages 5 and 6). The fluid inclusion data show that the first two pulses of mineralization have different characteristics, but both belong to the epithermal category (stage 2: 172.9~277.2°C, 7.4~17.0 wt% NaCl eq.; stages 3 and 4: 142.1~321.0°C, 2.7~17.96 wt% NaCl eq.). The H–O isotopic compositions of stages 3 and 4 quartz (δDV-SMOW: –174‰~−120‰, δ18OH2O: 1.59‰~11.34‰) are similar to those of stages 3 and 4 minerals (δDV-SMOW: –165‰~−150‰, δ18OH2O: 6.14‰~13.03‰), whereas they are different from stage 1 and 2 (δDV-SMOW: –108.3‰~−103.6‰, δ18OH2O: 1.92‰~3.82‰) and stage 5 and 6 (δDV-SMOW: –165‰~−138‰, δ18OH2O: −12.91‰~0.82‰) minerals from the Zhaxikang deposit. Additionally, stage 2 sulfides have δ34S values of 5.4‰~11.2‰ that are similar to stage 2 sulfides in the Zhaxikang deposit (7.8‰~12.2‰), and these δ34S values overlap those of many SEDEX-type deposits. The δ34S values also show a decreasing trend from stage 2 through stages 3 and 4 to stage 5 in Keyue and Zhaxikang deposits, which may relate to the overprint by later mineralization events. The Pb isotopic data (206Pb/204Pb: 18.530~19.780, 207Pb/204Pb: 15.674~15.939, and 208Pb/204Pb: 38.618~40.559) show a significant crustal contribution. However, the minerals from different pulses of mineralization also exhibit slightly different Pb isotopic characteristics. These inferences from fluid inclusions and isotope are also demonstrated by geological and mineralogical evidence. Overall, the Keyue deposit is an epithermal deposit and has mainly experienced three pulses of mineralization.

Geologia e geocronologia do Charnockito São Francisco Xavier e do Gabronorito Ribeirão do Paiol (MG/SP) e suas possíveis relações com processos crustais extensionais nos orógenos Brasília Meridional e Ribeira Central

Na região entre as cidades de São Francisco Xavier, Monteiro Lobato, Campos do Jordão e São Bento do Sapucaí, estão expostas rochas metamórficas de fácies granulito de média pressão a anfibolito, associadas a Nappe Socorro e ao Terreno Embu. O Gabronorito Ribeirão do Paiol e o Charnockito São Francisco Xavier constituem corpos magmáticos encaixados em metagranitoides do Batólito Serra da Água Limpa. O Gabronorito Ribeirão do Paiol é isotrópico, tem textura inequigranular seriada hipidiomórfica a xenomórfica e idade de cristalização U-Pb LA-ICPMS de 534 ± 6 Ma. O Charnockito São Francisco Xavier possui textura isotrópica, equigranular a inequigranular seriada hipidiomórfica a xenomórfica e idade de cristalização entre 579 ± 5 Ma (núcleo dos grãos) e 562 ± 8 Ma (borda dos grãos), compatível com a evolução da Faixa Ribeira Central. O geotermômetro baseado em saturação de zircônio em rocha total aponta temperatura de cristalização de aproximadamente 880ºC para o charnockito. Os dois corpos exibem valores de εNd (t) negativos (-7,32 para o gabronorito e -12,41 para o charnockito), indicando contaminação e /ou origem crustal do magma gerador de ambos.

Natural versus anthropogenic influences on the chemical composition of bulk precipitation in the southern Apennines, Italy: A case study of the town of Potenza

