Total Alkali Silica Research Articles

Geochemical and isotopic studies of the Cretaceous igneous rocks in the Yeongdong Basin, Korea: Implications for the origin of magmatism in pull-apart basin

The Yeongdong Basin is one of the pull-apart basins in the southwestern part of the Korean Peninsula that has developed during Cretaceous sinistral fault movement. The bimodal igneous activities (basalts and rhyolites) in the basin appear to be closely associated with the development. Here, we discuss the origin of the igneous rocks using chemical and radiogenic isotope data. Basaltic (48.4–52.7 wt% SiO2) and rhyolitic (70.3–70.8 wt% SiO2) rocks have transitional alkalinity in a total alkali-silica diagram. The rhyolitic rocks have relatively high K2O contents (5.2–6.0 wt%) for given SiO2. The basaltic rocks have a distinct negative anomaly of Nb and Ta in a MORB-normalized spider diagram, indicating a significant amount of crustal component in the magma. The basaltic rocks plot generally within the calc-alkaline basalt field in the trace element discrimination diagrams. The eNd(T) values of he basaltic rocks (−13.6 to −14.3) are slightly higher than those of the rhyolitic rocks (−14.7 to −15.2), and the initial Sr isotopic ratios of the former (0.7085–0.7093) are much lower than those of the latter (0.7140–0.7149). Nevertheless, the initial Nd and Sr isotope ratios of the igneous rocks in the Yeongdong Basin are similar to those of the neighboring Cretaceous igneous rocks in the Okcheon belt. The Pb isotope ratios plot within the field of Mesozoic granitoids outside of the Gyeongsang Basin in Pb−Pb correlation diagrams. Since a basaltic magna requires the mantle source, the enriched isotopic signatures and negative Nb anomaly of the basaltic rocks suggest two possibilities for their origin: enriched mantle lithospheric source, or depleted mantle source with significant amount of crustal contamination. The slightly more enriched isotopic signatures of rhyolites suggest that they originated from differentiation of the basaltic magma with some crustal contamination, or from direct partial melting of the lower crust. Although larger exposed volume of the rhyolites than the basals may support the latter possibility, distinct difference in isotopic ratios between the rhyolites and the Precambrian basement rocks favor the former one.

Alkalic intrusive rocks associated with the Mount Kare gold deposit, Papua New Guinea; comparison with the Porgera intrusive complex

The Mount Kare gold deposit is located 18 km southwest of the giant Porgera gold mine (proven and probable reserves of 51.5 million metric tons at 0.23 oz Au/t) in the highlands of Papua New Guinea. Both deposits are spatially associated with late Miocene alkalic intrusive complexes emplaced in Mesozoic-Tertiary shelf sedimentary sequences near the edge of the Australasian plate. Close geologic similarities between the two deposits, including the presence of quartz-roscoelite-Au veins and breccias, have spurred recent exploration efforts at Mount Kare.The Mount Kare and Porgera intrusive complexes consist of hypabyssal suites of comagmatic, volatile-rich mafic intrusions containing clinopyroxene, olivine, plagioclase, hornblende, chromian spinel, and apatite in melagabbros and mafic porphyries, with plagioclase, hornblende, clinopyroxene, titanian magnetite, and apatite in more evolved rocks (listed in decreasing order of abundance). Analcite occurs as an alteration phase of the groundmass and fills miarolitic cavities in evolved leucogabbros at Mount Kare but has not been observed in the Porgera intrusive complex. Least altered igneous rocks from both suites are nepheline-normative and are characterized by high volatile, alumina, large ion lithophile, and high field strength element contents. They display light rare earth element enrichments ([La/Yb] cn = 9-28 at Mount Kare, 14-20 at Porgera), but relatively low concentrations of K and Ti on mantle-normalized diagrams. The rocks are mostly sodic ([Na 2 O - 2] > or = K 2 O) according to the total alkali-silica classification scheme. Nd, Sr, and Pb isotope compositions and K-Ar ages of hornblende and biotite from the two suites are virtually indistinguishable (epsilon Nd = 6-7, 87 Sr/ 86 Sr [asymp] 0.7036, 208 Pb/ 204 Pb [asymp] 38.5, 207 Pb/ 204 Pb [asymp] 15.55, 206 Pb/ 204 Pb [asymp] 18.65; K-Ar age of Mount Kare intrusive complex = 6.0 + or - 0.1 Ma 2gma ], Porgera intrusive complex = 6.0 + or - 0.3 Ma 2gma ]). These combined characteristics indicate a dominantly alkalic intraplate affinity for the parental magma at both locations, although a weak subduction zone signature is also suggested by the relatively high volatile and alumina and low Ti abundances. It is proposed that magmatic activity was related to changes in geodynamic configuration occurring immediately prior to early Pliocene collision between the Australasian plate and an island-arc system. Minor differences, such as divergent trajectories of Pb isotope crustal contamination arrays, the occurrence of analcite only at Mount Kare, and the widespread presence of excess 40 Ar in igneous amphiboles only at Porgera, indicate that, although related, the Mount Kare and Porgera intrusive complexes developed independently of one another.Disseminated auriferous pyrite from the Mount Kare Au deposit is associated with phyllic alteration of intrusive and sedimentary rocks. A single K-Ar analysis of illitc from altered rock records an age of 5.5 + or - 0.1 Ma (2Sigma ), which falls in the middle of the range of illite and roscoelite ages from the Porgera Au deposit (5.1-6.1 Ma). This result, combined with slightly older ages of igneous activity, suggests a close temporal and genetic association between magmatism and mineralization, at both locations. In addition, the narrow range of Pb isotope compositions of galenas from veins at Mount Kare is identical with that from Porgera, suggesting that similar ore-forming processes were involved at the two locations.Recognition of closely similar igneous lithologies and ore types at Mount Kare and Porgera supports theories of a genetic relationship between volatile-rich, mafic, alkalic magmatism and meso- to epithermal Au mineralization, as previously proposed at Porgera. It is suggested that both of these deposits belong to the alkalic epithermal group, to which Cripple Creek (Colorado), Emperor (Fiji), and Ladolam (Lihir Island, Papua New Guinea) also belong. Igneous centers with these characteristics and located in complex convergent tectonic settings should be considered as prospective targets for precious metal exploration.

