Potassic Volcanic Rocks Research Articles

Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline

The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and Nb/Y and low Zr/Nb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group II rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher Zr/Nb, and lower Nb/Y. Group III rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and II rocks, and contain elevated Ba contents. Groups I–III are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and II basalts. Groups I–III are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

The shoshonite porphyry Cu-Au association in the Goonumbla District, N.S.W., Australia

The Goonumbla porphyry copper-gold deposit in N.S.W., Australia, is hosted by late Ordovician (439.2 ± 1.2 Ma)shoshonitic igneous rocks. In terms of their petrography, the rocks vary from andesitic to dacitic lavas and tuffs which are partly intruded by monzonite stocks; they are characterized by high and variable Al2O3 (13.4–19.9 wt%), very high K2O values (up to 6.8 wt%), and high K2O/Na2O ratios (0.58–1.48), which are typical for the shoshonite association. The rocks also have enriched LILE concentrations (Ba up to 1200 ppm, Sr up to 1350 ppm), low HFSE (TiO2 < 0.67 wt%, Zr < 125 ppm, Nb < 10 ppm, Hf < 3.4 ppm), and very low LREE (La < 22.4 ppm, Ce < 31 ppm), which are typical for potassic volcanic rocks formed in alate oceanic-arc setting. Mineral chemistry of selected magmatic mica and apatite phenocrysts from host rocks reveals relatively high SrO and BaO contents (micas: ∼ 0.15 wt% and up to 0.28 wt%, respectively; apatites: up to 0.28 wt% and 0.19 wt%, respectively) and very high halogen concentrations. Micas are characterized by up to 3.9 wt% F and 0.14 wt% Cl, whereas apatites have up to 3.6 wt% F and 0.68 wt% Cl. These very high halogen contents compared to those from barren intrusions imply that the shoshonitic magmatism was the source of mineralization. Copper-gold mineralization consists mainly of bornite, chalcopyrite, chalcocite and minor pyrite and tetrahedrite. Native gold occurs mainly as minute grains within silicates of the host rocks, and more rarely as fine inclusions in the sulphides. Mineralization is accompanied by wallrock alteration comprising a spatially restricted potassic type and a regional propylitic alteration type. Thus, the porphyry copper-gold deposit in the Goonumbla district can be viewed as an additional example of a worldwide association between potassic/shoshonitic magmatism and base- and precious-metal mineralization. More specifically, it appears to be the oldest recorded example of a shoshontie-associated porphyry Cu-Au deposit from a late oceanic-arc setting, a possible modern analogue being Ladolam at Lihir Island, Papua New Guinea

Geochemical discrimination between shoshonitic and potassic volcanic rocks in different tectonic settings: A pilot study

Potassic volcanic rocks (PVRs), such as shoshonites, are an important constituent of young volcanic suites in five main tectonic settings:Continental arcs (e.g. Andes);Post-collisional arcs (e.g. Alps);Oceanic arcs, subdivisible into initial (e.g. Marianas) andlate (e.g. Sunda Arc);and Within-Plate (e.g. inland N. American Cordillera). Since some PVRs are associated with world-class gold and base metal deposits, and can be important in tectonic reconstruction of ancient terranes, new diagrams based on immobile elements (Al, Ti, P, Y, Zr, Nb, La, Ce, Hf) are presented for discriminating the tectonic setting of PVRs from their geochemistry; spidergrams and previously published discrimination diagrams based on `normal' basalts proved unsuitable. Diagrams were derived, in part via multigroup linear discriminant analysis, using a database of fresh, primitive, Cenozoic PVRs (497 analyses), itself carefully screened from a newly erected database of 2,222 PVRs of all ages and compositions. To maximise discrimination, the diagrams are hierarchical: the first applies to all five settings and removes the most distinctive group (Within-Plate), while the remainder target more subtle distinctions. Overall, concentrations of most LILE, LREE and HFSE, and ratios such as Zr/Al and Ti/Al, tend to be highest in Within-Plate PVRs, intermediate in Continental and Postcollisional Are PVRs, and lowest in Oceanic Arc PVRs. In detail, Postcollisional are distinguished from Continental Arc PVRs by higher Ce/P, and lower Zr/Ce, Ti/Nb ratios of the latter; late from initial Oceanic arc PVRs by higher Hf, La and P of the former. These geochemical differences have a firm theoretical foundation in petrogenetic processes during magma emplacement and/or in source heterogeneities related to subduction or mantle metasomatism.

