Petrology and geochemistry of early cretaceous bimodal continental flood volcanism of the NW Etendeka, Namibia. Part 2: Characteristics and petrogenesis of the high-Ti latite and high-Ti and low-Ti voluminous quartz latite eruptives

As a result of their relative concentration towards the respective Atlantic margins, the silicic eruptives of the Parana (Brazil)-Etendeka large igneous province are disproportionately abundant in the Etendeka of Namibia. The NW Etendeka silicic units, dated at similar to132 Ma, occupy the upper stratigraphic levels of the volcanic sequences, restricted to the coastal zone, and comprise three latites and five quartz latites (QL). The large-volume Fria QL is the only low-Ti type. Its trace element and isotopic signatures indicate massive crustal input. The remaining NW Etendeka silicic units are enigmatic high-Ti types, geochemically different from low-Ti types. They exhibit chemical affinities with the temporally overlapping Khumib high-Ti basalt (see Ewart et al. Part 1) and high crystallization temperatures (greater than or equal to980 to 1120degreesC) inferred from augite and pigeonite phenocrysts, both consistent with their evolution from a mafic source. Geochemically, the high-Ti units define three groups, thought genetically related. We test whether these represent independent liquid lines of descent from a common high-Ti mafic parent. Although the recognition of latites reduces the apparent silica gap, difficulty is encountered in fractional crystallization models by the large volumes of two QL units. Numerical modelling does, however, support large-scale open-system fractional crystallization, assimilation of silicic to basaltic materials, and magma mixing, but cannot entirely exclude partial melting processes within the temporally active extensional environment. The fractional crystallization and mixing signatures add to the complexity of these enigmatic and controversial silicic magmas. The existence, however, of temporally and spatially overlapping high-Ti basalts is, in our view, not coincidental and the high-Ti character of the silicic magmas ultimately reflects a mantle signature.

The igneous charnockite—high-K alkali-calcic I-type granite—incipient charnockite association in Trivandrum Block, southern India

The Pan-African (640 Ma) Chengannoor granite intrudes the NW margin of the Neoproterozoic high-grade metamorphic terrain of the Trivandrum Block (TB), southern India, and is spatially associated with the Cardamom hills igneous charnockite massif (CM). Geochemical features characterize the Chengannoor granite as high-K alkali-calcic I-type granite. Within the constraints imposed by the high temperature, anhydrous, K-rich nature of the magmas, comparison with recent experimental studies on various granitold source compositions, and trace- and rare-earth-element modelling, the distinctive features of the Chengannoor granite reflect a source rock of igneous charnockitic nature. A petrogenetic model is proposed whereby there was a period of basaltic underplating; the partial melting of this basaltic lower crust formed the CM charnockites. The Chengannoor granite was produced by the partial melting of the charnoenderbites from the CM, with subsequent fractionation dominated by feldspars. In a regional context, the Chengannoor I-type granite is considered as a possible heat source for the near-UHT nature of metamorphism in the northern part of the TB. This is different from previous studies, which favoured CM charnockite as the major heat source. The Occurrence of incipient charnockites (both large scale as well as small scale) adjacent to the granite as well as pegmatites (which contain CO2, CO2-H2O, F and other volatiles), suggests that the fluids expelled from the alkaline magma upon solidification generated incipient charnockites through fluid-induced lowering of water activity. Thus the granite and associated alkaline pegmatites acted as conduits for the transfer of heat and volatiles in the Achankovil Shear Zone area, causing pervasive as well as patchy charnockite formation. The transport Of CO2 by felsic melts through the southern Indian middle crust is suggested to be part of a crustal-scale fluid system that linked mantle heat and CO2 input with upward migration of crustally derived felsic melts and incipient charnockite formation, resulting in an igneous charnockite - I-type granite - incipient charnockite association.

A refined solution to the first terrestrial Pb-isotope paradox

The first terrestrial Pb-isotope paradox refers to the fact that on average, rocks from the Earth's surface (i.e. the accessible Earth) plot significantly to the right of the meteorite isochron in a common Pb-isotope diagram. The Earth as a whole, however, should plot close to the meteorite isochron, implying the existence of at least one terrestrial reservoir that plots to the left of the meteorite isochron. The core and the lower continental crust are the two candidates that have been widely discussed in the past. Here we propose that subducted oceanic crust and associated continental sediment stored as garnetite slabs in the mantle Transition Zone or mid-lower mantle are an additional potential reservoir that requires consideration. We present evidence from the literature that indicates that neither the core nor the lower crust contains sufficient unradiogenic Pb to balance the accessible Earth. Of all mantle magmas, only rare alkaline melts plot significantly to the left of the meteorite isochron. We interpret these melts to be derived from the missing mantle reservoir that plots to the left of the meteorite isochron but, significantly, above the mid-ocean ridge basalt (MORB)-source mantle evolution line. Our solution to the paradox predicts the bulk silicate Earth to be more radiogenic in Pb-207/Pb-204 than present-day MORB-source mantle, which opens the possibility that undegassed primitive mantle might be the source of certain ocean island basalts (OIB). Further implications for mantle dynamics and oceanic magmatism are discussed based on a previously justified proposal that lamproites and associated rocks could derive from the Transition Zone.

