Metaluminous Affinity Research Articles

New U–Pb zircon ages of plagiogranites from the Nagaland–Manipur Ophiolites, Indo-Myanmar Orogenic Belt, NE India

We report new U–Pb zircon ages and whole-rock chemistry of plagiogranites from the Nagaland–Manipur Ophiolites, exposed in the Indo-Myanmar Orogenic Belt, NE India, which represents the southeastern extension of the Indus–Yarlung–Tsangpo Suture of Neo-Tethyan origin. The plagiogranites (SiO 2 = 57 and 76 wt%) are characterized by high sodium (Na 2 O = 5.9 – 7.2 wt%) and low potassium (K 2 O = 0.1 – 0.6 wt%), coupled with low Rb (1.8 – 9.5 ppm), low Rb/Sr (<0.1) and A/CNK < 1 (molar). Geochemically they are oceanic plagiogranites with trondhjemitic compositions and metaluminous affinities. Their rare earth element (REE) patterns exhibit depletion in light REE (La N /Sm N = 0.50 – 1.13) relative to heavy REE (Sm N /Yb N = 0.59 – 0.88), with low ΣREE and incompatible trace element contents that indicate derivation from a depleted mantle source. High ratios of large ion lithophile elements to high field strength elements with pronounced Nb, Ta and Ti depletions furthermore suggest the presence of the subduction component in the source region. Zircon U–Pb geochronology yielded mean 206 Pb/ 238 U ages between 116.4 ± 2.2 and 118.8 ± 1.2 Ma that record crustal formation in the Early Cretaceous. Collectively the data indicate that the Nagaland–Manipur Ophiolites are broadly coeval and geochemically comparable with Neo-Tethyan ophiolites elsewhere in the Indus–Yarlung–Tsangpo Suture.

Geochronology and geochemistry of igneous rocks in the Bailingshan area: Implications for the tectonic setting of late Paleozoic magmatism and iron skarn mineralization in the eastern Tianshan, NW China

The late Paleozoic Bailingshan intrusions and volcanic rocks are located in the Aqishan–Yamansu arc belt in the southern part of the eastern Tianshan and are associated with an important group of iron skarn deposits. The exposed intrusive rocks are mainly granodiorite, monzonitic granite, and granite. Zircon U–Pb dating of the Tugutublak Formation tuffaceous dacitic lava yields an age of 324Ma, whereas dates of the Bailingshan granodiorite, monzonitic granite, and granite yields ages of 317Ma, 313Ma, and 307Ma, respectively. The results indicate that the Bailingshan granitoids were emplaced soon after the eruption of the Tugutublak dacite. All these rocks studied show calc-alkaline to high-K calc-alkaline and metaluminous affinities, with A/CNK values ranging 0.83–1.10. They are enriched in Rb, K, and Pb, depleted in Nb, Ta, Ti, and P, and contain low Sr/Y (4.16–23.7) and Sr (109.0–347.0 ppm) values, displaying typical arc geochemical affinities. The tuffaceous dacitic lava has low Nb/Ta (10.3–14.1) values, a wide range of Mg# (6–64), positive zircon εHf(t) (3.2–7.5) values, and elevated whole-rock εNd(t) (2.03–4.41), but low ISr values (0.70427–0.70530), indicating that the source magma may have been derived from the juvenile lower crust with minor mantle input. The Bailingshan I-type intrusions also exhibit a mixed source signal, as constrained by Nb/Ta ratios, Mg#, and isotopes characteristics. Because the granodiorite, monzonitic granite, and granite intrusions have higher zircon εHf(t) (3.3–7.5, 11.8–13.5, and 10.2–14.4, respectively) and εNd(t) (3.90, 5.78, and 5.94, respectively) values than those of the tuffaceous dacitic lava, it is suggested that mantle-derived materials may have played a more prominent role with their petrogenetic evolution. Integrating our new geological, age, geochemical and isotopic data we propose that the Aqishan–Yamansu iron skarn belt may have formed in a back-arc position or within an intra-arc basin generated by the southward subduction of the Kanggur oceanic plate beneath the Yili–Central Tianshan block during the late Paleozoic, with felsic-intermediate magmatism occurring during the basin inversion.

