high-Mg Adakites Research Articles

Geochemistry and U-Pb zircon geochronology of Late-Mesozoic lavas from Xishan, Beijing

Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of the Tiaojishan Formation and the Donglintai Formation from Xishan, Beijing, reveal that ages of upper lavas of Tiaojishan Formation and Middle of Donglintai Formation are 137.1±4.5 Ma(2σ) and 130－134 Ma, respectively. The fomer is slightly older than the latter and the age difference between these two formations is less than 5 Ma. These lines of evidence prove that the two volcanoes erupted within a short time. The age of the Tiaojishan Formation from Xishan, Beijing is distinctively different from that of the Chende Basin. This indicated that the ages of Tiaojishan lavas varied in different regions. The Tiaojishan Formation consists of typical adakite (SiO 2 =56%, Na 2 O = 3.99－6.17, Na 2 O/K 2 O = 2.2－3.1, Sr = 680－1074´10 -6 , Y = 13.2－16.3´10 -6 , Yb = 1.13－1.52´10 -6 , Sr/Y = 43－66), high-Mg adakite and high-Mg andesite (Mg# = 54－55). Features of continental crust of adakite from the Tiaojishan Formation and its syngeneric middle silicic vocanic rocks, such as typical Nd-Ta negative abnormality and Pb possive abnormality, indicate that these lavas are originated from partial melts of continental crust. These results suggest that the adakite from the Tiaojishan Formation of Xishan, Beijing derived from thickened eclogitic lower crust and lithosphere beneath the North China craton at mesozoic that was foundered into the aesthenosphere, and subsequenctly partially melted and interacted with mantle olivine during melts upward migration. The age of lavas from the Tiaojishan Formation restrained the foundation which should last at least until 137 Ma. Lavas of the Donglintai Formation are rhyolith and andesite with normal Mg# and thus they did not interact with the mantle. These lavas represent remobilized melts of lower crust material caused by mantle aesthenosphere upwelling migration induced by foundation.

Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths

Peridotite xenoliths from the Quaternary Cerro del Fraile basalts, southernmost South America, sample the mantle less than 25 km east of the Andean Austral Volcanic Zone (AVZ), an arc segment characterized by melting of a young, ‘hot’, subducted slab and the eruption of adakites. Many of these peridotite xenoliths are modified by either modal and/or cryptic Na-rich metasomatism, which produced elevated Sr/Y, La/Yb and La/Nb ratios typical of slab melts. Some of the metasomatized xenoliths, derived from a relatively deep and hot portion of the mantle, contain an interconnected network along mineral grain boundaries of high-Mg#, low-Y andesitic glass with major and trace element composition similar to the high-Mg adakites erupted in the AVZ. We interpret this adakitic glass to be a quenched slab melt that has infiltrated the mantle wedge from below. The texture and chemistry of this quenched melt and surrounding mantle minerals suggest that selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine is an important process in producing high-Mg adakites from primary low-Mg slab melts.