Search

The timing and magnitude of rotation of Fiji in relation to the tectonic evolution of the SW Pacific region has been investigated by extensive (126 sites) paleomagnetic sampling of Late Miocene and younger units. Generally, samples yielded well-defined characteristic remanence directions which indicate that the main Fiji platform rotated by 135±17.5° counterclockwise commencing at, or before, 10 Ma. This very large and rapid rotation ceased abruptly at 3 Ma, coincident with the onset of spreading on the major N-S aligned spreading centers in the North Fiji Basin. Latest Miocene to Pliocene paleomagnetic results indicate that Viti Levu, the main island itself, has suffered no internal disruption by localized rotations after 5 Ma. We present a reconstruction for the origin of Fiji which differs markedly from previous models such that Fiji, Tonga, and Lau form a continuous N-S aligned arc and that arc reversal was initiated at or near Fiji itself.

New compositional data are presented for recent (

Magnetite—apatite deposits associated with the Atacama Fault Zone of northern Chile are interpreted here, on field criteria, as being the products either of hydrothermal fluids with a strong magmatic signature, or of late-stage Fe-rich magmas mixed with an aqueous fluid. Even in the Chilean iron belt, apatite-rich magnetite deposits are a rarity. Variations in F- and Cl- contents in apatites, strongly zoned with respect to halogens, are indicative of primary variations in fHF and fHCI in the hydrothermal fluids. Small variations in halogen fugacities in the aqueous fluid are capable of buffering large variations in halogen content within apatite crystals in equilibrium with that fluid. The recorded halogen zonation profiles are inconsistent with crystallization of the apatites simply from a volatile-rich, late-stage fractionation Fe-rich magma, or its derived magmatic vapour. It is more likely that they are the result of mineral—fluid buffering with a fluid that represents the mixing of a magmatically-derived aqueous fluid with a meteoric fluid that has variably scavenged Ca and Cl from within the country rocks. The source magma of the former is probably an Fe-P enriched acidic magma, derived by fractionation of primary calc-alkaline basic magmas.

The Fiji platform has a thick, youthful crust of ensimatic origin. Stratigraphy within the Fiji islands indicates a history restricted to the Cenozoic, with volcanic rocks, principally submarine fragmental types with related sediments, dominating the succession. There are also carbonate sediments and plutonic rocks of gabbroic to tonalitic composition which were intruded during the Tholo orogeny, an important middle to late Miocene tectonic event.Mineral deposits in Fiji include various types of massive sulfide; manganese ores; disseminated porphyry-type Cu-Au deposits; epithermal-telethermal Au-Te-Ag deposits; veins, skarns, and limestone replacements related to plutonism; and residual deposits such as iron sands and bauxite.Preorogenic rocks are generally arc tholeiites, but following the Tholo orogeny there was great chemical diversity--possibly owing to anatexis and contamination--and eruption of arc tholeiite, calc-alkaline andesite, shoshonitic rocks, and oceanic-type basalts. Contamination of magmas by watery metamorphic fluids is thought to be the cause of relative enrichment of chalcophile elements in postorogenic rocks of late Miocene to early Pliocene age. Nearly all the major mineral deposits in Fiji are associated with these rocks.Volcanic and structural factors exert a local control on mineralization. On a regional scale plate collision in Outer Melanesia led to the Tholo orogeny and deep fracturing of the Fiji platform--factors of fundamental importance to Fiji mineralization; however, a simple correlation of island-arc mineral deposits and subduction is not apparent.