Felsic Volcanics in South Khetri Copper Belt, Rajasthan and their Metallogenic Significance

Abstract Felsic volcanics in the south Khetri Copper Belt (KCB) occur interlayered with fine grey banded phyllitic quartzite which often contains fine carbonaceous matter. The volcanics include crystal tuff, ultrafinely laminated grey tuff and pumice. They have been recognised in field on the basis of vesicles and rare amygdules; and under microscope by embayed and resorbed quartz phenocrysts. Glass is altered and incipiently recrystallised. Devitrification textures like spherulites are intact in rare cases. The relatively unaltered tuffs fall in dacitic field of Total Alkali-Silica (TAS) diagram. The tuffs are intimately associated with baryte and ironstone layers at places. Recent work in the south KCB indicates that sulfide mineralisation is polymetaHic. It is stratabound hosted by felsic volcanics and fine carbonaceous phyllite and quartzite. The mineralised zones show wall-rock alteration in the form of profuse silicification and sericitisation. These features when seen in conjunction with the occurrence of concordant baryte and ironstone layers, indicate that the sulfide mineralisation is volcanic-exhalative type.

The construction of the Total Alkali-Silica chemical classification of volcanic rocks

This paper describes the reasoning that lies behind the construction of the field boundaries in the Total Alkali-Silica diagram (TAS) for the chemical classification of volcanic rocks. It shows that by utilizing nomenclature in common use by petrologists in combination with a large computer database of geochemical analytical information derived from the published literature, it was possible to construct a Total Alkali-Silica diagram for volcanic rocks of only 15 fields, which has straight-line boundaries and necessitates the definition of only 11 points to construct the diagram. One of the principal constraints on the positioning of the boundaries between the various named fields was to minimize the degree of overlap between adjacent fields.

A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram

This contribution summarizes and brings up to date the recommendations made by the IUGS Subcommission on the Systematics of Igneous Rocks for the classification of volcanic rocks when modal analyses are lacking. The classification is on a non-genetic basis using the total alkali-silica (TAS) diagram, and is as nearly consistent as possible with the QAPF modal classification. The diagram is divided into 15 fields, two of which contain two root names which are separated according to other chemical criteria, giving the following 17 root names: basalt, basaltic andesite, andesite, dacite, rhyolite, trachybasalt, basaltic trachyandesite, trachyandesite, trachyte, trachydacite, picrobasalt, basanite, tephrite, phonotephrite, tephriphonolite, phonolite and foidite. Using Na-K criteria, trachybasalt may be further divided into the sub-root names hawaiite and potassic trachybasalt, basaltic trachyandesite into the sub-root names mugearite and shoshonite, and trachyandesite into the sub-root names benmoreite and latite.

A proposal by the IUGS Subcommission on the Systematics of Igneous Rocks for a chemical classification of volcanic rocks based on the total alkali silica (TAS) diagram

The IUGS Subcommission on the Systematics of Igneous Rocks recommends that volcanic rocks should be classified on a non‐genetic basis using the total alkali silica (TAS) diagram. This is divided into 14 fields, three of which contain two root names which are separated according to other chemical criteria, giving the following 17 root names: basalt, basaltic andesite, andesite, dacite, rhyolite, alkali rhyolite, trachybasalt, trachyandesite, trachyte, alkali trachyte, picrobasalt, basanite, tephrite, phonotephrite, tephriphonolite, phonolite and foidite. Using other simple parameters, trachybasalt may be further divided into the sub‐root names of hawaiite and potassic trachybasalt, and trachyandesite may be further divided into the sub‐root names of mugearite, shoshonite, benmoreite and latite.