Felsic Granodiorite Research Articles

Archean crustal generation and Neoproterozoic partial melting in the Ivindo basement, NW Congo craton, Republic of Congo: Petrology, geochemistry and zircon U–Pb geochronology constraints

We present systematic petrographic descriptions, geochemical and geochronological data from Souanké, Bomalinga, Elogo and Zoula areas in the Ivindo basement to understand the evolution of NW Congo Craton, Republic of Congo. Hornblende tonalite, biotite granodiorite, usually with hornblende-bearing enclaves, and granitic leucosome veins are ubiquitous in all areas. Specifically, coarse-grained muscovite-garnet-bearing pegmatite cross-cutting ancient mafic tonalite and felsic granodiorite in the Souanké area. Overall, three well-defined chemical patterns are described: (1) low SiO2 (57–63 wt%), high MgO + FeO (9–19 wt%) and metaluminous composition with intermediate LREE/HREE ratios and slightly positive Eu anomaly for tonalite, (2) middle SiO2 (67–73 wt%), MgO + FeO (7–1.5 wt%) and peraluminous composition with intermediate LREE/HREE ratios for granodiorite and (3) high SiO2 (72–75 wt%), low MgO + FeO (∼<1 wt%) and peraluminous character with low LREE/HREE ratios (flat patterns) and negative Eu anomaly in granitic veins and pegmatites. Regardless of the study area, the hornblende tonalites present similar ages (2884 ± 2.7, 2875 ± 5.6 and 2888 ± 3.2 Ma) that are near coeval with the ages presented by biotite granodiorites (2896 ± 8.9, 2880 ± 5.9 and 2875 ± 6.4 Ma), while granitic leucosomes have slightly younger ages (2850 ± 7.3, 2870 ± 8.2 and 2858 ± 7.2 Ma). Lastly, garnet-bearing pegmatite presents a Neoproterozoic/Cambrian transition age of 540 ± 1.6 Ma. Different enrichment LREE/HREE ratios combined with negative Nb–Ta and Ti anomalies may suggest a subduction-like setting for the generation of near coeval Mesoarchean tonalite-granodioritic crust. Whereas field relationships and flat REE patterns in the muscovite-garnet-bearing pegmatite indicate a younger ∼540 Ma crustal melting in the NW Congo Craton likely related to West Gondwana assembly. Further, these results confirm that the ∼2.9 Ga represents a significant period of continental reoworking within a magmatic arc setting.

Geochronology and geochemistry of Devonian magmatism in the Frontal cordillera (Argentina): geodynamic implications for the pre-Andean SW Gondwana margin

ABSTRACT Whereas north of 33°S Devonian calcalkaline magmatism is notably absent, a Devonian arc is well developed to the south of 33°S. The Carrizalito, Pampa de los Avestruces, and Papagayos plutons are located in the Frontal Cordillera between 34° and 35°S. U-Pb zircon SHRIMP and LA-MC-ICP-MS data from the plutons yield Early Devonian ages of 389 ± 3 and 409 ± 3 for Carrizalito, 413 ± 2 Ma for Pampa de los Avestruces, and 346 ± 3, 393 ± 5 and 412 ± 3 Ma for Papagayos. The younger zircon age from the Papagayos pluton suggests that the Devo nian magmatism continued during the Early Carboniferous. Carrizalito and Pampa de los Avestruces plutons display metaluminous to slightly peraluminous granodiorite composition, with SiO2 ranging from 65.1 to 67.8 wt.% and from 67.9 to 69.7 wt.%, respectively, and define a magnesian calc-alkalic trend. The Papagayos pluton is metaluminous to slightly peraluminous and slightly more evolved in composition (felsic granodiorite to monzogranite), with a SiO2 content that varies between 68.4 to 70.5 wt.%, defining a magnesian with ferroan enrichment and calc-alkalic to alkali-calcic trend. These geochemical characteristics are similar to those reported for the Carboniferous arc granitoids in the Frontal Cordillera and Western Sierras Pampeanas at 27°–30°S and the archetypical calc-alkaline granitoids of the Peruvian Coastal batholith, indicating an Early Devonian magmatic arc. Rare earth element patterns ([La/Yb]N = 10–20) with mostly slight to moderate negative Eu anomalies (Eu/Eu* = 0.62–0.80) and geochemical ‘spider-diagrams’ are similar to arc granitoids. Combined whole-rock Nd and Sr isotope data from the largest Carrizalito and Pampa de los Avestruces plutons (εNdt = −4.3 and −4.6; 87Sr/86Srt = 0.70704 and 0.70762; TDM = ca. 1.5 Ga) together with in situ U–Pb and Hf isotope data from magmatic zircon (εHft ranging from −16.5 to −5.4; TDM = ca. 1.7 Ga), suggest a common older continental lithosphere source. The presented data indicate tectonic segmentation in the pre-Andean margin controlling the presence or absence of Devonian subduction-related magmatism. This segmentation is best explained by changes in the tectonic plate configuration occurred in a long-lived convergent margin.