Laser-ICP-MS U–Pb zircon ages and geochemical and Sr–Nd–Pb isotopic compositions of the Niyasar plutonic complex, Iran: constraints on petrogenesis and tectonic evolution

We conducted geochemical and isotopic studies on the Oligocene–Miocene Niyasar plutonic suite in the central Urumieh–Dokhtar magmatic belt, in order better to understand the magma sources and tectonic implications. The Niyasar plutonic suite comprises early Eocene microdiorite, early Oligocene dioritic sills, and middle Miocene tonalite + quartzdiorite and minor diorite assemblages. All samples show a medium-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of large-ion lithophile elements (LILEs, e.g. Rb, Ba, Sr), enrichment of light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs, e.g. Nb, Ta, Ti, P). The chondrite-normalized rare earth element (REE) patterns of microdiorite and dioritic sills are slightly fractionated [(La/Yb)n = 1.1–4] and display weak Eu anomalies (Eu/Eu* = 0.72–1.1). Isotopic data for these mafic mantle-derived rocks display ISr = 0.70604–0.70813, ϵNd (microdiorite: 50 Ma and dioritic sills: 35 Ma, respectively) = +1.6 and −0.4, TDM = 1.3 Ga, and lead isotopic ratios are (206Pb/204Pb) = 18.62–18.57, (207Pb/204Pb) = 15.61–15.66, and (208Pb/204Pb) = 38.65–38.69. The middle Miocene granitoids (18 Ma) are also characterized by relatively high REE and minor Eu anomalies (Eu/Eu* = 0.77–0.98) and have uniform initial 87Sr/86Sr (0.7065–0.7082), a range of initial Nd isotopic ratios [ϵNd(T)] varying from −2.3 to −3.7, and Pb isotopic composition (206Pb/204Pb) = 18.67–18.94, (207Pb/204Pb) = 15.63–15.71, and (208Pb/204Pb) = 38.73–39.01. Geochemical and isotopic evidence for these Eocene–Ologocene mafic rocks suggests that the magmas originated from lithospheric mantle with a large involvement of EMII component during subduction of the Neotethyan ocean slab beneath the Central Iranian plate, and were significantly affected by crustal contamination. Geochemical and isotopic data of the middle Miocene granitoids rule out a purely crustal-derived magma genesis, and suggest a mixed mantle–crustal [MASH (melting, assimilation, storage, and homogenization)] origin in a post-collision extensional setting. Sr–Nd isotope modelling shows that the generation of these magmas involved ∼60% to 70% of a lower crustal-derived melt and ∼30% to 40% of subcontinental lithospheric mantle. All Niyasar plutons exhibit transitional geochemical features, indicating that involvement of an EMII component in the subcontinental mantle and also continental crust beneath the Urumieh–Dokhtar magmatic belt increased from early Eocene to middle Miocene time.

Geochronology–geochemistry of the Cida bimodal intrusive complex, central Emeishan large igneous province, southwest China: petrogenesis and plume–lithosphere interaction

The Cida complex is situated in the Panxi region and is predominantly composed of mafic-ultramafic and syenitic rock units; minor amounts of intermediate rocks occupy the contact zone between the two major rock types. The intermediate unit is mineralogically heterogeneous and typically exhibits a mottled structure. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb zircon dating shows that the mafic-ultramafic rocks and syenitic rocks formed almost coevally (243 ± 0.77 Ma and 240.5 ± 0.76 Ma, respectively). These ages may represent the end phase of the Emeishan large igneous province (ELIP) magmatism. Most of these three rock types possess alkaline and metaluminous affinities. The mafic-ultramafic, syenitic, and intermediate units have K2O + Na2O contents of 1.85–5.16, 6.55–10.46, and 9.55–11.54 wt.%, and SiO2 contents of 40.06–46.70, 61.74–68.54, and 51.57–54.13 wt.%, respectively. The mafic-ultramafic unit displays ocean-island basalt (OIB)-like primitive-mantle-normalized incompatible element patterns, coupled with low initial 87Sr/86Sr ratios (0.7048–0.7064), positive ϵNd(t) (0.32–2.23), and zircon ϵHf(t) (4.53–14.17) values, consistent with a mafic plume-head origin, whereas one exceptional sample with negative ϵNd(t) (–0.22) can be interpreted as due to the involvement of considerable amounts of enriched subcontinental lithospheric mantle. The relatively low (La/Yb) N ratios (3.40–7.69) reflect a spinel-facies lherzolite source. The syenitic unit is characterized by enrichment in large ion lithophile elements (e.g. Rb, K, Pb) and light rare earth elements (LREEs), relative to high field strength elements (e.g. Nb, Ta, P, Ti) and heavy rare earth elements (HREEs), respectively. These features, together with their metaluminous affinities, low SiO2 contents, lower initial 87Sr/86Sr ratios (0.7043), positive ϵNd(t) (0.18), and zircon ϵHf(t) (2.63–10.09) values as well as modelling of REEs, can be plausibly explained by crustal partial melting of juvenile basic materials beneath the Yangtze Block. In contrast, the field, petrographic observations, and geochemical signatures (e.g. the linear correlations between FeO* and MgO, K/Ba and Rb/Ba ratios) suggest that the intermediate unit may have resulted from magma mixing between the syenitic and basaltic magmas that in turn had evolved from a parental mafic-ultramafic liquid. Thus, the formation of the Cida complex can be attributed to the plume–lithosphere interaction plus partial melting of juvenile basic lower crust in response to heating of underplated plume-derived basaltic magma.