This paper presents new data on the chemical composition of precipitation in a selected area of the southern Apennines of Italy; these data are used to assess natural and anthropogenic contributions to the precipitation. The town of Potenza was used as a representative urban site, and the major and minor element (Na+, K+, Ca2+, Mg2+, NH4+, Cl−, NO3−, SO42−, and Li+, NO2−, F−, Br−, PO43−) and trace element (Zn, Fe, Al, Sr, Mn, Ba, Cu, Cr, V, As, Pb, Ni, and Cd) concentrations of bulk precipitation from three different sampling sites within the study area were determined between June 2011 and May 2012. The majority of the collected rainwater has pH values higher than 5.6. The composition of this rainwater was dominated by Ca2+ the element with the highest total volume-weighted mean concentration (TVWA), followed by Cl−, SO42−, Na+, NO3− , K+, Mg2+, and NH4+. The TVWA of the trace metals decreased in the order Zn>Fe>Al>Ba>Sr>Mn>Pb>Cu>As. Neutralising factor (NFXi) values indicate that Ca2+ is the dominant neutralising cation within this rainwater, with lower contribution from NH4+ and Mg2+. The precipitation analysed during this study has a negligible marine influence as determined using Cl− concentrations as a proxy for the abundance of sea salts. Precipitation Fi values (marine fraction of element i) indicate that Na+ is the element with the highest seawater component, and we found partial marine contributions for the SO42−and Mg2+ concentrations within this rainwater. Enrichment factors (EF) of selected elements were calculated to identify the sources of non-crustal elements: K+, Mg2+, Fe, and Ca2+ are the only elements dominantly sourced from the crust, whereas Mn and Ba have highly variable EF values (10–100) suggesting that a small proportion of these elements was derived from anthropogenic sources in addition to a more significant crustal contribution. Sr, Cu, Pb, Zn, and As are highly enriched with respect to average crustal compositions, confirming that the concentrations of these elements within precipitation are significantly controlled by human activities.

Early Cretaceous (100–105 Ma) Adakitic magmatism in the Dachagou area, northern Lhasa terrane, Tibet: implications for the Bangong–Nujiang Ocean subduction and slab break-off

We performed U–Pb zircon dating and whole-rock major and trace element geochemical analyses of monzonites and granodiorites from the Dachagou area, northern Lhasa terrane, Tibet. Zircon U–Pb dating of two plutons yielded magmatic crystallization ages of 100 and 105 Ma, indicating that the intrusive rocks were emplaced in the Early Cretaceous. The monzonites and granodiorites are characterized by high Al2O3 and Sr contents, low Yb and Y contents, and high Sr/Y ratios, and secondarily by low K2O contents, high Mg # values, and relatively high Cr and Ni abundances. Major and trace element geochemical analyses indicate that the rocks have adakitic signatures, suggesting generation by partial melting of an oceanic slab. However, there are some differences between the monzonites and granodiorites, which imply that the monzonitic magmas were contaminated by enriched mantle and crustal components and that the granodioritic magmas were contaminated by enriched mantle melts without a significant crustal contribution. Based on these data and the regional tectonic setting, we propose that the oceanic slab break-off model best explains adakitic magma generation in the Dachagou area. Identification of adakites in the area provides important evidence of slab break-off during the southward subduction of the Bangong–Nujiang Ocean plate.

Archaean andesite petrogenesis: Insights from the Grædefjord Supracrustal Belt, southern West Greenland