Geochronological and geochemical studies of adakites from Tethyan Belt, Western Pakistan: A clue to geodynamics and Cu-Au mineralization

ABSTRACT Large porphyry Cu-Au deposits are associated with Early Miocene intrusive rocks in Tethyan belt, discovered along Chagai magmatic arc in Western Pakistan, adjacent to Southeast Iran. Two types of rocks were discriminated as granodiorite and monzodiorite from Saindak area. The granodiorites are associated with regional large Cu-Au mineralization, while the monzodiorites are mostly ore-barren. LA-ICP-MS U-Pb dating yielded weighted mean 206Pb/238U ages of 22.16–24.2 Ma for granodiorites and ca. 22.6 Ma for a monzodiorite. These intrusive rocks in the Saindak area are all calc-alkaline rocks, with enriched LILEs and depleted HFSEs, and without Eu negative anomalies. The felsic granodiorites are characterized by high Sr contents and Sr/Y ratios, with intermediate to high (La/Yb)N ratios that identify them as typical high-silica adakites. In contrast, the basaltic-andesitic monzodiorites are just normal arc-related rocks, showing less fractionated REE patterns than the granodiorites, with systematically lower LREE and higher MREE and HREE. Low K2O/Na2O ratios and decoupled Sr/Y-(La/Yb)N ratios indicate the characteristics of slab-derived adakites for the granodiorites, most likely originated through partial melting of the subducted Neotethys oceanic crust beneath Eurasian continent followed by subsequent mantle interaction. The almost simultaneously melting of subcontinental lithospheric mantle induced by dehydration of Neotethys plate gave rise to the formation of the barren monzodiorites. These intrusive rocks in the Saindak area are characterized by similar Sr-Nd-Pb isotopes with high 87Sr/86Sri ratios, slightly negative εNd(t) values and radiogenic Pb isotopes, plotting in the field between the MORB and EM-II mantle endmembers or the Average Cadomian Lower Crust, suggesting subducting sediments or old continental crustal materials have contributed into the compositions of these rocks by source enrichment or crustal contamination. The slab-melting derived adakite is favourable for regional massive Cu-Au mineralization in the Saindak area.

Geochemistry of Ekenkpon and Nkporo shales, Calabar flank, SE Nigeria: implications for provenance, transportation history and depositional environment