Importance of water for Archaean granitoid petrology: a comparative study of TTG and potassic granitoids from Barberton Mountain Land, South Africa

A new model for Archaean granitoid magmatism is presented which reconciles the most important geochemical similarities and differences between tonalite-trondhjemite-granodiorite (TTG) and potassic granitoids. Trace element abundances reveal a strong arc magmatism signature in all studied granitoids from Barberton Mountain Land. Characteristic features include HFSE depletion as well as distinct enrichment peaks of fluid-sensitive trace elements such as Pb in N-MORB normalisation, clearly indicating that all studied granitoids are derived from refertilised mantle above subduction zones. We envisage hydrous basaltic (s.l.) melts as parental liquids, which underwent extensive fractional crystallisation. Distinctive residual cumulates evolved depending on initial water content. High-H2O melts crystallised garnet/amphibole together with pyroxenes and minor plagioclase, but no olivine. This fractionation path ultimately led to TTG-like melts. Less hydrous basaltic melts also crystallised garnet/amphibole, but the lower compatible element content indicates that olivine was also a liquidus phase. Pronounced negative Eu-anomalies of the granitic melts, correlating with Na, Ca and Al, indicate plagioclase to be of major importance. In the context of our model, the post-Archaean disappearance of TTG and concomitant preponderance of granites (s.l.), therefore, is explained with secular decrease of aqueous fluid transport into subduction zones and/or efficiency of deep fluid release from slabs.

Pumice rafting and faunal dispersion during 2001-2002 in the Southwest Pacific: record of a dacitic submarine explosive eruption from Tonga

A new influx of sea-rafted pumice reached the eastern coast of Australia in October 2002, approximately 1 year after a felsic, shallow-marine explosive eruption at a previously unknown volcano (0403-091) along the Tofua volcanic arc (Tonga). The eruption produced floating pumice rafts that first became stranded in Fiji in November 2001, approximately I month after the eruption. Strandings of sea-rafted pumice along shorelines have been the only record of products from this submarine explosive eruption at the remote, submerged volcano. Computed drift trajectories of the sea-rafted pumice using numerical models of southwest Pacific surface wind fields and ocean currents indicate two cyclonic systems disturbed the drift of pumice to eastern Australia, as well as the importance of the combined wave and direct wind effect on pumice trajectory. Pumice became stranded along at least two-thirds (>2000 km) of the coastline of eastern Australia, being deposited on beaches during a sustained period of fresh onshore winds. Typical amounts of pumice initially stranded on beaches were 500-4000 individual clasts per in, and a minimum volume estimate of pumice that arrived to eastern Australia is 1.25 x 10(5) m(3). Pumice was beached below maximum tidal/storm surge levels and was quickly reworked back into the ocean, such that the concentration of beached pumice rapidly dissipated within weeks of the initial stranding, and little record of this stranding event now exists. Most stranded pumice clasts ranged in size from 2 to 5 cm in diameter; the largest measured clasts were 10 cm in Australia and 20 cm in Fiji. The pumice has a low phenocryst content (3500 km) and period of pumice floatation (greater than or equal to1 year), confirm the importance of sea-rafted pumice as a long-distance dispersal mechanism for marine organisms including marine pests and harmful invasive species. Billions of individual rafting pumice clasts can be generated in a single small-volume eruption, such as observed here, and the geological implications for the transport of sessile taxa over large distances are significant. An avenue for future research is to examine whether speciation events and volcanicity are linked; the periodic development of globalism for some taxa (e.g., corals, gastropods, bryozoa) may correlate in time and/or space with voluminous silicic igneous events capable of producing >10(6) km(3) of silicic pumice-rich pyroclastic material and emplaced into ocean basins. (C) 2004 Elsevier B.V. All rights reserved.

ELA-ICP-MS U-Pb zircon geochronology of regional volcanism hosting the Bajo de la Alumbrera Cu-Au deposit: implications for porphyry-related mineralization