We present new whole-rock major, trace and platinum-group element data, as well as Sm–Nd and Lu–Hf isotope data for meta-volcanic rocks from the Mesoarchaean Grædefjord Supracrustal Belt (GSB), located within the Tasiusarsuaq terrane, southern West Greenland. We also present new in situ zircon U–Pb isotope data (by LA-ICP-MS) for associated felsic rocks. This region has experienced amphibolite to lower granulite facies metamorphism, causing re-equilibration of most mineral phases (including zircon).An intrusive tonalite sheet with a zircon U–Pb age of 2888 ±6.8Ma, yields a minimum age for the GSB. The Sm–Nd and Lu–Hf isotope data do not provide meaningful isochron ages, but the isotope compositions of the mafic rocks are consistent with the ca. 2970Ma regional volcanic event, which is documented in previous studies of the Tasiusarsuaq terrane. The major and trace element data suggest a significant crustal contribution in the petrogenesis of andesitic volcanic rocks in the GSB. The trace element variation of these andesitic leucoamphibolites cannot be explained by bulk assimilation–fractional-crystallisation (AFC) processes involving local basement. Rather, the observed patterns require binary mixing between basaltic and felsic end-member magmas with between 50% and 80% contributions from the latter (depending on the assumed felsic composition). Hf-isotope constraints point to contamination with pre-existing continental crust with an age of ca. 3250Ma. Basement gneisses of this age were previously described at two localities in the Tasiusarsuaq terrane, which supports the mixing hypothesis. Thus the felsic end-member likely represents melts derived from the local basement.Ultramafic rocks (18.35–22.80wt.% MgO) in GSB have platinum-group element (PGE) patterns that are similar to magmas derived from high-degree melting of mantle, but they have relatively enriched trace element patterns. We propose that the ultramafic rocks represent arc-related picrites or alternatively were derived by melting of metasomatised sub-continental lithospheric mantle.Overall these new geochemical data from the Mesoarchaean Grædefjord Supracrustal Belt and the petrogenetic mixing model in particular, are similar to observations from modern continental subduction zone environments, which also require large degrees of mixing with felsic basement melts. Therefore, we propose that the metavolcanic rocks formed in a modern-style subduction zone geodynamic setting, which due to the hotter Archaean mantle conditions allowed for substantial amounts of partial melting and magma mixing, rather than assimilating pre-existing continental crust.

High-K, calc-alkaline I-type granitoids from the composite Yozgat batholith generated in a post-collisional setting following continent-oceanic island arc collision in central Anatolia, Turkey

The composite Yozgat batholith consists of a S-I-A-type granitoid association intruding the supra-subduction zone-type (SSZ-type) central Anatolian ophiolite and medium- to high-grade metasedimentary rocks of the central Anatolian crystalline complex. These rocks are unconformably covered by Palaeocene to Early Eocene sedimentary rocks. The I-type granitoids are the most common rock association of this huge batholith. In an area between the towns of Şefaatli and Yerkoy, the southwestern part of the batholith can be subdivided into five mappable units: the Akcakoyunlu quartz monzodiorite (mafic; hornblende K-Ar cooling ages of 77.6–79.3 Ma); the Cankili monzodiorite (mafic; hornblende K-Ar cooling age of 71.1 Ma); the Adatepe quartz monzonite (mafic; hornblende K-Ar cooling age of 68.0 Ma); the Yassiagil monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 69.9–79.8 Ma) and the Karakaya monzogranite (felsic; hornblende + biotite K-Ar cooling ages of 71.3–77.0 Ma). All the lithological units, except the Karakaya monzogranite, include large K-feldspar megacrysts and various types of mafic microgranular enclaves in field outcrops, indicating mingling and mixing. In addition, microscopic textures showing the hybridization between the coeval mafic and felsic magma sources are present. Whole-rock major element geochemistry shows a high-K calc-alkaline, metaluminous, I-type composition with an aluminium saturation index (ASI) less than 1.10 and with CIPW diopside content in all the lithological units. Large ion lithophile elements (LILE), light rare earth elements (LREE), some high field strength elements (HFSE) (except Nb) enrichments and significant crustal contribution revealed by the oxygen and sulphur stable isotope compositions in the mafic and felsic I-type granitoid units are consistent with mafic lower crustal and metasomatized mantle sources the latter of which were metasomatized by earlier supra subduction zone (SSZ)-derived fluids during the development of the SSZ-type central Anatolian ophiolite.

Chemical composition of rainwater collected at a southwest site of Mexico City, Mexico