Geochemical and petrogenetic features of plagiogranite dykes in the Koziakas ophiolitic mélange (W. Thessaly, Greece)

Plagiogranite and dolerite dykes with boninitic affinities intrude thrust-bounded blocks of serpentinized harzburgite at the eastern part of the Koziakas ophiolitic melange. Geochemically they are characterized as oceanic plagiogranites with trondhjemitic composition and metaluminous affinities. Low ΣREE and trace element contents indicate derivation from a depleted mantle source. Geochemical modelling shows that the Koziakas plagiogranites could have originated by 5–20 % partial melting of a gabbro precursor. Locally, an early-stage high-T hydrothermal alteration event seems to have caused the anorthitization of magmatic plagioclase. This hydrothermal event was overprinted by a low-T retrograde alteration. The overall field data and geochemical affinities suggest a supra-subduction zone origin for the studied plagiogranite dykes, which is compatible with the geotectonic environment of the Koziakas ophiolite.

Upper Carboniferous retroarc volcanism with submarine and subaerial facies at the western Gondwana margin of Argentina

During Late Carboniferous times a continental magmatic arc developed at the western margin of Gondwana in South America, as several marine sedimentary basins were formed at the same time in the retroarc region. North of 33°S, at Cordón Agua del Jagüel, Precordillera of Mendoza, Argentina, a volcanic sequence crops out which was emplaced in a submarine environment with some subaerial exposures, and it is intercalated in marine sediments of Agua del Jagüel Formation, which fills of one of these retroarc basins. This paper presents, for the first time, a facies analyses together with geochemical and isotopic data of this volcanic suite, suggesting its deposition in an ensialic retroarc marine basin. The volcanic succession comprises debris flows with either sedimentary or volcanic fragments, base surge, resedimented massive and laminated dacitic–andesitic hyaloclastite, pillow lava, basic hyaloclastite and dacitic–andesitic lavas and hyaloclastite facies. Its composition is bimodal, either basaltic or dacitic–andesitic. The geochemistry data indicate a subalkaline, low K calk-alkaline and metaluminous affinity. The geochemistry of the basalts points to an origin of the magmas from a depleted mantle source with some crustal contamination. Conversely, the geochemistry of the dacites–andesites shows an important participation of both crustal components and subduction related fluids. A different magmatic source for the basalts than for the dacites–andesites is also supported by Sr and Nd isotopic initial ratios and Nd model ages. The characteristics of this magmatic suite suggest its emplacement in an extensional setting probably associated with the presence of a steepened subduction zone at this latitude during Upper Carboniferous times.

Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions

Many basaltic flood provinces are characterized by the existence of voluminous amounts of silicic magmas, yet the role of the silicic component in sulphur emissions associated with trap activity remains poorly known. We have performed experiments and theoretical calculations to address this issue. The melt sulphur content and fluid/melt partitioning at saturation with either sulphide or sulphate or both have been experimentally determined in three peralkaline rhyolites, which are a major component of some flood provinces. Experiments were performed at 150MPa, 800–900 � C, fO2 in the range NNO – 2 to NNO þ 3 and under water-rich conditions. The sulphur content is strongly dependent on the peralkalinity of the melt, in addition to fO2, and reaches 1000ppm at NNO þ 1 in the most strongly peralkaline composition at 800 � C. At all values of fO2, peralkaline melts can carry 5–20 times more sulphur than their metaluminous equivalents. Mildly peralkaline compositions show little variation in fluid/melt sulphur partitioning with changing fO2 (DS � 270). In the most peralkaline melt, DS rises sharply at fO2 > NNO þ 1 to values of >500. The partition coefficient increases steadily for Sbulk between 1 and 6wt % but remains about constant for Sbulk between 0� 5 and 1wt %. At bulk sulphur contents lower than 4wt %, a temperature increase from 800 to 900 � C decreases DS by � 10%. These results, along with (1) thermodynamic calculations on the behaviour of sulphur during the crystallization of basalt and partial melting of the crust and (2) recent experimental constraints on sulphur solubility in metaluminous rhyolites, show that basalt fractionation can produce rhyolitic magmas having much more sulphur than rhyolites derived from crustal anatexis. In particular, hot and dry metaluminous silicic magmas produced by melting of dehydrated lower crust are virtually devoid of sulphur. In contrast, peralkaline rhyolites formed by crystal fractionation of alkali basalt can concentrate up to 90% of the original sulphur content of the parental magmas, especially when the basalt is CO2-rich. On this basis, we estimate the amounts of sulphur potentially released to the atmosphere by the silicic component of flood eruptive sequences. The peralkaline Ethiopian and Deccan rhyolites could have produced � 10 17 and � 10 18 g of S, respectively, which are comparable amounts to published estimates for the basaltic activity of each province. In contrast, despite similar erupted volumes, the metaluminous Parana´–Etendeka silicic eruptives could have injected only 4� 6 · 10 15 g of S in the atmosphere. Peralkaline flood sequences may thus have greater environmental effects than those of metaluminous affinity, in agreement with evidence available from mass extinctions and oceanic anoxic events.

