Amounts Of Volcanic Rocks Research Articles

Distinctive features of Late Palaeozoic massive sulphide deposits in South China

More than 20 sediment-hosted massive sulphide deposits occur in Late Palaeozoic basins in South China. These deposits are accompanied by a certain amount of volcanic rocks in the host sequence and are economically important for their Cu, Pb, Zn, Au and Ag reserves. The deposits and their host strata were commonly intruded by Mesozoic granitoids. Remobilisation of sedimentary ores and magmatic hydrothermal overprinting processes resulted in the coexistence of massive sulphides with vein-, skarn- and porphyry-type orebodies in the same region or within a single deposit. The ore-containing basins occur in different tectonic settings. The Lower Yangtze basin occurs on a passive continental margin, where the deposits are high in Cu and Au with minor Pb and Zn and recoverable Ag, Co and Mo. The ores have a lower concentration of radiogenic lead, and δ 34S values close to zero. Fluid inclusions are highly saline and Na-rich. Fluids and metals of the Lower Yangtze Region are interpreted to have been derived essentially from deep sources including the Precambrian basement. By contrast, basins of the Nanling Region formed in an intracontinental setting developed on a folded Caledonian basement. These deposits are higher in Pb, Zn, Sn and W, as well as Cu, with recoverable Ag, Sb, Hg, U, Bi, Tl and Mo. The ores are characterised by a higher concentration of radiogenic lead and a wide variation of δ 34S composition. Fluid inclusions have lower salinities and higher K +/Na + ratios. Fluids are considered to have been sourced substantially from seawater by convection. Metals for the Nanling deposits were essentially derived from the Caledonian basement by leaching. The contrast in ore composition between these two regions appears to have been controlled by differences in basement composition of the ore-forming basins.

Geology of the Inyo Mountains Volcanic Complex: Implications for Jurassic paleogeography of the Sierran magmatic arc in eastern California

An ∼3.1-km-thick volcanic complex exposed in the southern Inyo Mountains, east-central California, records Jurassic subaerial depositional environments along the east flank of the Sierran arc. This complex, which we name the Inyo Mountains Volcanic Complex, is subdivided into lower, middle, and upper stratigraphic intervals. The 200–580-m-thick lower interval comprises predominantly epiclastic strata deposited on alluvial fans and adjacent river flood plains that were inclined northeast. Mafic lava flows and rare reworked tuff in this interval record the onset of Jurassic(?) volcanism in this part of the arc. The 300–700-m-thick middle interval is composed predominantly of intermediate to silicic lava flows and tuffs representing a major episode of volcanism ending at ca. 169 Ma that is contemporaneous with emplacement of numerous plutons in the region. The >2260-m-thick upper interval is composed of epiclastic strata with minor intercalations of volcanic rock. Most of this interval accumulated on low-gradient flood plains that hosted evaporative lakes and that were episodically invaded by alluvial fan complexes. Three new U-Pb age determinations constrain the lower half of the upper interval to have been deposited during the interval from ca. 169 Ma to 150 Ma. The uppermost part of the complex remains undated but probably accumulated prior to 140 Ma. The Inyo Mountains Volcanic Complex is part of a belt of volcanic complexes that are the easternmost preserved Jurassic complexes of the Sierran arc. These complexes share sufficient similarities to suggest that they represent a distinctive arc-flank depositional province significantly different from that represented by coeval volcanic complexes preserved in roof pendants farther west, closer to the magmatic axis of the arc. Similarities among arc-flank complexes include predominantly to exclusively subaerial settings, substantial (>30%) portions of epiclastic strata, and existence at times of north- to northeast-inclined paleoslopes. We infer on the basis of the varying types and amounts of volcanic rocks that whereas most complexes in the arc-flank province were rarely if ever proximal to major eruptive centers, complexes in two areas (White Mountains and eastern Mojave Desert) were at times located in or adjacent to such centers. These differences lead us to speculate that the east flank of the Jurassic arc consisted of eastward-projecting volcanic salients separated by arc recesses—typified by the Inyo Mountains area—in which epiclastic deposition was dominant.

ON THE VOLCANIC ROCKS IN THE JAPANESE PALEOZOIC, THIRD REPORT, PERMIAN

Permian deposits widely developing in the southern Kitakami mountains have been divided into the Sakamotosawa series, the Kanokura series and the Toyama subsystem in ascending order. Volcanic rocks and their associated pyroclastic sediments are rather rare in the Permain deposits of those mountains, however, there exist thin liparitic or dacitic tuffs and “schalstein” in the Sakamotosawa series ranging from the Pseudoschwagerina to the lower half of the Parafusulina zone in age. Except for the Sakamotosawa series, volcanics and their pyroclastics have never been found in the Permian systems in that area The Kanokura series is further diyided into, in ascending order, the Kattisawa and Iwaisaki stages. Lepidolina multiseptata horizon of the Iwaisakistage may be possibly correlative to the middle to upper Neoschwagerina zone, while the Lepidolina gigantea horizon of the same stage may be nearly equivalent to the Yabeina globosa zone. Outside the Kitakami mountains, a large amount of volcanic rocks and their...