Measurements of the trace metals Cd, Cr, Mn, Ni, Pb, V and Al in soluble and insoluble rain fractions and SO 4 − 2 , NO 3 −, Cl −, HCO 3 −, Ca 2+, Mg 2+, Na +, K +, NH 4 + and H + in soluble fractions were performed in rainwater collected at a southwest site of Mexico City during the rainy seasons of 2001 and 2002. Aluminum presented the highest volume-weighted mean concentration (VWMC) in both insoluble and soluble fractions. In the insoluble fractions, the VWM of the other trace metals decreased in the order Mn, Pb, Ni, V, Cr and Cd, and in the soluble fractions in the order Mn, V, Ni, Pb, Cd and Cr. Ammonium presented the higher VWMC, followed by SO 4 2−, NO 3 −, HCO 3 −, Ca 2+, Cl −, H +, Na +, Mg 2+ and K +. Air mass back trajectories were associated to the concentrations of trace metals and of SO 4 2−, Ca 2+, Mg 2+, NH 4 + and H + observed during each rainy day. Trace metal concentrations were not clearly related to wind direction. Enrichment factors related to the relative abundance of elements in crustal material were calculated using Mg as reference. The high enrichment factors (EF c) suggested that, in general, trace metals and major ions had an anthropogenic origin. Aluminum, K +, and Ca 2+ were the only elements that had a significant crustal source. Factor analysis (Principal Component Analysis) with Varimax normalized rotation grouping the elements analyzed into three factors. Factor 1 indicated a crustal contribution for Ca 2+, K +, Mg 2+ and anthropogenic sources for SO 4 2−, NH 4 + and V. Factor 2 indicated a high loading for Al, Ni and Mn, that indicate possible contribution of anthropogenic sources but with a significant crustal contribution for Al. Factor 3 indicated an anthropogenic origin for H + and NO 3 −. Pearson's correlations show that Al correlated with all the metals, including Ca 2+ and Mg 2+. The solubility of trace metals did not depend on rainwater pH. As it was expected, Al presented the highest wet deposition flux.

Oxygen and sulfur isotope geochemistry revealing a significant crustal signature in the genesis of the post-collisional granitoids in central Anatolia, Turkey

Late Cretaceous granitoid rocks from central Anatolia comprise S-I-A-type plutons derived from the collisional stages of the Neo-Tethyan convergence system in central Turkey. These granitoids intrude the tectonic imbrication zone consisting of blocks of supra-subduction zone-type (SSZ-type) central Anatolian ophiolite and crustal metasediments which are present in the İzmir-Ankara-Erzincan suture zone. The plutons are overlain by Late Palaeocene-Early Eocene or younger detrital sediments. Granitoid formation is thought to be related to magma generation processes occurring in a post-collisional lithospheric detachment-related geodynamic setting that resulted from slab break-off or lithospheric delamination. Whole-rock S and quartz/feldspar O isotope data from these plutons yields a broad range of values, and both parameters indicate a nearly exclusively supracrustal origin for the S-type granites, as well as a significant crustal contribution in the genesis of the hybrid I-type and A-type granitoids. The more mafic I-type and A-type granitoid rocks of any given suite have lower S and O values, indicative of their larger degree of mantle component. The combined stable isotope geochemical compositions, when coupled with major, trace and REE geochemistry and regional geology, provide evidence that the significant crustal contribution originated from a metasomatized mantle layer which was affected by earlier SSZ-derived fluids, and then accreted into the subcontinental lithosphere as collision occurred. The partial melting of such a metasomatized mantle layer in a post-collisional extensional geodynamic setting, supplied either by the slab break-off or lithospheric delamination mechanisms, provided the significant crustal signature in the hybrid magmas of the Late Cretaceous I-type and A-type granitoids in central Anatolia, Turkey.

Interrelations between coeval mafic and A-type silicic magmas from composite dykes in a bimodal suite of southern Israel, northernmost Arabian–Nubian Shield: Geochemical and isotope constraints

Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ 18O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648. The initial 143Nd/ 144Nd ratios, expressed by ε Nd( T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [ δ 18O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of 18O/ 16O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ 18O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ 18O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally.

