Present-day Volcanism Research Articles

Tertiary andean magmatism in Chile and Argentina between 28°S and 33°S: Correlation of magmatic chemistry with a changing Benioff zone

The distribution and character of Tertiary magmatism in the non-volcanic region of shallow subduction between 28°S and 33°S in the Andes correlates with other geological and geophysical evidence suggesting that the subduction zone in this region has shallowed over the last 18 Ma with a major change occurring between 11 and 8 Ma. Geologic and geochemical data suggest that the magmatic source region under the Chilean Cordillera (29.0–30.5°S) changed at 16–18 Ma from garnet-poor to garnet-rich coincident with cooling due to shallowing of the subduction zone and thickening of the crust. Magmas older than 16–18 Ma appear to have fractionated at shallower crustal levels and interacted less with the pre-existing crust than have younger magmas. Except for small silicic ignimbrites, volcanism ended in the Cordillera at about 11 Ma. Magmatic activity spread to east to the Calingasta Valley and Precordillera in Argentina between 18 and 7 Ma (exact ages uncertain) as the subduction zone shallowed. The chemistry of the andesitic to rhyolitic composition rocks in this region are consistent with derivation from a thickened crust over a shallowing subduction zone. After 8 Ma, the principal magmatic activity migrated far to the east to the Pocho (7.8-4.9 Ma) and San Luis regions in the Sierras Pampeanas. The volcanic rocks (54–70% SiO 2) in this area are geochemically distinct from recent volcanic rocks in other parts of the Andes, but are consistent with eruption over a deep segment of the seismic zone. Present-day volcanism in the “flat slab” region is lacking in the west due to insufficient heat for melting in the mantle above the slab, and in the east because the slab has already lost the components to flux melting in the mantle above the slab by the time temperatures are high enough to produce melting.

THE HYDROLOGY OF ANTARCTICA

The hydrology of the Southern Ocean floor, the ice sheet covering Antarctica, and the bedrock underlying it is a matter of great interest. This article attempts to shed some light on the problem of the hydrology of this part of the world in the form of prognoses, because the available direct information on the subsurface waters of this vast region is completely inadequate. From the standpoint of their practical utilization, the fresh waters at the base of the Antarctic ice sheet and the waters below the permafrost of the larger islands of Antarctica are the most promising. The large artesian basins of Antarctica with a thick mantle of sedimentary rocks are of interest for their oil and gas potential. One can be certain also of obtaining thermal waters from some of the artesian structures of this continent. Special attention should be paid to the fumaroles and solfataras in areas of present-day volcanism, and to the rift zones, where thermal waters may also be obtained. 16 references.

Continent—arc collision and reversal of arc polarity: New interpretations from a critical area

Northern New Guinea has been regarded as a region where the polarity of an island arc reversed following collision with the Australian continent in the Tertiary. However, the evidence for this reversal is not compelling. Because present-day volcanism off the north coast of mainland Papua New Guinea is associated with a steeply northward-dipping Benioff zone (almost vertical) and late Cainozoic volcanoes in the central highlands to the south cannot be related to any present-day Benioff zone, a more acceptable interpretation is that, following collision, the northward-dipping slab beneath the arc became suspended nearly vertically. The active marginal basin lying to the north of the arc is unlikely to be subducted southwards beneath the mainland, because the lithosphere beneath marginal basins appears to be neither thick nor cold enough for the initiation of subduction. Polarity reversal, therefore, may not be the inevitable consequence of continent—arc collisions. Instead, the downgoing slab may steepen, equilibrate with the surrounding mantle and lose its identity. Continuing convergence may be taken up at other plate boundaries and the accreted arc may never again become active.

Comparative petrogenesis of Archaean and modern low-K tholeiites. A critical review of some geochemical aspects

Low-K tholeiites are the most abundant component of both Archaean greenstone belt volcanism and present-day volcanism. This paper compares the geochemistry of Archaean and modern low-K tholeiite associations, paying attention to elements which resist secondary modification during low-grade metamorphism. Allowance is made for differing degrees of fractionation among the suites considered by the use of variation diagrams versus Mg number. The Archaean basalts may be distinguished from modern mid-ocean ridge basalts and marginal basin basalts in having contents of Ti, Zr and P which are about half those in the modern suites, the comparison being referred to a common Mg number. Higher contents of Cr, Ni and Co and lower concentrations of Al discriminate the Archaean suite from modern island-arc tholeiites. When referred to a common Mg number, the Archaean tholeiites are found to have Ni contents at least twice as high as any modern low-K tholeiite. These comparisons indicate that no universally satisfactory analogue of the Archaean tholeiite association exists among modern low-K tholeiites of any type. It is therefore inappropriate to seek information on the tectonic environment of greenstone belt volcanics by the use of geochemical discriminants developed from compilations of analyses of Phanerozoic basalts from known tectonic associations. Semi-quantitative calculations suggest that the greater concentrations of Ni in the Archaean tholeiites cannot be explained in terms purely of higher melting temperatures and greater degrees of melting. Melting at higher pressures is a possible explanation, but since this alternative requires the existence of lower geothermal gradients in the Archaean source region than obtain at modern ocean ridges, it is regarded as unattractive. It is concluded that the Archaean source region must have possessed higher concentrations of Ni than in the modern mantle, possibly in the form of disseminated metallic particles reflecting incomplete core separation.

Relation of Macdonald Volcano to migration of volcanism along the Austral Chain

The growth of island chains by the linear migration of volcanic activity is manifest, to varying degrees, in the surface geology of a number of island groups. In Hawaii, the classic example, this pattern is attested by present-day volcanism, geomorphology, and geochronology. The recent discovery of the currently active Macdonald volcano at the southeastern extension of the Austral chain supports the validity of a linear volcanic migration pattern for that group as well.