Petrogenesis Of Volcanic Rocks Research Articles

Petrology and petrogenesis of volcanic rocks from the Taupo Volcanic Zone: a review

Taupo Volcanic Zone (TVZ) volcanic rocks comprise three major compositional series: high-alumina basalt (HAB), andesite, and rhyolite, plus a fourth, minor, dacitic series resulting from mixing of andesite and rhyolite magma. Relatively rare HABs originate as near-primary magma from depleted, chemically and isotopically homogeneous mantle. They erupted exclusively through thin, extensional crust, and have evolved by limited crystal fractionation and crustal assimilation. Andesite volcanoes broadly define the present-day active volcanic front, particularly in the southern and northern sectors of the TVZ. The rocks are generally high-silica, but range in composition from basaltic andesite to dacite. Nine petrologically distinct andesite types are recognised; none are directly related to HAB, the majority originating as AFC derivatives of low-alumina basalt. Rhyolite is volumetrically dominant in the TVZ (> 90% by volume), occurring predominantly in the central part but also offshore to the north as lava, ignimbrite and airfall deposits. Although geochemical and isotopic data, and experimental studies have placed some constraints on petrogenetic models, these remain controversial. Crustal anatexis of meta-greywacke basement can be dismissed as a major source, but basement rocks may be involved through secondary contamination. Least-squares mixing models using geochemistry and a variety of stable and radiogenic isotopes show that TVZ rhyolite could be generated by AFC of a mafic parent. However, these models cannot easily explain the apparent absence of large volumes of intermediate rocks and mafic residues. Melting of pre-existing volcanics or lower crustal granulites is also possible, but the existence of such rocks beneath the TVZ is not proven. Although HAB, andesite and rhyolite have coherent major-element compositions, and their occurrence can be explained in terms of crystal fractionation/crustal contamination/AFC models, all three have distinctive trace-element and isotopic characteristics that cannot be convincingly explained by any single-stage petrogenetic model.

Late Silurian – Early Devonian rifting during dextral transpression in the southern Gaspé Peninsula (Quebec): petrogenesis of volcanic rocks

The Upper Silurian – Lower Devonian volcanic rocks in the southern Gaspé Peninsula of the Quebec Appalachians crop out at the northeast end of the Connecticut Valley – Gaspé Synclinorium. These shallow marine and subaerial sequences reach a thickness of up to at least 2000 m and comprise two groups: (1) the Late Silurian volcanic rocks, which are mainly transitional alkalic–tholeiitic basalts with steeply sloping REE patterns; (2) the Early Devonian volcanic rocks, which include a significant proportion of intermediate rocks in addition to tholeiitic basalts. Compared with the Silurian rocks, the Devonian basalts have lower abundances of strongly incompatible trace elements such as Ba, Th, Ta, Nb, and light REE and relatively flat heavy REE patterns. Basalts of both groups display negative Nb and Ta anomalies (relative to Th and La).Although the basalts of both sequences were derived from lithospheric mantle, the Silurian basalts were generated from garnet peridotite at ~ 80 km depth while the Devonian basalts appear to have resulted from a larger degree of melting of spinel peridotite at a shallower depth (~ 60 km). Devonian intermediate rocks are probably the result of mixing of the basaltic magma with upper crustal material through assimilation – fractional crystallization processes. The basalts are interpreted to have formed in a northwest-trending rift zone located in the Quebec Reentrant during dextral transpression along the Appalachian Orogen. Rotation during and after the volcanism reoriented the rift zone to a northeast trend. The high density layer at the base of the crust under the Magdalen Basin may be the former magma chamber for the Silurian–Devonian volcanism. The change from transitional to tholeiitic volcanism at the Silurian–Devonian boundary suggests that the stretching value (ratio of final to initial surface area) increased from < 2 to > 2 at that time. This boundary is also coincident with the Salinic disturbance that is inferred to have been produced by erosion of the thermally uplifted block associated with rifting.

Comparison of 230Th- 238U disequilibrium systematics in lavas from three hot spot regions: Hawaii, Prince Edward and Samoa

230Th- 238U disequilibrium systematics reveal several important characteristics of the mantle source regions and petrogenesis of volcanic rocks in the presumed hot spots of Hawaii, Marion Island (Prince Edward hot spot), and Samoa. The ( 230Th 232Th ) activity ratios of lavas from these three hot spots (1.06 ± 0.07, 1.04 ± 0.08, and 0.81 ± 0.06, respectively) imply that the source regions are each nearly homogeneous with Th U weight ratios of 2.9, 3.0, and 3.8. For Marion Island and Mauna Kea, Hawaii, negligible secular variation occurs in the ( 230Th 232Th ) initial ratios. This supports other evidence for very short transfer time between source and surface. Significant residence time at depth prior to eruption cannot be ruled out for the Samoan lavas we have studied; however, the data for one of these flows deviate from the proposed ( 230Th 232Th )- 87Sr 86Sr correlation ( Condomines et al., 1981a) in the opposite sense from that expected for such residence. If it is assumed that the measured ( 230Th 232Th ) ratios of the young lavas reflect Th U in their mantle sources, then the observed variations among these three hot spots, combined with those reported by other workers for Iceland, the Azores and Tristan de Cunha, suggest that these sources are characterized by Th U ratios ranging from values similar to that of MORB source (~2.5) to values similar to those of bulk earth (~3.8). Mixing of different proportions of depleted and enriched mantle may be responsible for the observed range.