EM1 signature from within the post-Archaean subcontinental lithospheric mantle: Isotopic evidence from the potassic volcanic rocks in NE China.

Combined geological and geochemical evidence suggests that the age of the basement through which the Wudalianchi, Erkeshan, and Keluo (WEK) volcanic rocks in NE China were erupted is post-Archaean. Sr, Nd, and Pb isotope analyses demonstrate that the Miocene to Recent WEK potassic volcanic rocks were derived from an enriched mantle source which had been metasomatised during the Proterozoic. The primary potassic magmas are the products of partial melting of either this enriched mantle source or, more probably, a mixture of this enriched source with an OIB source. In either case, a post-Archaean subcontinental lithospheric mantle with an EM1 signature has made a significant contribution to the WEK potassic rocks.

Sr, Nd, Pb Isotope geochemistry and magma evolution of the potassic volcanic rocks, Wudalianchi, Northeast China

Wudalianchi volcanic rocks are the most typical Cenozoic potassic volcanic rocks in eastern China. Compositional comparisons between whole rocks and glasses of various occurrences indicate that the magma tends to become rich in silica and alkalis as a result of crystal differentiation in the course of evolution. They are unique in isotopic composition with more radiogenic Sr but less radiogenic Pb.87Sr /86 Sr is higher and143Nd/144Nd is lower than the undifferentiated global values. In comparison to continental potash volcanic rocks, Pb isotopes are apparently lower. These various threads of evidence indicate that the rocks were derived from a primary enriched mantle which had not been subjected to reworking and shows no sign of incorporation of crustal material. The correlation between Pb and Sr suggests the regional heterogeneity in the upper mantle in terms of chemical composition.

On the orgin of the Italian potassic magmas: 1. A discussion contribution

In their controversy about the origin of high and variable 18 O 16 O ratios in the Central Italian potassic volcanic rocks B. Turi, H.P. Taylor and coworkers as well as P.M. Holm and N.C. Munksgaard do not discuss pertinent evidence which clearly favours the latter group's view about a continental-crustal-derived component in the magma sources. However, neither Holm and Munksgaard nor others advocating a similar model have so far appreciated an important implication from this conclusion: Primitive magma compositions are not constrained to low silica, high Mg and Ca compositions. Volume relationships suggest that many voluminous high-silica ignimbrite eruptions may be primitive in the sense of non-evolved. The role of a subduction zone in the genesis of these potassic magmas is re-assessed. Results of a previously published new metasomatic model are discussed that can reconcile apparently conflicting evidence for subduction zone or rift environment for these alkaline magmas. The model suggests release of fluids from a hotspot in the astenosphere and their infiltrating and metasomatising a succession of subducted sediments and overlying mantle. Subsequent rifting causes partial melting of the metasomatised domains and alkaline volcanism. This model should not be uncritically generalised but neither must isotopic signatures of high-potassium rocks be considered as necessarily mantle-indigenous.

