Volcano-plutonic Association Research Articles

Sources of basaltoid magmas in rift settings of an active continental margin: Example from the bimodal association of the Noen and Tost ranges of the Late Paleozoic Gobi-Tien Shan rift zone, southern Mongolia

The bimodal volcanoplutonic (basalt-peralkaline rhyolite with peralkaline granites) association of the Noen and Tost ranges was formed 318 Ma ago in the Gobi-Tien Shan rift zone of the Late Paleozoic-Early Mesozoic central Asian rift system, the development of which was related to the movement of the continental lithosphere over a mantle hot spot. A specific feature of the Late Paleozoic rifting was that it occurred within the Middle-Late Paleozoic active continental margin of the northern Asian paleocontinent. Continental margin magmatism was followed after a short time delay by the magmatism of the Gobi-Tien Shan rift zone, which was located directly in the margin of the paleocontinent. Such a geodynamic setting of the rift zone was reflected in the geochemical characteristics of rift-related rocks. The distribution of major elements and compatible trace elements in the rift-related basic and intermediate rocks corresponds to a crystallization differentiation series. The distribution of incompatible trace elements suggests contributions from several sources. This is also supported by the heterogeneity of Sr and Nd isotopic compositions of the rift-related basaltoids: eNd(T) ranges from 4.4 to 6.7, and (87Sr/86Sr)0, from 0.70360 to 0.70427. The geochemical characteristics of the rift-related basaltoids of the Noen and Tost ranges are not typical of rift settings (negative anomalies in Nb and Ta and positive anomalies in K and Pb) and suggest a significant role of the rocks of a metasomatized mantle wedge in their source. In addition, there are high-titanium rocks among the rift-related basaltoids, whose geochemical characteristics approach those of the basalts of mid-ocean ridges and ocean islands. This allowed us to conclude that the compositional variations of the rift-related basaltoids of the Noen and Tost ranges were controlled by three magma sources: the enriched mantle, depleted mantle (high-titanium basaltoids), and metasomatized mantle wedge (medium-Ti basaltoids). The medium-titanium basaltoids were formed in equilibrium with spinel peridotites, whereas the high-titanium magmas were formed at deeper levels both in the spinel and garnet zones. It terms of geodynamics, the occurrence of three sources of the rift-related basaltoids of the Noen and Tost ranges was related to the ascent of a mantle plume with enriched geochemical characteristics beneath a continental margin, where its influence caused melting in the overlying depleted mantle and the metasomatized mantle wedge. The formation of rift-related andesites in the Noen and Tost ranges was explained by the contamination of mantle-derived basaltoid melts with sialic (mainly sedimentary) continental crustal materials or the assimilation of anatectic granitoid melts.

Faciologia da Associação Vulcano-Plutônica Taquarembó, Cambriano do Escudo Sul-rio-grandense, RS - Brasil

The Taquarembo Plateau, situated in the southwestern part of Sul-rio-grandense shield, is part of Camaquã Basin, that was formed at the end of the post-collisional stage in Brasiliano/Pan-Africa event, Neoproterozoic in age. This plateau is composed of volcanic and shallow intrusions, related to a silica-saturated alkaline series, ranging from alkaline metaluminous basalt to comendiitic flows, known as Taquarembo Volcano-Plutonic Association. The faciological identification leads to the reconstruction of the volcanic pile, formed from the base to the top by: particulated flows (ignimbrites and reo-ignimbrites); ressedimented syneruptive deposits; surge and fall horizons, closely associated with alkaline metaluminous lava flows (basalts, mugearites, rhyolites); and peralkaline lava flows (comendiites), and shallow intrusions of quarz monzonite and syenite that crosscut the volcanic pile. The facies identification suggests a highly explosive volcanic regime, under sub-aerial condition, in a close association with the feeder necks.