The Neoproterozoic Dubr intrusives, Central Eastern Desert, Egypt: petrological and geochemical constraints on the evolution of a mafic-felsic suite

The Neoproterozoic magmatism in the Dubr area took place in two suites namely: (1) a mafic suite of a gabbro-diorite complex and (2) a felsic suite of a granodiorite and monzogranite composition. Pressures estimated for the gabbro and diorite rocks show that they have originated over a wide range of pressures (1.5-6.5 kbar). The gabbro and diorite rocks predict crystallization temperatures ranging from 820 to 940 °C and 660 to 710 °C, respectively. The rare-earth element abundances of the Fe--tholeiite gabbro-diorite complex display fractionated {(La/Lu) n = 1.53-2.96} and LREE-enriched {(La/Sm) n = 1.12-2.62} patterns. The REE patterns of the gabbro-diorite complex suggest that it can be generated from a spinel-bearing mantle source. Transition metal contents of the gabbro-diorite complex are variable and low (Ni, 9-133 ppm; Cr, 20-325 ppm; Sc, 17.7-34 ppm), indicating that none of the samples are primitive. The gabbro-diorite complex may have formed via relatively high degree partial melting (25-30 %) of an enriched mantle source followed by fractional crystallization of olivine, Cr-spinel with minor clinopyroxene and plagioclase. Chemical evidences imply that crustal contamination is likely to be a minor factor in trace element variation of the gabbro-diorite complex. The granodiorite and monzogranite are calc-alkaline showing metaluminous affinity for the granodiorite and a slightly peraluminous nature for the monzogranite. They display trace element and REE characteristics of I-type granites formed in a subduction-related arc environment. The hornblende of the granodiorite and monzogranite yielded pressures of an average 3 kbar and 2.5 kbar, respectively. Amphibole-plagioclase geothermometer yielded crystallization temperatures ranging from 680 to 700 °C and 650 to 675 °C for the granodiorite and monzogranite, respectively. The co-linear trends on variation diagrams for the granodiorite and monzogranite indicate a genetic link between them through fractional crystallization. It appears likely that the restite-unmixing model is responsible for the petrological and chemical features of the granodiorite. The most probable source for the granodiorite is a dioritic rock. The monzogranite was derived as a result of crystallization of plagioclase, biotite, hornblende, quartz, ilmenite and magnetite phases from a granodioritic melt.

Petrological and geochemical constraints on the tectonomagmatic evolution of the late Neoproterozoic granitoid suites in the Gattar area, North Eastern Desert, Egypt

Late Neoproterozoic granitoid rocks from the Gattar area, north Eastern Desert of Egypt, belong to three distinctive magmatic suites, namely: (1) calc-alkaline older granitoids, (2) mildly alkaline subsolvus younger granitoids, and (3) alkaline hypersolvus younger granitoids. These suites are genetically unrelated. The older granitoids consist of three main varieties: quartz-monzodiorite, quartz-monzonite and granodiorite. They show enrichment of LIL elements, high LREE/HREE ratio, with little or no Eu anomalies, and HFS elements are generally low, and further, exhibit metaluminous (ASI < 1) affinity with I-type features. The parental magma for this suite is most probably of mantle derivation. The most mafic quartz monzodiorite represents 25 % batch melting of garnet lherzolite lower crustal source followed by fractional crystallization producing quartz-monzonite and granodiorite emplaced in a volcanic arc environment. The Gattar younger granitoids have metaluminous to slightly peraluminous and alkaline affinities. They can be classified as mildly alkaline subsolvus and alkaline hypersolvus granites. The younger granitoids cannot be related to the older granitoids by simple fractionation from one parental magma. Hypersolvus granites have higher contents of Rb, Y, and Nb and lower contents of TiO 2 , Sr, Ba and REE and have more pronounced negative Eu anomalies than the subsolvus granites. Subsolvus granites have post-orogenic (A2-subtype) features, while hypersolvus granites have anorogenic (Al-subtype) characters. Mildly alkaline subsolvus granites have been generated by fractional crystallization of mainly K-feldspar, plagioclase, biotite, hornblende and accessory phases such as allanite, titanite and zircon from a mafic magma in order to generate their chemical variations. The alkaline hypersolvus granites, on the other hand, could be generated by about 10 % batch melting of amphibolite in the lower crust. The presence of leached biotite-hematite-fluoride clots in the hypersolvus granites suggests that fluid fractionation and fluorine complexing have played some role during the evolution of the hypersolvus granites.