Sm-Nd isotope and trace element evidence for heterogeneous igneous protoliths of Variscan mafic blueschists in the East Krkonoše Complex (West Sudetes, NE Bohemian Massif, Czech Republic)

Mafic blueschists of the East Krkonoše Complex (EKC, Krkonoše-Jizera Block, central West Sudetes) are glaucophane-bearing metabasites partly overprinted to greenschist facies assemblages. Major and trace element concentrations and Sm-Nd radiogenic isotopes were studied with the aim to evaluate the chemical affinities of their igneous protoliths of Early Ordovician age (485 ± 4 Ma, [1]), and draw inferences on their paleotectonic setting. The distribution of immobile incompatible trace elements patterns (e.g., Th/Nb from 0.07 to 0.1) and Nd isotope signatures (?Ndi = +5.9 to +7.7) of most mafic samples correspond to those of modern basalts of MORB—and OIB-types, respectively, free of significant interactions with materials from the continental crust. Basalts of this compositional diversity may be generated during the early stages of sea-floor spreading, but may also build sea-floor volcanoes (seamounts) situated close to mid-ocean ridges. One sample displays evidence for significant crustal contribution (Th/Nb = 0.4, ?Ndi = + 3.2), while another one, with deep Nb anomaly (Nb/Nb*= 0.33) combined with LREE depletion (147Sm/144Nd = 0.2018) and strongly radiogenic Nd isotope signature (?Ndi = + 7.4), is reminiscent of supra-subduction lavas formed in the absence of recycling of sediments of continental provenance. The igneous precursors of the EKC mafic blueschists were extracted from diverse mantle sources, that probably reflect a range of tectonic environments: incipient sea-floor spreading subsequent to lithospheric break-up heralded by bimodal volcanism [2], oceanic seamounts and, possibly, fragments of supra-subduction oceanic crust. The occurrence of such a heterogeneous igneous assemblage, combined with the presence of HP-LT metamorphic overprinting, suggests that all these mafic bodies were scrapped off subducted oceanic crust and its attached passive margin, and incorporated into an accretionary complex during the final closure of an oceanic domain, in Late Devonian - Early Carboniferous times.

Timing of post-collisional H-type to A-type granitic magmatism: U?Pb titanite ages from the Alpine central Anatolian granitoids (Turkey)

The last stages of the continental collision during the closure of the Neotethyan ocean in central Anatolia are characterized by post-collisional H- and A-type granitoids intruding both the metamorphic country rocks and allochthonous ophiolitic rocks of the central Anatolian crystalline complex. Available Rb–Sr and K–Ar whole-rock and mineral age data on the H- and A-type granitoids in central Anatolia are inconsistent. To better constrain the geological relevance and the timing of the change in the chemical character of magmatism in the wake of the Alpine orogeny in Anatolia, we re-evaluated the geochemical characteristics and dated titanite from representative H- (Baranadag quartz-monzonite: BR) and A-type (Camsari quartz-syenite: CS) granitoids by the U–Pb method. BR is a high-K calc-alkaline intrusion with mafic microgranular enclaves and shows enrichment of LILE relative to HFSE. The alkaline CS displays higher SiO2, Na2O+K2O, Fe/Mg, Rb, Th and HFSE with corresponding depletion in CaO, MgO, Fe2O3tot, P2O5, Ba, Sr, and Ti. Chondrite-normalized REE patterns of the BR and CS samples have LREE-enriched and flat HREE patterns, whereas CS differs from BR by higher LREE enrichment and lower MREE and HREE contents. Mineralogical and geochemical characteristics suggest that BR and CS were not products of the same magma source. BR was derived from a subduction-modified depleted hybrid-source and CS had an enriched mantle source with significant crustal contribution. The U–Pb titanite ages of the H-type central Anatolian granitoids (BR) and the A-type granitoids (CS) are 74.0±2.8 and 74.1±0.7 Ma, respectively. The coeval evolution of post-collisional/calc-alkaline H- to A-type magmatism was possibly associated with source heterogeneity and variable involvement of continental materials during the evolution of these granitoids. These new age data constrain the timing of the onset of a post-collision extensional period following the Alpine thickening within the passive margin of the Tauride–Anatolide platform, which occurred before the opening of the latest Cretaceous central Anatolian basins.