The Cretaceous Nkporo and Ekenkpon Shales within the Calabar Flank were investigated using Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) and Atomic Emission Spectroscopy (ICP-AES) to deduce the provenance, transportation history and depositional environment. The results show the dominance of SiO2 (45.27 – 46.45%; 44.50 – 54.83%), Al2O3 (22.27 – 23.57%; 19.20 – 20.20%) and Fe2O3 (8.30 – 9.04%; 5.64 – 7.30%) constituting the bulk major oxides of Nkporo and Ekenkpon Shales respectively. The Index of Chemical Variation (ICV) ranges from 0.42 – 0.56 and 0.57– 0.68 for Nkporo and Ekenkpon Shales respectively, suggesting moderate and high degree of weathering. The enrichment of Sr (124 – 350ppm; Nkporo Shale and 176 – 856ppm; Ekenkpon Shale) compared to Post Australian Archean Shale (PAAS; 14.6ppm) could be attributed to feldspars in the source area, and is easily incorporated in the clay minerals of the shales. Also the values of Th and U which are 15.00 – 17.20ppm; 14.6- 23.2ppm and 3.5-4.2ppm; 1.9-6.2ppm respectively in Nkporo and Ekenkpon sediments show enrichment compared to PAAS of 14.6ppm and 3.1ppm for respective values of Th and U. The Eu anomaly of 0.04 to 0.07 suggests felsic source rocks for both formations while plots of TiO2 vs. Al2O3 indicate an intermediate to dominantly felsic granodiorite source for both shales, with little input from felsic volcanic provenance plus traces of quartzose sedimentary rocks. The mobility of Na, Ca, and K due to progressive weathering of the shales is evident in the bivariate plots of Na2O wt% vs. PIA, CaOwt% vs. PIA and K2O vs. PIA. Values of Th/U ratios range from 3.37 – 4.91(Nkporo) and 3.10 – 7.68 (Ekenkpon), indicating moderate to high weathering and reworking of sediments. It is envisaged that the area is associated with passive to active continental margin tectonics, where sediments were mainly sourced from felsic rocks of the adjoining terrain and deposited in oxic, continental to transitional marine environment based on Al2O3– K2O +CaO+MgO-Fe2O3+MgO; AKF plot.KEYWORDS: Cretaceous, Calabar Flank, provenance, transportation history, depositional environment.

Early Cambrian U-Pb zircon age and Hf-isotope data from the Guasayán pluton, Sierras Pampeanas, Argentina: implications for the northwestern boundary of the Pampean arc

An Early Cambrian pluton, known as the Guasayán pluton, has been identified in the central area of Sierra de Guasayán, northwestern Argentina. A U-Pb zircon Concordia age of 533±4 Ma was obtained by LA-MC-ICP-MS and represents the first report of robustly dated Early Cambrian magmatism for the northwestern Sierras Pampeanas. The pluton was emplaced in low-grade metasedimentary rocks and its magmatic assemblage consists of K-feldspar (phenocrysts)+plagioclase+quartz+biotite, with zircon, apatite, ilmenite, magnetite and monazite as accessory minerals. Geochemically, the granitic rock is a metaluminous subalkaline felsic granodiorite with SiO2=69.24%, Na2O+K2O=7.08%, CaO=2.45%, Na2O/ K2O=0.71 and FeO/MgO=3.58%. Rare earth element patterns show moderate slope (LaN/YbN=8.05) with a slightly negative Eu anomalies (Eu/Eu*=0.76). We report the first in situ Hf isotopes data (εHft=-0.12 to -4.76) from crystallized zircons in the Early Cambrian granites of the Sierras Pampeanas, helping to constrain the magma source and enabling comparison with other Pampean granites. The Guasayán pluton might provide a link between Early Cambrian magmatism of the central Sierras Pampeanas and that of the Eastern Cordillera, contributing to define the western boundary of the Pampean paleo-arc.

Petrogenetic connections between ash-flow tuffs and a granodioritic to granitic intrusive suite in the Sierra Nevada arc, California

Trace-element analyses of zircons from ash-flow tuffs and granitic rocks in the east-central Sierran Nevada, combined with U-Pb zircon ages and trace-element abundances in whole rocks, allow us to interpret a shared magmatic heritage for widespread eruption of ash-flow tuffs and assembly of a typical Sierran granodioritic to granitic intrusive suite. The tuffs have 5%-15% phenocrysts of quartz, feldspar, biotite, and amphibole, silica contents ranging from 70% to 74%, and have variable alkali abundances but are consistently light rare-earth enriched with weak negative Eu anomalies, suggesting chemical classification as hydrothermally altered, arc-type, low-silica rhyolites. The tuffs yielded U-Pb zircon crystallization ages of 232, 224, and 219 Ma, indicating that explosive volcanism began before and continued throughout emplacement of granodioritic to granitic plutons in the underlying 226-218 Ma Scheelite Intrusive Suite, one of the oldest and largest intrusive suites in the Sierra Nevada batholith. Zircon crystals from ash-flow tuffs and contemporaneous felsic granodiorite and granite show distinct chemical zoning and extensive overlap in trace-element abundances, indicating crystallization of zircons from very similar melt compositions despite dissimilar late cooling histories. These data provide evidence for a unified volcano-plutonic model for Sierran arc magmatism that describes a long-lived, multi-stage magma system composed of an incrementally emplaced, granodioritic to granitic intrusive suite in the upper crust, overlain by widespread deposits of ignimbrite eruptions generated by thermal rejuvenation of felsic crystal mushes.