Olympic Dam ore genesis; a fluid-mixing model

Numerical modeling shows that fluid mixing was probably the dominant ore-forming process in the Olympic Dam Cu-U-Au deposit. The deposit is hosted by the Olympic Dam Breccia Complex, which is within the Roxby Downs Granite. The granite and the breccia complex are coeval with the Gawler Range Volcanics-Hiltaba Suite volcano-plutonic association, and all are products of a major Middle Proterozoic thermal event on the Gawler craton, South Australia. In the Olympic Dam Breccia Complex, early magnetite (+ or - hematite), chlorite, sericite, siderite, and minor pyrite, chalcopyrite, and uraninite mineralization (association I) is extensively overprinted by hematite, sericite, chalcocite, bornite, pitchblende, barite, fluorite, and chlorite (association II). The paragenetically latest major mineral association consists of hematite, or hematite + granular quartz + or - barite (association III). All three mineral associations locally display complex overlapping and indistinct boundaries. Rock relations, breccia textures, ore mineral textures, and mineral parageneses all provide evidence of repetitive brecciation and mineralization events, indicating that ore genesis was complex and multistage. The mineral associations and their zonation, combined with fluid inclusion and isotopic data, indicate that mixing of a hotter magmatic or deeply circulated meteoric water and a cooler meteoric water was probably responsible for ore genesis. Ore mineral textures, the abundance of hematite, and the close association of sulfides and pitchblende with hematite, all suggest that ore precipitation was caused by reduction of sulfate coupled with oxidation of iron during mixing. Fluid inclusion salinities, and the absence of evidence for boiling during precipitation of associations II and III, are consistent with the cooler meteoric water having originated as saline ground water or playa lake water within the volcanic succession inferred to have been extensively developed above and in the vicinity of the Olympic Dam Breccia Complex. U-Pb geochronology and textural studies show that mineralization accompanied brecciation, dike intrusion, and regional mafic and felsic volcanism on the Gawler craton...

Th [sbnd]Pb versus U [sbnd]Pb isotope systematics in allanite from co-genetic rhyolite and granodiorite: Implications for geochronology

U Th Pb isotopic analyses were performed on single allanite crystals from a rhyolite of the Atesina volcanic complex (AVC) and a granodiorite of the Cima d'Asta pluton (CAP) which form the largest calc-alkaline volcano-plutonic association of Late Hercynian age in the South Alpine domain (northern Italy). The best approximation to the time of emplacement of the AVC and CAP is given by essentially coeval 232Th 208Pb ages of 276.3 ± 2.2 Ma (95% confidence limit; n = 2) and 275.5 ± 1.5 Ma ( n = 4) for AVC rhyolite and CAP granodiorite, respectively. Although the AVC/CAP magmatites have been overprinted by a Triassic hydrothermal event, the Th Pb isotopic system in allanite remained virtually undisturbed, whereas the U Pb systems display variable degrees of open-system behaviour. The allanite Th Pb ages are neither affected by initial radioactive disequilibrium nor by the inheritance of radiogenic Pb components, which underlines the importance of the Th Pb isotopic system for accurate dating of Th-enriched minerals. The isotopic results demonstrate that even hydrothermally altered magmatites can successfully be dated by this technique.

A revised age for the granites of the central Somuncura Batholith, North Patagonian Massif

The age of the granites of the La Esperanza region of the Somuncura Batholith, North Patagonian Massif has been revised, based in part on RbSr whole-rock data obtained by reanalyzing samples from a previous study. The new ages are 258 ± 15 Ma for the Prieto Granodiorite and 259 ± 16 Ma for the Donosa Granite, both from the older La Esperanza plutonic complex, and 239 ± 4 Ma for the Calvo Granite, from the younger volcano-plutonic Dos Lomas complex. The initial 87Sr/ 86Sr ratios are all in the range 0.7070–0.7076. The ages probably correspond stratigraphically to Late Permian and Early Triassic for the two complexes, respectively, consistent with traditional geologic interpretation. Together with recently published Triassic ages from the Batholith of Central Patagonia, it is clear that the acidic volcano-plutonic associations of northern Patagonia are very latest Paleozoic and Mesozoic in age. They are not obviously related to terrane collision but are part of a Permo-Triassic acid magmatic province that extends throughout the central Andes and that preceded, or was associated with, the early rifting of Gondwana.

Oriental Moroccan Neogene volcanism and strike-slip faulting

The microtectonic analysis of faults and of volcano-plutonic associations in the Eastern Rif shows that the distribution of a large number of Miocene to Quaternary volcanic emission points is tectonically controlled. Their localisation, parallel to the strike-slip faults orientated N 070–090, N 040–050 and N-S suggests that the volcanic emissions could have occured along tension gashes making an angle of 40–50° to the initial strike-slip plane. The new orientation, of the principal compressional stress to N 040-000 during upper Miocene, reactivates faults striking N 070, N 040–050 and N-S; the volcanic emissions with a NW-SE, NE-SW and N-S directions, occur along gashes developed in this system. During the Plio-Quaternary, the counterclockwise rotation of the main stress to NW-SE, initiate strike-slip movement along the N-S, N 090 and N 120–140 faults; the volcanic emissions take place on ⪡en échelon⪢ tension gashes developed in this system. In this area, subjected since 10 M.a. to a compressional regime, the crustal mega shear zones develop scattered fissures and tension gashes which initiate partial fusion at depth by pressure release. The gaps in volcanic activity may correspond to the changes in the stress field.