U-Pb zircon age of 548 Ma for the leptynites (high-grade felsic rocks) of the central part of the Maures Massif. Geodynamic significance of the so-called leptyno-amphibolitic complexes of the Variscan belt of western Europe

Abstract U-Pb zircon and Rb-Sr geochronological, and Sm-Nd isotopic studies have been carried out on the so-called leptyno-amphibolitic complex of the central part of the Maures Massif. The emplacement of the protolith of the felsic end-member (« leptynites ») has been dated at 548 Ma, an age much older than those (lower Ordovician) previously obtained on other leptyno-amphibolitic complexes. Rb-Sr data obtained on whole rocks and on mineral separates give an age of 348 Ma for the amphibolite-facies metamorphism. Nd isotopes indicate that the amphibolites display clear-cut mantle-derived signatures, whereas a significant crustal contribution is recorded in the three analyzed felsic facies. One of these acidic terms can be interpreted in terms of a simple mixing between two components, respectively similar to the amphibolites and to the two other felsic samples. These latter involve another mantle source, distinct from that of the amphibolites, and comparable to that of continental alkali basalts. These data indicate that the central part of the Maures Massif and the southern Massif Central were possibly part of the same pre-Variscan structural unit. The lack of evidence for a clear genetic relationship between the respective protoliths of the two end-members of the leptyno-amphibolitic complex raises once again the problem of the geodynamic significance of these formations.

Geochemistry of the Mt Wright Volcanics from the Wonominta Block, northwestern New South Wales

The Mt Wright Volcanics are located in the Wonominta Block of northwestern New South Wales. Detailed regional mapping has shown that the block is a composite tectonic unit and that the metavolcanic rocks described as the Mt Wright Volcanics may have been emplaced at different time from Late Proterozoic (northern section: Packsaddle, Nundora) to Early Cambrian (southern section: Mt Wright). Geochemical investigations, including major and trace elements, as well as analyses of relic clinopyroxene, show that the rocks have affinities with alkali basalt with light‐rare‐earth‐element‐enriched compositions. An intra‐plate extensional environment (such as rift‐ and/or plume‐related) is considered most likely for the formation of the rocks. Though metamorphosed to various degrees, the rocks apparently retain much of their primary Sr isotopic character (initial 87Sr/86Sr about 0.7032) and, apart from their age, resemble the Tertiary intraplate volcanism in eastern Australia. The Nd isotope analyses yield remarkably similar results between the two sections of the Mt Wright Volcanics, with 143Nd/144Nd between 0.51260 to 0.51271 and εNd(T) 4.7 ±0.4 (calculated to 525 Ma). A kaersutite‐bearing xenolith found in the northern section of the volcanic sequence has a Nd isotope composition more depleted than its hosts with εNd(T) of 7.7. The isotope results suggest that the Mt Wright Volcanics were derived from a depleted mantle source without significant crustal contribution. It is proposed that the Mt Wright Volcanics possibly represent the products of a rifting event that led to the breakup of the Proterozoic supercontinent during Early Cambrian in eastern Australia.

The evolution of a calc-alkaline basic to silicic magma system: Geochemical and Rb-Sr, Sm-Nd, and [formula omitted] isotopic evidence from the Late Hercynian Atesina-Cima d'Asta volcano-plutonic complex, northern Italy

Geochemical and Sr-Nd-O isotopic data presented for basaltic andesitic to rhyolitic and for quartz noritic to monzogranitic rock suites from the Late Hercynian calc-alkaline Atesina volcanic complex (AVC) and the Cima d'Asta pluton (CAP), Southern Alps (northern Italy), provide information on both the primary magmatic processes and the effects of (mainly Triassic) hydrothermal overprint. Fluid infiltration led to mobilization of major and trace elements (K 2O, Na 2O, CaO, Rb, Sr, and Ba), opensystem behavior in total-rock Rb-Sr, and shift in δ 18 O to elevated values (total rock up to 16.6%. and volcanic matrix up to 17.8%.). Oxygen isotopic disequilibrium between quartz-feldspar pairs suggests water-rock interaction at medium/low temperatures. The δ 18 O values of quartz, the REE characterized by regular LREE enrichment/HREE depletion, and the Sm-Nd isotopic signatures, however, remained virtually unaffected by secondary processes. The initial ϵ Nd values (at 270 Ma) of the AVC and CAP magmatites are restricted to overlapping ranges of −3.6 to −6.5 and of −2.7 to −6.5, respectively, indicating significant crustal contribution; these values and associated T DM model ages of 1.1–1.6 Ga agree well with those of typical South Alpine lower crustal magmatites. The AVC and CAP rocks do not follow the “normal” trend of increasingly crustal Nd isotopic signatures with progressive degree of magma evolution expected for a single-stage AFC-type process, but instead display an inversion of this relationship. Geochemical and isotopic constraints favor a model of a large-scale MASH-type melting and mixing zone at or near the base of the continental crust. Distinct elemental enrichment/depletion and REE crossover patterns displayed by high-silica as compared to less silicic AVC rhyolites suggest subsequent magma evolution within a shallow-level compositionally zoned chamber.