ELA-ICP-MS U-Pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the Bajo de la Alumbrera porphyry Cu-Au deposit, NW Argentina, are cogenetic with stratigraphically well-constrained volcanic and volcaniclastic rocks of the Late Miocene Farallon Negro Volcanic Complex. Zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1.5 million years. Volcanism occurred in a multivent volcanic complex in a siliciclastic intermontane basin. The complex evolved from early mafic-intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions. Zircons from the basal mafic-intermediate lavas have ages that range from 8.46 +/- 0.14 to 7.94 +/- 0.27 Ma. Regionally extensive silicic explosive volcanism occurred at similar to8.0 Ma (8.05 +/- 0.13 and 7.96 +/- 0.11 Ma), which is co-temporal with intrusion of the earliest mineralized porphyries at Bajo de la Alumbrera (8.02 +/- 0.14 and 7.98 +/- 0.14 Ma). Regional uplift and erosion followed during which the magmatic-hydrothermal system was probably unroofed. Shortly thereafter, dacitic lava domes were extruded (7.95 +/- 0.17 Ma) and rhyolitic diatremes (7.79 +/- 0.13 Ma) deposited thick tuff blankets, across the region. Emplacement of large intermediate composition stocks occurred at 7.37 +/- 0.22 Ma, shortly before renewed magmatism occurred at Bajo de la Alumbrera (7.10 +/- 0.07 Ma). The latest porphyry intrusive event is temporally associated with new ore-bearing magmatic-hydrothermal fluids. Other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at Bajo de la Alumbrera. However, their emplacement continued (from 7.10 +/- 0.06 to 6.93 +/- 0.07 Ma) after the final intrusion at Bajo de al Alumbrera. Regional volcanism had ceased by 6.8 Ma (6.92 +/- 0.07 Ma). The brief history of the volcanic complex hosting the Bajo de la Alumbrera Cu-Au deposit differs from that of other Andean provinces hosting porphyry deposits. For example, at the El Salvador porphyry copper district in Chile, magmatism related to Cu mineralization was episodic in regional igneous activity that occurred over tens of millions of years. Bajo de la Alumbrera resulted from the superposition of multiple porphyry-related hydrothermal systems, temporally separated by a million years. It appears that the metal budget in porphyry ore deposits is not simply a function of their longevity and/or the superposition of multiple porphyry systems. Nor is it a function of the duration of the associated cycle of magmatism. Instead, the timing of processes operating in the parental magma body is the controlling factor in the formation of a fertile porphyry-related ore system.

Volcanic setting of the Bajo de la Alumbrera porphyry Cu-Au deposit, Farallon Negro Volcanics, Northwest Argentina

The late Miocene Farallon Negro volcanics, comprising basaltic to rhyodacitic volcano-sedimentary rocks, host the Bajo de la Alumbrera porphyry copper-gold deposit in northwest Argentina. Early studies of the geology of the district have underpinned the general model for porphyry ore deposits where hydrothermal alteration and mineralization develop in and around porphyritic intrusions emplaced at shallow depths (2.5-3.5 km) into stratovolcanic assemblages. The Farallon Negro succession is dominated by thick sequences of volcano-sedimentary breccias, with lavas forming a minor component volumetrically. These volcaniclastic rocks conformably overlie crystalline basement-derived sedimentary rocks deposited in a developing foreland basin southeast of the Puna-Altiplano plateau. Within the Farallon Negro volcanics, volcanogenic accumulations evolved from early mafic to intermediate and silicic compositions. The younger and more silicic rocks are demonstrably coeval and comagmatic with the earliest group of mineralized porphyritic intrusions at Bajo de la Alumbrera. Our analysis of the volcanic stratigraphy and facies architecture of the Farallon Negro volcanics indicates that volcanic eruptions evolved from effusive to mixed effusive and explosive styles, as magma compositions changed to more intermediate and silicic compositions. Air early phase of mafic to intermediate voleanism was characterized by small synsedimentary intrusions with peperitic contacts, and lesser lava units scattered widely throughout the district, and interbedded with thick and extensive successions of coarse-grained sedimentary breccias. These sedimentary breccias formed from numerous debris- and hyperconcentrated flow events. A later phase of silicic volcanism included both effusive eruptions, forming several areally restricted lavas, and explosive eruptions, producing more widely dispersed (up to 5 kin) tuff units, some tip to 30-m thickness in proximal sections. Four key features of the volcanic stratigraphy suggest that the Farallon Negro volcanics need not simply record the construction of a large steep-sided polygenetic stratovolcano: (1) sheetlike, laterally continuous debris-flow and other coarse-grained sedimentary deposits are dominant, particularly in the lower sections; (2) mafic-intermediate composition lavas are volumetrically minor; (3) peperites are present throughout the sequence; and (4) fine-grained lacustrine sandstone-siltstone sequences occur in areas previously thought to be proximal to the summit region of the stratovolcano. Instead, the nature, distribution, and geometry of volcanic and volcaniclastic facies suggest that volcanism occurred as a relatively low relief, multiple-vent volcanic complex at the eastern edge of a broad, > 200-km-wide late Miocene volcanic belt and oil ail active foreland sedimentary basin to the Puna-Altiplano. Volcanism that occurred synchronously with the earliest stages of porphyry-related mineralization at Bajo de la Alumbrera apparently developed in an alluvial to ring plain setting that was distal to larger volcanic edifices.