Structure and growth of the Izu‐Bonin‐Mariana arc crust: 2. Role of crust‐mantle transformation and the transparent Moho in arc crust evolution

Abstract

Evolution of arc crust and subarc mantle in the Izu‐Bonin‐Mariana (IBM) intraoceanic arc‐trench system is examined by petrological modeling of arc magma generation and differentiation. Characteristic seismic structural features of the IBM arc highlighted in this modeling include the presence of (1) a middle crust with a P wave velocity (Vp) of 6.0–6.5 km/s, (2) a 6.5–6.8 km/s Vp layer at the top of the lower crust, (3) a high‐velocity (Vp = 6.8–7.2 km/s) lower crust, and (4) an uppermost mantle exhibiting rather low velocities (Vp = 7.4–7.7 km/s). The formation of the middle crust, which is considered to have an intermediate composition, is examined by (1) the mantle‐derived basalt model including anatexis of the initial mafic lower crust or mixing of mantle‐melting‐derived basaltic magma with crust‐melting‐derived felsic magma and (2) the mantle‐derived andesite model including differentiation of boninitic magma. The Vp calculated for the inferred compositions for each layer of the IBM crust on the basis of the basalt model is consistent with the observed values, whereas the andesite model cannot account for the characteristic Vp of the middle crust, the uppermost lower crust, and the uppermost mantle. The results further suggest that the volume of mafic restite and cumulates that are “crustal residues” resulting from the evolution of middle and upper arc crust is, at least, 3 to 9 times greater than that of the seismically defined IBM lower crust. One possible explanation to overcome this apparent dilemma is that the mafic to ultramafic crustal components are transformed to subarc mantle. During this process the subarc Moho is chemically transparent and permeable to the crustal components. This crust‐mantle transformation could play the major role in the creation of mature arc crust with intermediate compositions similar to continental crust.

Yin and yang of continental crust creation and destruction by plate tectonic processes

“…Much of this seems to have disappeared, most likely by foundering into the upper mantle. Tatsumi et al (2008) concluded that middle Izu-Bonin-Mariana arc crust of intermediate composition was generated from mantle-derived basalts via complex multistage magmatic processes. These include anatexis of amphibolite-facies mafic lower crust to generate felsic melts and mixing of felsic melts with mantle-derived basaltic magma ( Figure 6).…”

“…magmatically adds to pre-existing continental crust, such as that beneath the Andes. Juvenile continental crust is mostly extracted from the mantle as mafic (basaltic or boninitic) magma, which must be further refined by anatectic remelting of the crust and delamination (foundering) to yield andesitic material approximating the bulk composition of continental crust (Rudnick and Gao 2003;Tatsumi 2005;Tatsumi et al 2008). Although juvenile continental crust is likely to be mafic at first, it inherits from subduction zone processes the distinctive geochemical characteristics of subduction-related magmas: enrichment in silica and LIL elements (e.g.…”

“…Petrologic modelling to explain the composition of arc lavas indicates that more lower crust -composed of olivine-and pyroxene-rich cumulates -should have been produced than is observed from geophysical imaging. Tatsumi et al (2008) concluded that on average 160% as much subcrustal pyroxene-rich materials as juvenile crust underlies intra-oceanic arcs ( Figure 6). Much of this seems to have disappeared, most likely by foundering into the upper mantle.…”

“…Formation and refinement of juvenile arc crust, like that of Izu-Bonin-Mariana. (A) Model of arc crust evolution from Tatsumi et al (2008). Incipient arc magmatism replaces the pre-existing oceanic crust to create the initial mafic arc crust.…”

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

“…The origin of I-type granitoids in arcs is very much discussed, and proposed models can be grouped in two endmember processes: partial melting of amphibolites in the lower arc crust (e.g., Clemens 1990;Clemens and Vielzeuf 1987;Tatsumi et al 2008Tatsumi et al , 2009 or hydrous high-to medium-pressure magma differentiation with limited amount of assimilation in the lower crust (e.g., Davidson et al 2007;Jagoutz et al 2009;Sisson et al 2005). We have previously discussed the origin of the Kohistan arc granitoids based on a more limited dataset and from the perspective of processes in the lower arc crust exposed in the Southern Plutonic Complex and the Chilas Complex (Jagoutz 2010;Jagoutz et al 2009Jagoutz et al , 2011.…”