Mantle sources and magma‐continental crust interactions during early Red Sea‐Gulf of Aden rifting in southern Yemen: Elemental and Sr, Nd, Pb isotope evidence

Large‐scale magmatic activity, ranging from late Oligocene to Quaternary, is associated with the Red Sea‐Gulf of Aden rifting throughout the Arabian passive margin. The Southern Yemen area represents the southernmost extremity of this magmatic range, facing the Afar area, and provides a means of studying the magmatic records of early stages of rifting (30–16 Ma) in a plume‐related context. We investigate major and trace elements, and Nd, Sr and Pb isotopes of a bimodal series of transitional affinity consisting of (1) thick olivine‐basalt traps overlain by ignimbritic rhyolites, (2) basaltic, rhyolitic, trachytic and peralkaline dykes with a prevailing N120–140° E orientation, and (3) gabbroic, syenitic and granitic plutons. Major and some trace element variations from basalts to felsic rocks are consistent with low‐pressure fractional crystallization. Mass balance calculations using major elements suggest the fractionation of clinopyroxene (6–9%), olivine (∼6%), plagioclase (42–43%), magnetite (∼12%), apatite (1–2%) ± alkali feldspar (16%). However, LILE (large ion lithophile element) enrichment and high initial 87Sr/86Sr ratios (up to 0.7074 and 0.710 in rhyolites and pantellerites, respectively) require the felsic rocks to be generated through significant crustal assimilation. Nd and Sr isotopic ratios of the rhyolitic traps can be reproduced by bulk mixing between magmas similar to the underlying basaltic unit and the Arabian Proterozoic basement. On the other hand, an assimilation‐fractional crystallization process is required to account for the isotopic diversity of the rhyolitic and peralkaline dykes. Rhyolites can be derived from a basaltic liquid by a moderate fractionation rate (F = 0.47) and a high crustal assimilation rate (R = 0.45), whereas the pantellerites require more significant fractionation rate (F = 0.07) and a very low assimilation rate (R = 0.05). Elemental and isotopic signatures of the basalts do not support a significant crustal contribution in their formation and their isotopic diversity (87Sr/86Sr from 0.7034 to 0.7051, 143Nd/144Nd from 0.512676 to 0.513045 and 206Pb/204Pb from 17.96 to 18.66) mainly reflects mantle source heterogeneities. Sr‐Nd‐Pb isotopic data are consistent with a binary mixing between depleted and enriched source regions. The depleted end‐member corresponds to an asthenospheric reservoir approaching that producing mid‐ocean ridge basalts at Gulf of Aden/Red Sea spreading centers. The enriched reservoir, intermediate between enriched mantle I and II end‐members, is supposed to be located within old subcontinental lithosphere related to the Pan‐African orogenic events. Unlike the modern volcanics from Afar, no HIMU (high U/Pb ratio) signature has been recognized in our sampling. This rules out any significant chemical influence of the Afar plume upon early rift‐related volcanism in Southern Yemen and suggests a continental rift initiation of passive type. However, one can suspect that the Afar plume may have supplied the excess heat required to produce so voluminous traps and to trigger melting in the lithospheric mantle, making the distinction between passive and active rifting more ambiguous.