Felsic Suite Research Articles

Fractional crystallization of the basaltic suite of Usu volcano, southwest Hokkaido, Japan, and its relationships with the associated felsic suite

REE concentrations were determined for four basalts, two basaltic andesites, three dacites and one rhyolite from Usu volcano, SW Hokkaido, Japan, and are reported with their mineralogical data. The basic rocks are tholeiitic, whereas the felsic rocks are calc-alkaline. No intermediate andesite has been erupted. The REE abundances of the basic rocks increase with increasing FeO ∗ MgO ratios and the mineral compositions are consistent with low-pressure fractionation. The variation of the REE concentrations can also be explained by low-pressure fractionation. In contrast, the chemical features of the felsic rocks can be hardly accounted for by low- or high-pressure fractionation; the felsic rocks are more depleted in HREE than the most differentiated basaltic andesite. A model calculation is suggestive of their origin: the felsic rocks can be produced by remelting of an intrusive body of tholeiitic basic magma.

Petrogenesis of a Late Precambrian (575?600 Ma) bimodal suite in Northeast Africa

Late Precambrian crustal evolution in the North Eastern Desert of Egypt occurred in a strongly extensional tectonic environment and was accompanied by abundant bimodal igneous activity. The extrusive and intrusive expressions of this magmatism, known as the Dokhan Volcanics and Pink Granites, respectively, were studied in detail from two areas. The Dokhan Volcanics and associated feeder dikes consist of a “mafic” suite dominated by andesites (∼60% SiO2) and smaller volumes of basalt and a “felsic” suite composed of rhyolite tuffs, ignimbrites and hypabyssal intrusions (∼72–78% SiO2). The rocks of the mafic suite display calc-alkaline trends on an AFM diagram but are enriched in incompatibles such as TiO2, P2O5, K2O, Rb, Sr, Ba, Zr, Y, Nb, and LREE. Rare earth element patterns are steep, with (Ce/Yb)n = 7.7 to 16.8. They contain moderate Ni (60 ppm) and Cr (95 ppm), indicating limited low-P fractionation. The melts of the mafic suite are interpreted to have formed either by ≤25% batch melting of eclogite or by ∼10% batch melting of LREE-enriched garnet lherzolite. The rocks of the felsic suite include Dokhan rhyolites and the epizonal Pink Granites. These contain 72–78% SiO2, are metaluminous and peraluminous, and have the high K2O/Na2O and FeO*/(FeO*+MgO) characteristic of post-tectonic, “A-type” granites. They are moderately enriched in incompatible elements, but their REE patterns overlap with those of the mafic suite, from which they can be distinguished by deep europium anomalies (Eu/Eu*=0.08–0.64) and flat HREE patterns=((Yb/Er)n=0.90–1.16). They share with the rocks of the mafic suite isotopic characteristics of depleted mantle, precluding anatexis of much older continental crust. The europium anomalies covary with Sr contents and indicate that plagioclase control was important, while the flat HREE patterns preclude residual garnet in the source. Hence the felsic melts could not have formed by anatexis of garnet-bearing mafic lower crust. Such melts could have formed by anatexis of amphibolite-facies crust, an interpretation which is not favored because the melts are not saturated with P2O5. Alternatively, the felsic melts may have formed via low-P fractional crystallization of the mafic melts, with about 2/3 removal of mostly plagioclase and amphibole along with minor apatite and zircon. This may have been accompanied in the latest stages of magmatic evolution by liquid-state fractionation such as thermo-gravitational diffusion or halide complexing.

隠岐島後火山岩類のストロンチウム同位体比

The isotopic composition of strontium and the abundances of rubidium and strontium in volcanic rocks from Dogo of the Oki Islands and Takashima in the northwest Kyushu, West Japan, and Ulrungdo of Korea, have been determined. The rubidium and strontium contents for alkaline basalts range from 27.6 to 51.2 ppm and 444 to 723 ppm, and 148 to 208 ppm and 3.7 to 205 ppm for intermediate to felsic suites, respectively. The alkaline basalts are divided into two groups with 87Sr/86Sr ratios of the restricted ranges of 0.70481-0.70496 and 0.70540-0.70575, respectively. However, the 87Sr/86Sr ratios of intermediate to felsic rocks of Dogo are relatively high and variable ranging from 0.70706 to 0.71019, which probably reflect the contamination and/or produced by body or partial melting of the basement rocks in this area without crustal assimilation of basaltic magma. In addition, the lead isotopic results indicate that the melting of Precambrian basement rocks possibly yields less radiogenic lead. In the southwest Japan, the 87Sr/86Sr ratios of Cenozoic basaltic rocks are clearly different between the San-in and the northwest Kyushu regions, which includes Jeju island, The higher 87Sr/86Sr ratios of basalts from the San-in region than that of basalts from the northwest Kyushu region also reflect the different properties of the upper mantle, which means there is regional heterogeneity of Sr isotopic ratios under the southwest Japan arcs. Furthermore, the relatively high and variable 87Sr/86Sr ratios of volcanic rocks are particularly concentrated in the southwestern Japan arcs which has probably more continental properties than the northeastern Japan arcs.

