LAVA sequence

Additional support for a lower crustal melting model comes from thermal modeling experiments, in which the thermal effects of multiple basaltic intrusions injected into basaltic lower crust are calculated (Jackson et al., 2005). There are three important results from these experiments, First, that significant melting can take place in the lower crust of a basaltic lava sequences, second that the melt can be extracted from its source, and finally that the melt is TTG in composition. 

The summit contains a small crater about 100 m wide. The volcaiuc rocks of Peter I Island consist of basalt and trachyte flows with nunor amounts of volcanic breccia and lavas of intermediate composition. Some of the flows are highly vesicular. Pahoehoe structures have been reported, but no pillow lavas have been seen (i.e., the flows are snbaerial). The lava sequences are locally cut by basalt dikes and plugs. 

Menaichizawa, Sasahata), is composed mostly of brecciated andesite lavas and andesitic hyaloclastics. This formation is conformably overlain by the formation (Hotakizawa, Sunakobuchi) composed of thick sequence of basaltic lavas and tuff breccias with minor intercalations of mudstone and felsic tuff. 

The Harvey Group consists of three units Harvey Mountain, Cherry Mountain, and York Mills Formations. The upper Harvey Mountain formation consists of felsic volcanic rocks, typically flow-banded massive rhyolitic lava accompanied by ash-fall tuff. Rocks of the middle Cherry Mountain formation are characterized by the predominance of ash-fall and ash-flow tuffs at the top and volcanogenic sedimentary rocks at the base. The lower York Mills formation is a sequence of interbedded clastic, volcanogenic and minor volcanic rocks, that latter consisting of felsic volcanic-clast conglomerate, volcanogenic sandstone, siltstone and shale, felsic crystal lithic lapilli tuff and flow-banded rhyolite. 

Abstract Nash creek is the largest Zn-Pb deposit in the Tobique-Chaleurs Zone, and contains indicated and inferred resources of 7.8 Mt grading 2.72% Zn, 0.55% Pb and 18.26 g/t Ag and 1.2 Mt grading 2.66% Zn, 0.52% Pb and 18 g/t Ag, respectively. The deposit is hosted by Early Devonian bimodal volcanic and minor interlayered sedimentary rocks that were deposited in a trans-pressional rift. The host rocks sequence includes the Archibald Settlement Formation (rhyolite flows domes and related rocks), and the Sunnyside Formation (mafic lava flows and interlayered volcaniclastic and sedimentary rocks). 

The Tyrone Volcanic Group sequence comprises basaltic pillow lavas, tuffs of basic to intermediate composition, rhyolites, cherts, siltstones and dark grey mudstones representing up to three volcanic cycles. From base to top of each cycle and through the sequence as a whole, the Tyrone Volcanic Group becomes progressively more acidic in composition. 

The geologic timescale for the Earth is based mostly on stratigraphy (the depositional succession of layers from bottom to top). Most strata are sedimentary rocks. The relative ages of the sedimentary layers are determined by the sequence of deposition and by the fossils that they contain. It is not possible to use radioactive isotopes to measure the time that a sediment was deposited, because deposition does not reset radiometric clocks. However, the absolute ages of sediment layers and the fossils they contain have been determined by measuring radiometric ages of volcanic ash layers in the sequence or lavas that crosscut sedimentary strata. 

The lowest exposed products of the cone have an age of about 550 ka. However, tephra recovered from the nearby areas have shown a somewhat older age of about 580 ka, which most probably represents the beginning of the volcanic activity at Roccamonfina. The youngest lava from the main cone has an age of about 370 ka. The BLT and WTT sequences result from various eruptions that occurred from about 380 to 230 ka. The activity inside the caldera and along the cone flanks developed between about 300 and 100 ka (Chiesa et al. 1995 Giannetti and De Casa 2000 and references therein). 

The volcanic sequence consists of dominant pyroclastic deposits and minor lava flows, which cover an area of about 150 km2. Activity took place between about 0.8 and 0.1 Ma (La Volpe et al. 1984 Guest et al. 1988 La Volpe and Principe 1991). The oldest activity (0.8-0.7 Ma) was characterised by phonolitic to trachytic ignimbritic eruptions and emplacement of some lava domes. The following activity was dominated by... 

The BIF of the greenstone belts of Southern Africa are very old (older than 3.3- yr). Beukes (1973) mentions that in the lower part of the section of the Hooggenoeg formation of this belt, carbonaceous cherts completely devoid of iron appear in a thick sequence of metamorphosed tholeiitic basalts with intercalations of ultrabasic and acid lavas. The first signs of iron appear in the upper part of the section in a banded chert directly underlying the BIF. In this chert, small segregations of iron-rich material are closely associated with black bands of carbonaceous chert or replace them. 

Fewer reliable osmium isotopic data exist for Precambrian samples owing to the problems of disturbance of rhenium and osmium during alteration and metamorphism. Key points used to define mantle evolution are indicated in Figure 7. The osmium isotopic constraints on the composition of the Archean mantle are, in many cases, derived from Late Archean komatiite suites. Komatiites are high-MgO lavas that often form differentiated flow sequences, such that Re-Os isochrons can be determined chromite-rich layers are commonly present and provide a robust estimate of the initial osmium isotopic... 

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