High Large Ion Lithophile Element Research Articles

Petrogenesis and regional tectonic significance of Late Devonian mafic intrusions in the Meguma Zone, Nova Scotia

Late Devonian (377–368 Ma, 40Ar/39Ar; 376 Ma, U–Pb) mafic intrusions in the Meguma Zone crop out as dykes, plugs, and synplutonic bodies of gabbro, diorite, or lamprophyre. All of the intrusions have similar lithologie characteristics and hydrous ferromagnesian mineral assemblages, and they appear to represent a genetically related series of mafic bodies with similar petrogenetic histories in the crust of the Meguma Zone. The intrusions show wide chemical variation of SiO2 (45.7–65.7 wt.%), Al2O3 (8.9–26.5 wt.%), MgO (2.8–26.5 wt.%), CaO (1.2–11.2 wt.%), and K2O (0.1–4.4 wt.%), and they have calc-alkaline, high-K calc-alkaline, and shoshonitic characteristics. Large-ion lithophile elements (LILE) are present at variable but high concentrations (e.g., Ba = 62–1920 ppm, Sr = 176–2567 ppm) relative to most high field strength element (HFSE) abundances (e.g., Y = 10–37 ppm, Zr = 8–421 ppm), and light rare-earth elements (LREE) have much higher concentrations than heavy rare-earth elements (HREE) (La/Lu = 24–330). Initial Sr isotopic ratios (0.7044–0.7079) and εNd values (−4.36 to 3.69) are highly variable. Scatter on major oxide variation diagrams probably results from the fractionation of all the major modal phases in the intrusions (olivine, augite, hornblende, and (or) plagioclase), and the cumulate characteristics of some bodies support this suggestion. Nevertheless, parallel patterns for the intrusions on mid-ocean ridge basalt (MORB) normalized spider diagrams support the notion of similar mafic parent melts, and Sr–Nd isotopic data identify contamination by continental crust in only one of the intrusions. The most primitive picrite contains approximately basaltic HFSE in conjunction with HREE at 5–11CN, perhaps suggesting that the magmas emanated from depleted peridotite or pyroxenite, but high alkalies, LILE (<60MN), and LREE (10–100CN), and elevated initial Sr ratios in all of the intrusions, also require the existence of an enriched source component. Troughs in the spider diagrams at Ta, Nb, and Ti, and Sr–Nd isotopic values comparable with modern island-arc basalts, suggest that fluids derived from subducted ocean lithosphère metasomatized the mantle. Tectono-magmatic discriminators imply a continental margin arc environment rather than an island arc, and the intrusions record either Early Devonian subduction of Iapetus Ocean lithosphère beneath the Avalon terrane, Middle Devonian subduction of Theic Ocean lithosphère beneath the Meguma terrane, or an inherited subduction signature formed during a much older event.

A review of Archean volcanic-hosted massive sulfide and sulfate mineralization in Western Australia

Archean volcanic-hosted massive sulfide mineralization in Western Australia ranges in age from ca. 3.46 to 2.70 Ga. Barite-rich, Zn-Pb-Cu mineralization is hosted by relatively large ion lithophile element-rich calc-alkaline volcanic rocks in the ca. 3.46 Ga Warrawoona Group and ca. 3.3 Ga Gorge Creek Group in the eastern Pilbara craton. The high lead contents and 100 Zn/Zn + Pb ratios of between 60 and 80 are typical ofPhanerozoic mineralization hosted by silicic volcanic rocks, rather than of most Archean massive Cu-Zn sulfide deposits. The abundance of sulfate in these deposits, combined with evidence for evaporitic sulfates elsewhere in the Warrawoona Group, also suggests at least local oxidation of reduced suliur in the upper layer of the ocean at this time. Lead-rich, Zn-Pb-Cu mineralization also occurs in the ca. 2.99 Ga Whim Creek Group in the western Pilbara craton. This volcanic and sedimentary sequence formed in a rapidly subsiding basin within older continental crust, and the volcanic rocks are characterized by relatively high large ion lithophile element contents. In the east Yilgarn craton, Cu-Zn mineralization occurs in a bimodal assemblage of tholeiitic basalts and rhyolites at Teutonic Bore, in the Norseman-Wiluna belt. This deposit was mined during the 1980s. In the western Yilgarn block, Cu-Zn mineralization occurs in an assemblage of ca. 3.0 Ga. deep-water, calc-alkaline volcanic and sedimentary rocks in the Golden Grove belt. Mineralization in this belt is typically associated with magnetite and includes the Scuddles and Gossan Hill deposits. Alteration at these deposits is similar to that of Mattabi-type deposits in the Canadian Archcan and includes iron-rich chlorite and carbonates in poorly developed, or laterally extensive, feeder zones. The lead content of the host volcanic sequence and mineralization in this belt is typically much lower than that of the Pb-rich Pilbara mineralization. The Scuddles deposit with a mine recoverable reserve of about 10 million tons of 11 percent Zn, 0.8 percent Pb, and 1.2 percent Cu is currently being mined.