Provenance and Depositional Mode of Upper Cretaceous Chatsworth Formation Conglomerates, Simi Hills, California: ABSTRACT

Conglomerates of the Upper Cretaceous Chatsworth Formation occur as lenses of concentrated clasts in channels and as clasts dispersed at the base of thick, coarse-grained, graded sandstone beds. The matrix of the conglomerates consists of grains ranging from silt to granule size (4 mm) and comprises between 20 and 78% of any one conglomerate unit. The matrix composition ranges from 30 to 80% quartz, 25 to 60% feldspar, and 5 to 20% lithic fragments, with accessory biotite up to 5%. The conglomerate clast population is composed principally of clasts in the pebble size range. Five distinct rock types are recognized within the conglomerate clast population: blue-black argillite, 4 to 20%; felsic volcanites, 4 to 28%; felsic plutonites, 16 to 24%; arkosic sandstones and silts one, 0 to 12%; and a group of genetically related quartz-rich clasts, 17 to 46%. The quartz-rich clasts include sandstones and siltstones with continuous textural gradation from well-preserved sandstone through partially recrystallized sandstone with sutured grains, into metamorphosed, foliated quartz sandstone and quartz schist types. In addition, a conglomerate unit may contain between 0 and 14% authigenic rip-up clasts. The Chatsworth Formation, as a whole, is recognized to be a deep-sea fan complex upon which the primary depositional agent for sand was turbidity currents. Lenses of concentrated pebble conglomerates originated as debris flow, whereas beds of dispersed pebble clasts are of turbidity current origin. Paleocurrent data and the conglomerate clast composition for the Chatsworth Formation indicate that its detritus was derived from a source terrane to the south of the Simi Hills. The Santa Monica Mountains basement complex contains a large mass of argillite and felsic plutonite, but contains no felsic volcanite or quartz-rich suite of rocks. The basement in the northern Peninsular Ranges includes representatives of the principal rock types recognized in the clasts of the Chatsworth Formation conglomerates and, therefore, it is the best possible choice for the provenance. Extensive Franciscan terrane also lies south of the Chatsworth conglomerates, but no Franciscan detritus is recognized in the Chatsworth Formation. End_of_Article - Last_Page 1686------------

Reply: Geochemical comparison of Woodlawn and Mount Painter Acid Volcanics, Southeastern Australia

Abstract Major‐ and trace‐element chemistry (including rare‐earth elements), total‐rock Rb‐Sr and U‐Pb and zircon U‐Pb data are used in an attempt to distinguish between two essentially coeval, felsic volcanic suites: the predominantly submarine Woodlawn suite which is associated with massive Cu‐Pb‐Zn sulphide mineralization and the terrestrial Mt Painter suite, with minor vein‐type mineralization. The Woodlawn samples are the unmineralized equivalents of the volcanics in the immediate ore environment. Alteration perturbs some of the major‐ and trace‐element chemistry, particularly Ca and alkalis, thereby precluding their usefulness. REE patterns exhibit a significant light to heavy rare‐earth enrichment with an average La/Yb of 12 in the Mt Painter volcanics compared with 5.6 in the Woodlawn volcanics. Both suites have a marked negative Eu anomaly, with that of the Woodlawn samples more pronounced (‐45.5) than in the Mt Painter volcanics (‐29.2). A hydrothermally‐altered sample from Woodlawn has apparent...

Age of the Paresis Complex, South-West Africa

MESOZOIC igneous activity in South-West Africa is shown as lava plateaux and dyke swarms of basaltic composition and as a group of central volcanic complexes representing a highly variable range of rocks1. The Paresis complex (20° 15′ S., 15° 42′ E.) is one of the latter and is made up of a consanguineous suite of syenites, bostonites, rhyolites and associated foyaitic rocks, as well as minor basalts and gabbros. From structural and geochemical evidence Siedner2,3 concluded that the basaltic rocks of Paresis represent locally preserved remnants of the regional Stormberg volcanics coeval with, but genetically un related to, the central felsic suite. The age of Paresis is thus significant because it dates both the complex and the regional volcanism. The only other published chronological datum on Mesozoic igneous centres in South-West Africa is a potassium–argon age of 190±8×106 yr on biotite from the Klein Spitzkop granite4.