Age and extent of potassic volcanism on the Colorado Plateau

Six K-Ar ages on phlogopite from minettes in and near the Buell Park diatreme establish that minette and coeval kimberlitic volcanism occurred 25 m.y. ago. Two altered phlogopites yield younger ages, while on phlogopite sample with an age of 34 m.y. may contain excess argon. Published apatite fission-track ages for inclusions in the kimberlitic tuff at Buell Park are too old, perhaps due to partial retention of pre-eruption tracks. Published ages for other minette localities on the Colorado Plateau are insufficient to precisely define the duration of volcanism, but some eruptions may be as old as 30 m.y. Potassic volcanic rocks similar to the felsic minettes at Buell Park erupted at about the same time at Chino Valley, Arizona, in the transition zone of the Colorado Plateau. The correlation between the two localities of unusual potassic rocks is evidence that these rocks may reflect mantle conditions associated with the stability of the Plateau, and that these conditions contrasted with those in adjacent tectonic provinces at least as early as 25 m.y. ago.

Les types paragénétiques des amphiboles sodiques dans les roches magmatiques

In sodic amphiboles of igneous origin, Fe3+ ions are more abundant than Al3+ in the occupancy of the octahedral sites, even in varieties showing a great substitution of Al for Si in tetrahedral sites. On the other hand the Mg/Fe2+ ratio ranges are much more extended. The magnesium-rich alkali amphiboles are essentially richteritic : (1) potassic richterites in some ultramafic xenoliths of kimberlites and in highly potassic volcanic rocks (orenditic lamproites) ; (2) richterite-magnesioarfvedsonite solid solutions in carbonatites and related rocks. The paragenetic types of iron-rich or intermediate alkali amphiboles follow three main fractionation trends. In the series that follow a miaskitic evolution, the amphibole changes hastingsite to taramite, where NaFe3+ mainly substitutes for CaFe2+. The agpaitic alkaline series show the sequence kataphorite arfvedsonite → riebeckite-arfvedsonite, corresponding to NaSi → CaAl and F- OH-substitutions ; the Ca/Al ratio hardly deviates from one in all the amphiboles of the sequence. The amphiboles from supersatured alkaline series follow a fractionation trend richterite-katophorite → arfvedsonite → riebeckite-arfvedsonite, governed by the two main substitutions NaR3+ → CaR2+ and NaSi → CaAl. The Ca/Al ratio in this case is around two. Therefore SiO2 activity, as well as temperature, total pressure and oxygen fugacity, plays a major role in controlling the solubility of aluminium in sodic amphiboles, as well as in hornblendes.

Oxygen isotope studies of potassic volcanic rocks of the Roman Province, Central Italy

The 18O/16O ratios of rocks and coexisting minerals were measured for 93 samples of leucite-bearing lavas, pyroclastics, and related volcanic rocks from the Quaternary Roman Co-Magmatic Province, Italy. The δ18O values were found to generally increase northward in the sequence: Ischia (5.8 to 7.0); Somma-Vesuvius and Phlegrean Fields (7.3 to 8.3); Alban Hills (7.3 to 8.7); M. Sabatini (7.3 to 9.7); Vico Volcano (7.4 to 10.2); and M. Vulsini (8.1 to 11.7). The northward increase in δ18O parallels a similar increase in 87Sr/86Sr, and these data indicate that the Roman magmas have interacted strongly with high-18O continental crust. A marked increase in δ18O occurs just north of Rome where the Roman Province begins to overlap the calc-alkaline, oversaturated Tuscan Magmatic Province. Therefore, some of the observed 18O/16O and 87Sr/86Sr enrichments in the Roman magmas may have been facilitated by direct mixing with the high-18O Tuscan magmas or because the high-18O country rocks underwent widespread heating during a couple of million years of Tuscan igneous activity. Although many of the Roman magmas underwent fractional crystallization without appreciable change in δ18O, contamination has produced a correlation between δ18O and SiO2 content at several of the volcanic centers; thus the trachytes are typically higher in 18O than the undersaturated rocks. The major features of the oxygen isotope data can be explained in terms of a simple two-component mixing model in which one end-member was a primary, strongly undersaturated magma derived from the upper mantle, with δ18O≈+6, 87Sr/86Sr≈0.704 to 0.705, and SiO2<44wt.%. However, none of the analyzed samples have these values, as they have all been contaminated to some extent. The closest approach is found in some of the leucitepyroxenite ejecta from the Alban Hills. The second end-member, derived from the continental crust, had a variable composition with δ18O≈+12 to +20, 87Sr/86Sr≧0.712 to 0.720, and SiO2≧65wt.%, and it mixed in much greater proportions in the volcanoes north of Rome than in those of the Alban Hills or the Naples area. The widespread interactions between the Roman magmas and the continental crust are probably due to (1) the fact that such low-SiO2 magmas always have a very strong tendency to interact with quartz-bearing rocks of the continental crust, and (2) in Italy, these magmas were emplaced into a tectonically very active area containing poorly consolidated sedimentary rocks, and in the northern part of the belt there had been a prior history of extensive calc-alkaline igneous activity.