The Nevados de Payachata volcanic region (18�S/69�W, N. Chile) II. Evidence for widespread crustal involvement in Andean magmatism

Volcanism extending over 11 Ma is represented in the rocks of the Nevados de Payachata region, culminating in the formation of two large composite stratocones within the last 500 000 years. Chemically distinct mafic magmas are erupted at a number of parasitic centers. These cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones. Two distinct trends are defined by the intermediate and evolved rocks; a high LILE (large ion lithophile element), TiO2 and Ce/Yb lineage among the youngest rocks (including the two major stratocones), and a more typical calc-alkaline trend among the older (>1 Ma) rock types. Within individual volcanic centers, differentiation involves fractionation of plagioclase, pyroxene and hornblende, with biotite and K-feldspar in the more-evolved rock types. Isotopic compositions (Sr, Pb, Nd, O) vary little with differentiation from basaltic andesite to rhyolite, or with age. Contamination during differentiation from basalt to rhyolite may occur, but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust. The majority of data are similar to “baseline” compositions (Cenozoic parental magmas) from other parts of the central Andes and may reflect a relatively homogeneous magma source (or source mixture) throughout this central volcanic zone (CVZ), which is distinct from the southern and northern Andes, and from island-arc volcanic rocks.

The transformation of amphibolite facies gneiss to charnockite in southern Karnataka and northern Tamil Nadu, India

Amphibolite facies metamorphic grade gives way southward to the granulite grade in southern Karnataka, as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes. Petrologic investigations in the transitional zone south of Mysore have established the following points: 1) The transition is prograde. Amphibole-bearing gneisses intimately associated with charnockite at Kabbal and several similar localities are not retrogressive after charnockite, as proved by patchy obliteration of their foliation by transgressive, very coarse-grained charnockite, high fluorine content of biotite and amphibole in gneisses, and high large-ion lithophile element contents in gneisses and charnockites. These features are in contrast to very low fluorine in retrogressive amphiboles and biotites, very low large-ion lithophile element contents, and zonal bleaching of charnockite, in clearly retrogressive areas, as at Bhavani Sagar, Tamil Nadu. 2) Metamorphic temperatures in the transitional areas were 700°–800° C, pressures were 5–7 kbar, and H2O pressures were 0.1–0.3 times total pressures, based on thermodynamic calculations using mineral analyses. Dense CO2-rich fluid inclusions in the Kabbal rocks confirm the low H2O pressures at the first appearance of orthopyroxene. Farther to the south, in the Nilgiri Hills and adjacent granulite massif areas, peak metamorphic temperatures were 800°–900° C, pressures were 7–9 kbar, and water pressures were very low, so that primary biotites and amphiboles (those with high F contents) are rare. 3) The incipient granulite-grade metamorphism of the transitional areas was introduced by a wave of anatexis and K-metasomatism. This process was arrested by drying out under heavy CO2 influx. Charnockites so formed are hybrids of anatectic granite and metabasite, of metabasite and immediately adjacent gneiss, or are virtually isochemical with pre-existing gneiss despite gross recrystallization to granulite mineralogy. These features show that partial melting and metasomatism are attendant, rather than causative, in charnockite development. Copious CO2 from a deep-crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis. Granulite metamorphism of both neosome and paleosome followed. The process is very similar to that deduced for the Madras granulites by Weaver (1980). The massif charnockites, for the most part extremely depleted in lithophile minor elements, show many evidences of having gone through the same process.

Basalts and related rocks from deep-sea drilling sites in the Central and Eastern Indian Ocean

Petrological and geochemical data are presented for basement rocks recovered from thirteen sites (211, 212, 213, 214, 215, 216, 253, 254, 256, 257, 259, 260 and 261) drilled on Legs 22, 26 and 27 of the Deep Sea Drilling Program. Basalts from Sites 212, 213, 253, 257, 259 and 261 have petrographic and geochemical characteristics of tholeiitic basalts from active spreading ridge axes in the major ocean basins. Except for the lower basalts at Site 253, all the basement rocks from four Ninetyeast Ridge sites (214, 216, 253, 254) are similar to oceanic island tholeiitic sequences such as on Amsterdam—St. Paul Islands. The geochemical data for the Ninetyeast Ridge rocks are consistent with the development of this ridge as a hot-spot trace. Basalts from deep ocean Sites 216 and 256 have geochemical characteristics more akin to tholeiitic basalts from spreading ridge axes close to major volcanic islands, i.e. they have high large ion lithophile element abundances relative to tholeiitic basalts found at spreading ridge axes away from islands. Alkali olivine basalts were recovered from Site 211 and they contain abundant amphibole. They are related to the volcanism which created the Cocos—Keeling—Christmas shoal areas.