Geochemistry and Petrogenesis of the Neoproterozoic Granitoids in the Central Eastern Desert, Egypt

Abstract Neoproterozoic rocks in the Umm Gheig Province (UGP), central Eastern Desert, comprise five plutonic rock suites, i.e. a gabbro-diorite suite (GDS), an older granitoid suite (OGS), an I-type younger granitoid suite (I-YGS), an A-type younger granitoid suite (A-YGS) and a garnet-bearing granitoid suite (GBGS). The GDS exhibits a tholeiitic affinity generated in a volcanic arc, subduction-related environment. The unfractionated REEpattern of the gabbro {(La/Lu) N =0.6}differs significantly from the fractionated pattern of the diorite {(La/Lu) N =5.07}. They are, however, characterized by flat HREE{(Gd/Lu) N =1.24 and 1.45, respectively}and the absence of a Eu anomaly (Eu/Eu*=1.03 for both). The rocks of the GDSdo not represent a primary magma, and the gabbro could be generated by high degree melting of a garnet-free source followed by crystallization of clinopyroxene and olivine. The dioritic melt could be produced by melting of gabbroic rocks. The granitoids can be divided into four general suites emplaced late- (OGS, I-YGS and GBGS) and post-tectonically (A-YGS), each of which shows some variations in their bulk chemical compositions. All the granitoid suites display ferriferous and metaluminous affinities and belong to the non-alkaline series:OGS and I-YGS represent calc-alkaline magmatism while A-YGSand GBGS encompasses highly fractionated calc-alkaline magmatism. There is substantial uniformity in REEpatterns between the OGS, I-YGS and A-YGS, with no real change in light to heavy REEratios. Chondrite-normalized REEpatterns of the GBGSis characterized by strong enrichment in HREE and pronounced negative Eu anomalies (Eu/Eu*=0.095) compared to the other granitoid suites. The geochemistry and petrological characteristics of the granitoid suites suggest close genetic relationships and it is suggested that they were formed from a single parent magma, over a relatively short period of time. Major and trace element variations in the different granitoid suites indicate that they can be modelled by fractional crystallization of plagioclase, K-feldspar, biotite and hornblende. Variation in REEpatterns are governed by fractionation of accessory phases such as apatite, monazite and zircon which occur as inclusions in phenocryst phases.

Pan African layered diorite‐tonalite‐granodiorite from Sinai massif: an open system fractionation at a subduction zone

ABSTRACTThis paper reports on the occurrence of layered Pan African dioritetonalite‐granodiorite (DTG) rocks. The layering is marked by alternation of melanocratic (M) layers (diorites and tonalites) and leucocratic (L) layers (tonalites and granodiorites). M‐samples have cumulus biotite+hornblende+relict pyroxene+plagioclase+K‐feldspars+magnetite+apatite, and have transitional calc‐alkaline and metaluminous affinities. They were derived from subduction‐related magma enriched in Rb, Ba, K and LREE and depleted in Sr and Nb. L‐samples have cumulus plagioclase+hornblende. They are enriched in Sr and depleted in Rb, Ba, K, Nb and LREE. They have calc‐alkaline and peraluminous affinites.The formation of the rhythmic layers of DTG composition can be attributed to periodical replenishment of pulses of basic magma into a more evolved acidic magma chamber under open system conditions. Field relations, mineralogy and element concentration among the M‐ and L‐layers indicate that at the subduction zone, the ascending magma was contaminated with lower crustal materials (marginal basin metasediments) which led to LILE‐enrichment, Nb‐depletion and transition from calc‐alkaline to alkaline and from metaluminous to peraluminous affinities as well.