Petrological significance of granitic inclusions from Pliocene-early Pleistocene pyroclastic flow deposits in central Hokkaido, Japan.

Mineral and chemical compositions of granitic inclusions in the Pliocene-early Pleistocene rhyolitic pyroclastic flow deposits of central Hokkaido are very similar to those of the host rhyolitic pyroclastic flow deposits, but different from those of the basement Tertiary granites. The Sr isotope ratios of both of the pyroclastic flow deposits and the granitic inclusions range from 0.7032 to 0.7040, which are notably lower than those of the Tertiary granites (0.7050-0.7054) and the metamorphic rocks (0.7044-0.7071) of the Hidaka Belt. Such evidence indicates that the granitic inclusions are not derived from the basement rocks, but are co-magmatic with the rhyolitic pyroclastic flows. The water vapor pressure and temperature of the magma which produced both the rhyolitic pyroclastic flows and the granitic inclusions are estimated to be 1 kb and 600°-750°C. Hence, it may be interpreted that the rhyolitic pyroclastic flows are eruptive equivalents of a shallow granitic pluton. The low Sr ratios suggest that the source material of the volcano-plutonic association may not be ascribed to the older upper crust, but to a certain kind of lower crust or upper mantle material.

Datation 87Rb-87Sr du massif granitique du Mendic et des porphyroïdes a l'est de la Montagne Noire —un exemple de relation entre pluton et volcans

In the East of Montagne Noire (South Massif Central), the granitic Mendic massif and the porphyroids have been dated as 510±20 m.y. with a corresponding initial ratio of 0.706±0.001. Within the limits of the analytical errors, we cannot distinguish between two separate isochrons. The Mendic-porphyroids represents a volcano-plutonic association. The age of 510 m.y. is somewhat younger than the one determined on the orthogneisses of the Central zone (530 m.y.). It can be connected to a post-orogenic magmatism of the cadomian orogenesis. At last the Sr of the minerals of the Mendic massif has been partially remobilised during the hercynian orogeny (285 m. y.).

The problems of the geology of gold

This review gives the latest ideas on various problems of the geology of endogenic deposits of gold; the history of modern knowledge on this subject is closely connected with the general problems of orogenesis, which even in the nineteen-thirties were discussed in the literature by representatives of different groups of petrologists, mineralogists and geochemists. The author singles out some of the most important problems which refer to: the location of gold ore deposits and their coincidence with the structures of the earth's crust; the connection between gold mineralization and volcano-plutonic associations; the causes of relationships between gold and silver; hydrothermal metamorphism and gold mineralization; metasomatosis and the process of ore formation; the role of electric fields in the process of ore formation; and some others. The results of investigations on modern volcanic processes and the products of volcanism are used.

Achievements of modern geological volcanology and its trends 1

The paper quotes concepts on the genesis of basaltic magmas, their possible sources and modes of formation, as well as on the depths of their origin and possible occurrence at depths of 60–150 km in the suboceanic mantle and at depths of 150–250 km in the subcontinental mantle. Magma differentiation taking place in the vent of the volcano usually proceeds from more acid to more basic lavas and only during the Paricutin eruption has a reverse sequence been observed. The problem of ignimbrites has been widely discussed from the point of view of their genesis and terminology, but still remains unsettled. A possibility is indicated for the formation of andesite by an assimilation of the sialic material by a basaltic magma. A wide development is shown of volcano-plutonic associations, mostly derivatives of an acid magma. An intense volcanic activity is taking place on the floor of the world ocean and especially of the Pacific Ocean, where it is associated with suboceanic ridges, with the peripheral geosynclinal belts and, finally, with the basins that represent a sort of platform. The intense volcanicity in the Pacific Ocean, as compared with the volcanism on continents, forces one to assume that its difference is caused by a different structure of the ocean floor and the floor of the continents, and particularly by a small thickness of the crust under the Pacific Ocean. The use of new methods (radiocarbon, paleomagnetic) contributes to the progress of our knowledge about the phenomena of volcanism. A record is made of the formation of nue´es ardentes during eruptions, as well as the formation of basaltic volcanoes, uplift domes (not only by the injection of acid lavas, but also of basic lavas).