Isotopic Constraints on the Petrogenesis of Jurassic Plutons, Southeastern California

The 165 Ma Eagle Mountain intrusion is a heterogeneous, enclave-bearing, metaluminous remnant of the Cordilleran Jurassic arc that cuts regionally metamorphosed pre-Mesozoic rocks in the southeastern Mojave Desert of California. The main phase of the intrusion consists of granodiorite to tonalite host facies, diorite mixed facies, and homogeneous monzogranite facies. The host facies contains microdiorite enclaves interpreted as intermingled masses of mafic magma. Late-phase leucogranite stocks cut the main phase. Mineral equilibria indicate emplacement at ∼6.5 km depth, with solidus temperatures ranging from 760°C for diorite to 700°C for felsic granodiorite. Although uniform radiogenic-isotope compositions (Sri = 0.7085, ϵNdi = −9.4) suggest derivation from a single source, no known source has the composition required. A hybrid source is proposed, consisting of various proportions of juvenile mantle and recycled lower crust. Calculations indicate that the source of the Eagle Mountain intrusion comprised >60% juvenile mantle and <40% recycled crust. On the basis of their isotopic compositions, other mafic Jurassic plutons in the region were derived from sources containing different proportions of mantle and crustal components.

The granitoids of the Moravian Zone of northeast Austria: Products of a Cadomian active continental margin?

Abstract A large number of Cadomian granitoids occur in the Bruno-Vistulicum (Brno unit). This unit is a pre-Variscan consolidated block adjoining the Central European Variscan orogen on the eastern side of the Bohemian Massif. It most probably represents a marginal part (a prong) of the Fennosarmatian Platform. Some of the Bruno-Vistulian granitoids are exposed within Austria in the so-called Moravian Zone, which is located at the western margin of the Bruno-Vistulicum. It was deformed and metamorphosed to various degrees during the Variscan orogeny. The granitoids of the Moravian Zone are quartz-diorites, tonalites, quartz-monzodiorites, granodiorites and granites. Felsic granodiorites and granites prevail. The granitic plutonism as a whole exhibits a great number of geochemical similarities to circum-Pacific continental-arc, I-type batholiths. Cadomian plutonic activity in the Bruno-Vistulicum probably occurred in connection with subduction of oceanic lithosphere beneath the southwestern margin of the Fennosarmatian continent.

San Jacinto Intrusive Complex: 1. Geology and mineral chemistry, and a model for intermittent recharge of tonalitic magma chambers