Report of a Society Ordinary General Meeting held at Burlington House on 13 November 1985. The meeting was organized by Dr M. Brown and Dr H. Colley. With the success of the Specialist Groups and the decline of the ‘traditional’ OGM, the Society has sponsored several one-day thematic meetings either as OGMs or as SpecialScientific Meetings. Additionally, specialist or thematic meetings have reduced the opportunities for interdisciplinary research on a regional scale to be presented and discussed. These reasons prompted the convenors to propose a meeting on ‘Current research in regional geology’, with papers from academic institutes, the BGS and overseas. Speakers presented recent unpublished research results, and both the substantial attendance at the meeting and the lively discussion after each paper suggest that this style of OGM is very worthwhile. The BGS also displayed a number of poster contributions. An introduction to the meeting was given by M. Brown, who also shared the chair during the morning session with H. Colley. In the first paper, R. L. Johnson presented a general overview of the work undertaken by the BGS overseas, particularly in Africa. The most interesting type of project undertakh by the BGS in Africa under funding from the Overseas Development Administration is that involving research and development in which success is judged in terms of the prospecting licences sought at the end of a project! P. J. Treloar followed with a detailed account of recent research’in the Magondi (mid-Proterozoic) and Zambesi (Pan-African) mobile belts of NW and N

The use of principal component analysis in studying chemical trends in volcanic rock suites is described. It is suggested that eigenvectors generated from a correlation matrix, rather than a covariance matrix, could be used in this context. In the latter situation many elements are swamped by silicon's numerical size and range. In the former situation the alkalies and titanium begin to show their true importance.

Petrographic study of stained Pleistocene-Recent biolithites from Erromango reveals an advanced state of diagenesis. Micrite is produced by grain diminuton of algal colonies, and probably by fragmental abrasion. Other fabrics developed include granular mosaics and simple syntaxial rims. In samples collected from carbonates sandwiched between volcanic ash bands, two phases of cementation are apparent. The first phase cement is composed of ferroan calcite, which is followed by a second phase of non-ferroan calcite cement. Ferroan calcite is previously unrecorded as a cement in Recent carbonates, and in addition it is very rare as a first stage cement in ancient carbonates. Diagenesis of these carbonate sediments has produced in a very short time a hard, compact, partially recrystallized rock. Reasons for this advanced state of diagenesis are unknown, but in all probability they are connected with the high heat values in the rocks resulting from contemporaneous volcanicity.

In the New Hebrides, an Eastern and a Western Belt of mainly Neogene volcanic and sedimentary rocks are bisected by a longer chain of active and recently extinct volcanoes consisting of primary and reworked volcanic rocks. The Miocene volcanic rocks of the Western Belt are predominantly andesitic, though they range in composition from high-alumina basalt to dacite. Basalt and hornblende-bearing andesite are cut by intrusions of microgabbro, hornblende microdiorite, and gabbro-diorite. The dominantly calc-alkaline andesitic rocks of the Western Belt possibly originated through partial melting of the mantle in the Benioff zone. The Miocene volcanic rocks of the Eastern Belt are more tholeiitic and basic than those of the Western Belt. Hornblende is absent and orthopyroxene is only present in andesite, which is mainly restricted to Pentecost Island. Tectonically remobilized serpentinite on Pentecost is the only ultrabasic plutonic rock found in the New Hebrides. Small gabbro, norite, and die rite intrusions are also present. Primitive tholeiite of the Eastern Belt probably originated at a shallower depth in the Benioff zone than the calc-alkaline volcanic rocks of the Western Belt. The volcanic rocks of the Central Chain include an older sequence of upper Miocene(?) or Pliocene silica- and alkali-rich submarine pyroclastics. Polarity reversal during this period might explain a subsequent switch to tholeiitic magma during the latest volcanic cycle through Quaternary into Holocene time. Associated derivatives, which occur in smaller quantities and range in composition from ultramafic cumulates to dacite, were produced by crystal fractionation from the parent magma. Though this process gave rise to temporal compositional variation at individual volcanic centers, there is no conclusive evidence of lateral spatial compositional zoning across the narrow active arc that might reflect derivation of magma from different depths in the Benioff zone. The active volcanoes within the northern interarc basin are younger than those in the south of the chain; they are predominantly or exclusively basaltic and often produced some mildly alkaline basic rocks. These features, together with a high heat flow and bathymetric evidence, are consistent with recent concepts on the formation of such expanding basins by crustal extension with addition of fresh oceanic crust along the active volcanic axis.