Melting experiments on hydrous low‐K tholeiite: Implications for the genesis of tonalitic crust in the Izu–Bonin – Mariana arc

Abstract

A series of water-de®cient partial melting experiments on a low-K tholeiite were carried out under lower crustal P±T±H 2 O conditions (900±1200°C, 0.7±1.5 GPa, 2 and 5 wt% H 2 O added) using a piston-cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classi®ed as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu±Bonin ± Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust ($ 50% melting at 1050°C and below 1.2 GPa). Estimated densities of pyroxenitic restites ($ 3.9 g/cm 3 ) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm 3 ). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900°C).

Petrogenesis of Ashigawa and Tonogi granitic intrusions, southern part of the Miocene Kofu Granitic Complex, central Japan: M-type granite in the Izu arc collision zone

“…Comparing the present model with melting experiments of various basaltic composition at relatively low pressures (<10kb) and variable H 2 O conditions, temperatures required for 40% melting of lower crustal rock ranges from 950˚C (10kb, 2% H 2 O, Fig. 2 of Nakajima and Arima, 1998) to 1125˚C (5kb, 1% H 2 O, Figure 3 of Springer and Seck, 1997).…”

“…These experiments suggest that tonalitic melts similar to the composition of TON 09 can be formed by substantial degrees of partial melting of tholeiitic basalt compositions over a wide range of temperatures (e.g. Beard and Lofgren, 1991;Wolf and Wyllie, 1994;Rapp and Watson, 1995;Nakajima and Arima, 1998). The Tanzawa plutonic complex, located south of the Ashigawa and Tonogi intrusions, consists of a tonalite suite and a gabbro suite.…”

“…The Tanzawa tonalite and gabbro were interpreted to have been formed by crystal fractionation and accumulation of a tonalitic parental magma (Kawate and Arima, 1998). Kawate and Arima (1998) and Nakajima and Arima (1998) suggested that the Tanzawa parental magma was derived by partial melting of the hydrous mafic lower crust of the Izu arc. The proposed Tanzawa parental magma composition (HO 3) is shown in Table 1 for comparison with the Ashigawa Tonogi parental magma composition (TON 09).…”

Seismological evidence for variable growth of crust along the Izu intraoceanic arc

“…This process can create an andesitic crust, but the total composition of the crust remains basaltic owing to the mafic composition of the lower crust. Delamination of the lower part of the crust would be necessary to return the lower mafic component to the mantle [Turcotte, 1989;Kay and Kay, 1993;Nakajima and Arima, 1998;Tatsumi, 2005]. An alternative process for the creation of andesitic crust in an intraoceanic arc setting was proposed by Kelemen [1995], whereby andesites form in the mantle through the interaction of rising silicic melt, derived from the subducted slab, with peridotitic rocks in the mantle wedge.…”

Collision Accretion Tectonics of the Proto-Izu-Mariana Arc: A Review

“…This strongly suggests that extensive tectonic erosion occurred, resulting in the coexistence of two rock units that were formed 100-150 km from each other horizontally, i.e., rocks on the volcanic front and rocks on the descending oceanic slab. Petrogenesis of juvenile crust and TTG rocks are being debate with two models: basaltic arc magma fractionation （Kawate and Arima, 1998） and delaminated mafic lower crust （Nakajima and Arima, 1998） . In the latter model, a difficulty is derived from density at the Moho depth and small size of delaminated residue.…”