Geological mapping within the San Jacinto Mountains of southern California has delineated three major and numerous minor plutons of the Cretaceous Peninsular Ranges batholith. Early minor intrusives emplaced into quartz‐rich metasedimentary sequences span the compositional range olivine gabbro to granite. The three large (to 250 km2) plutons span a limited compositional range between mafic tonalite (color index (CI) &gt; 15) and K‐feldspar‐poor granodiorite (CI ≃ 10). All units are composed of plagioclase (An30–40) (50–55%), quartz (20–30%), K‐feldspar (1–8%), biotite (10–15%), hornblende (0–5%), titanite (0–2%), and accessory zircon, apatite, allanite, and ilmenite. Variations in mineral abundances are geographically systematic only within the youngest major mass (unit III), which grades from marginal mafic tonalite to central K‐feldsparpoor granodiorite. Mineral foliations and banding, schlieren, and inclusion orientation within each unit usually parallel the nearest contact. Alignment of foliations and apparent flow‐sorting and scour features are interpreted as reflecting flow patterns within each chamber. Mafic synplutonic dykes (of quartz diorite and tonalite) intruded the tonalites and were broken up to form extensive inclusion trains. Dyke‐tonalite relations are interpreted as showing that (1) magma adjacent to the pluton wall had considerable yield strength, (2) magmatic flow adjacent to pluton walls was capable of moving material some distance (up to kilometers) to form the inclusion trains, and (3) the dykes represent conduits through which a considerable amount of liquid was added to the inflating magma chambers. Mineral compositions throughout the major plutons are relatively uniform. Mean plagioclase composition ranges from An40 in the most mafic tonalites to An30 in the most felsic granodiorites; the total microprobe‐observed range is An44 to An25 (and to An47 in a mafic inclusion). Mg/(Mg + Fe + Mn) of biotite and hornblende drop similarly from 0.44 to 0.36 reflecting changes in rock Mg/(Mg+Fe). The sole oxide mineral is almost pure ilmenite. The limited ranges of mineral compositions imply considerable stability of physico‐chemical conditions during the crystallization of each pluton. The field, petrographic, and mineral chemical data are interpreted in terms of an intermittently recharged continuously solidifying magma chamber.

Observations on the geology of the northern part of North-East Land (Spitsbergen)

Summary The paper describes the geology of the northern part of North-East Land, between 80° 30' N. and 80° N. and from 20° E. to 28° E., and refers also to rock outcrops on the east coast. The greater part of the interior is covered with land-ice. Oskar Kulling described the isoclinally folded succession of Hecla Hoek sediments in the north-west region, in which he found Lower Palaeozoic brachiopods near the top and a tillite below them, regarded as Eo-Cambrian. The subjacent beds are referred to the Pre-Cambrian and the succession known so far is more than 4000 metres thick; there are no strong unconformities. There was, therefore, Pre-Cambrian-Lower Palaeozoic continuity of deposition in the arctic part of the " Caledonian geosyncline ". The Hecla Hoek sediments are traced eastward in a pitching syncline between two anticlines, and the lowest divisions are identified. Fine-bedded shales and some other beds alternate with arenaceous rocks laid down in shallow water : some of the latter, low in the succession, bear ripple-marks and problematic impressions, some possibly of organic origin. Some of the shales display rhythmic bedding. The sediments as a whole accumulated in a neritic zone of a subsiding sea-floor, and the relative altitude of the land varied from time to time. The geosynclinal nature of the sea is evident, especially in the mountains of western Spitsbergen, built from its deposits at some time during the Lower Palaeozoic, and it is thought that the continental zone lay in the east, with its Archaean basement. In the mountains of Spitsbergen some rock-groups have arisen from the regional metamorphism of Hecla Hoek sediments, and they have been traced one into the other, notably by Fleming and Edmonds in Ny Friesland (North-East Spitsbergen). Granitization has also been invoked by Orvin and others. In North-East Land the lowest Hecla Hoek sediments are substantially without metamorphic grade. Beneath them lies a crystalline complex of schists and gneisses invaded by grey granites here regarded as granites sensu stricto , to be distinguished from pink-red granites—here recognized as felsic granodiorites—which penetrate not only the schists, gneisses and grey granites but at least the lower part of the Hecla Hoek sediments. The granodiorites are certainly pre-Upper Carboniferous; diorites may be associated with them, in addition to pegmatites and quartz veins. A broad classification and sequence of the igneous and metamorphic rocks is established. Thus it seems that in North-East Land the unaltered sediments are unconformable on the crystalline complex, from which rocks of low-grade metamorphism had previously been removed : the underlying rocks are, then, a core of greater age. An analogous core might have been present in Ny Friesland and have induced by its resistance to the stress of Lower Palaeozoic mountain-building the progressive grade of regional metamorphism in the adjacent Hecla Hoek sediments. There may thus be some comparison with structures in East Greenland. Obviously, not all metamorphic rocks in western Spitsbergen are Archaean, but consideration should be given to some dynamic function of already modified Archaean rocks in the Lower Palaeozoic (" Caledonian ") fold systems.