Kimberlite and Related Rocks: A Critical Reappraisal

A discussion of the most important petrographic features of kimberlite is presented and a new definition of kimberlite is proposed. The discussion emphasizes those features of kimberlite which are of petrogenetic significance and points out that alnoitic and lamprophyric rocks associated with alkaline rock-carbonatite complexes are not true kimberlites. Consideration of the proposed relation of kimberlites to mica peridotites, carbonatites, basalts, potassic volcanic rocks, and melilitites reveals that kimberlites are not simply or closely related genetically to any of these rocks.

Lineages and Fractionation Trends in Undersaturated Volcanic Rocks from the East Otago Volcanic Province (New Zealand) and Related Rocks

The petrology of the East Otago Volcanic Province (late Miocene), including the Dunedin Complex, is dominated by undersaturated sodic lavas and shallow intmsives whose compositions range from alkali basalt, basanite, and nephelinite to trachyte and phonolite. A range of relatively potassic volcanic rocks is also developed in the Province. The degree of undersaturation and ratios such as (FeO+Fe2O3):MgO and K2O:Na2O vary widely in rocks with comparable differentiation indices. Chemical and mineralogical data on whole rock-glass pairs and variants of shallow intrusives in the Province, assessed in conjunction with similar data from other volcanic provinces, demonstrate the production of trachytic liquids from alkali basaltic parents and of phonolitic liquids from basanitic parents. Fractionation trends in both sodic and potassic series, defined by variation in differentiation index and normative nepheline, indicate that the degree of undersaturation of the derivative salic liquids is dependent on that of the parent; the slope of the fractionation trend lines is influenced by additional factors, including Po2. The unequivocal production of phonolite from trachyte in the East Otago Province has not been demonstrated. The mafic variants of theralites from Waihola and a basanite-pegmatoid association from Omimi are similar in composition, but the most salic differentiates from these differentiation sequences display pronounced differences in their KO:Na2O ratios, tending towards leuco-theralitic (cf. lugarite) and malignitic compositions respectively. A multicomponent spectrum of fractionation lineages is suggested for individual differentiated bodies and for lava series. Sodic lineages include: (I) alkali basalt–hawaiite–mugearite–benmoreite–trachyte; (2) a more undersaturated series from basanite through nepheline hawaiite, nepheline mugearite, and nepheline benmoreite, to phonolite; (3) a nephelinite series, more restricted in compositional range. The relatively high Fe: Mg ratios characterizing the mugearitic variants are sometimes exhibited by members of the more potassic series in which, however, Fe:Mg ratios tend to be characteristically lower. This feature is correlated with the frequent occurrence of kaersutitic amphibole or its resorption products, indicative of intratelluric fractionation under relatively hydrous conditions and probably relatively high Po2. Certain nepheline trachyandesites from East Otago are described in detail. Trachy-basalt-trachyandesite-tristanite-trachyte and sanidine basanite-nepheline trachyandesite-nepheline tristanite-phonolite lineages are also proposed, and the nomenclature of these more series discussed.

Absarokite-Shoshonite-Banakite Series

Absarokite-Shoshonite-Banakite Series