Cenozoic Basaltic Volcanism Research Articles

Age relationships and tectonic implications of late Cenozoic basaltic volcanism in Northland, New Zealand

Abstract An episode of late Miocene ‐ Recent essentially basaltic volcanism is the latest in a sequence of magmatic events recognised in the tectonically complex geological development of the Northland Peninsula. New K‐Ar dates together with an extensive collection of new major and trace element chemical analyses prompt a reassessment of the significance of these late Cenozoic basalts. The main time/ space groupings recognised are Tertiary volcanics in the Kaikohe ‐ Bay of Islands, Puhipuhi, Ti Point, and Stony Batter areas and Quaternary basalts in the Kaikohe ‐ Bay of Islands and Whangarei areas and at Tara. Basalts in the Kaikohe ‐ Bay of Islands area are transitional to alkalic in character, while those in the south are transitional to tholeiitic, with the Ti Point and Stony Batter rocks being geochemically distinct. A consistent model for these observations is that the magmas originate from different levels of a layered mantle source in which the upper part carries a geochemical signature inherited from an earlier subduction event.

Trace element and isotopic variations in Kerguelen and Heard Island basalts

The Zhejiang–Fujian late Cenozoic volcanism took place sporadically in four volcanic belts and constitutes an important diffuse continental rift basalt province in eastern China. The volcanic rocks consist predominantly of basanite, alkali olivine basalt, olivine tholeiite and quartz tholeiite with subordinate nephelinite and rare alkali picrite basalt. Twenty-four new ages of the basaltic rocks and amphibole megacryst determined by 40Ar–39Ar incremental heating experiments demonstrate that the basaltic lava erupted from 0.9 to 26.4 Ma. Although some basanite dikes and nephelinite pipes in Inner and Inner Middle belts of the Zhejiang area are early Miocene, almost all the late Cenozoic basaltic volcanism occurred following cessation of South China Sea seafloor spreading (≤16 Ma). Based on these results, most late Cenozoic intraplate magmatism surrounding the South China Sea margins may be related to the migration of the South China Sea mid-ocean ridge system beneath SE China since mid-Miocene. However, in the Zhejiang–Fujian region, volcanic activities terminated gradually and propagated westward and northward due to the collision of the Luzon arc with the eastern edge of the Eurasia continent during late Miocene to Pleistocene.The Zhejiang–Fujian basalts exhibit trace element and isotopic affinities with OIB. Sr and Nd isotope compositions range from 0.703264 to 0.704235 and 0.512725 to 0.512961, respectively, similar to the composition of the Leiqiong basalts in South China. The enrichment of LREE coupled with depleted Sr–Nd isotopic compositions in the basaltic rocks imply that recent mantle metasomatism occurred shortly before the Cenozoic magmatism. The Sr–Nd–Pb isotopic compositions as well as high LILE/HFSE ratios found in some basaltic rocks showed that an EM2-type lead isotope signature existed in the continental lithospheric mantle in the Zhejiang–Fujian region. This character may have resulted from mantle metasomatism due to a paleo-subduction event. The Zhejiang–Fujian basaltic rocks were generated by partial melting and mixing of different proportions of depleted asthenospheric mantle (DMM or MORB) with EM2-type lithospheric mantle and have undergone different degree of fractional crystallization when the magma ascended to the surface.

CENOZOIC VOLCANIC ROCK SERIES IN SOUTHEASTERN CHINA (II)——TRACE ELEMENT GEOCHEMISTRY

On the premise of the sister paper, the trace element geochemistry of Cenozoic volcanic rocks of southeastern China is discussed in detail in this paper. The data presented in this paper indicate that the upper mantle beneath the area studied is enriched in the strongly HYG elements as compared with the chondritic mantle, and that the depth of Cenozoic basaltic magma sources progressively increases from the coast to the inland. The Cenozoic basaltic volcanism has occurred in a geotectonic setting of rifting continental margin; on the contrary, the Mesozoic volcanic rocks are predominated by the felsic volcanics erupting in a geotectonic setting of active continental margin. It can be inferred from the studies of ultramafic inclusion-derived geotherm that the Cenozoic rifting of eastern China continent has been mainly controlled by the thermal mantle diapirism.

Cenozoic basaltic volcanism of Kamchatka as an indicator of displacement of the crust

Cenozoic basaltic volcanism of Kamchatka as an indicator of displacement of the crust

Thrust faulting and crust--upper mantle structure in East Australia

Summary. The mid-crustal earthquake of 1973 March 9 (mb= 5.5, h≤ 20 km) located 60 km south-west of Sydney, Australia, provides unambiguous evidence of contemporary thrust faulting in South-eastern Australia — a region of high heat flow and Cenozoic basaltic volcanism. Aftershock locations suggest a steeply dipping fault in the depth range from 8 to 24 km with a lateral extent of about 8 km. The mechanism solution is consistent with a tectonic stress field that is dominated by east—west horizontal compression. A seismic moment of 5.7 ± 1023± 20 per cent dyne-cm was computed from surface-wave amplitudes. Minimum values of slip and stress drop, 2 cm and 1 bar respectively, were estimated from the moment and a fault size taken’ from aftershock locations. Refinement modelling by a controlled Monte Carlo technique was used to provide unbiased models directly from multimode group velocities. The dispersion of fundamental and higher mode surface waves recorded at the digital high-gain station at Charters Towers, Queensland, and the WWSSN station at Adelaide, South Australia, is satisfied by crust- and upper-mantle models which have neither pronounced S-wave low-velocity zones nor thick high-velocity lids within 140 km of the Earth's surface. These models have subcrustal shear velocities of 4.20–4.32 km/s which are 0.4–0.5 km/s slower than Canadian shield shear velocities (CANSD).