Growth of upper plate lithosphere controls tempo of arc magmatism: Constraints from Al-diffusion kinetics and coupled Lu-Hf and Sm-Nd chronology

Most magmatism occurs at mid-ocean ridges, where plate divergence leads to decompression melting of the mantle, and at volcanic arcs, where subduction leads to volatile-assisted decompression melting in the hot mantle wedge. While plate spreading and subduction are continuous, arc magmatism, particularly in continental arcs, is characterised by > 10-50 Myr intervals of enhanced magmatic activity followed by rapid decline (DeCelles et al., 2009). In some cases, such as the Andes, this pattern has recurred several times (Haschke et al., 2002). Abrupt changes in plate convergence rates and direction (Pilger, 1984) or repeated steepening and shallowing of subducting slabs (Kay and Coira, 2009) have been suggested as triggering flare-ups or terminating magmatism, but such scenarios may not be sufficiently general. Here, we examine the thermal history of deep crustal and lithospheric xenoliths from the Cretaceous Sierra Nevada batholith, California (USA). The deepest samples (similar to 90 km), garnet-bearing spinel peridotites, show cooling-related exsolution of garnet from high-Al pyroxenes originally formed at > 1275 degrees C. Modelling of pyroxene Al diffusion profiles requires rapid cooling from 1275 to 750 degrees C within similar to 10 Myr. Also suggesting deep-seated, rapid cooling is a garnet websterite from similar to 90 km depth with nearly identical Lu-Hf (92.6 +/- 1.6 Ma) and Sm-Nd (88.8 +/- 3.1 Ma) isochron ages to within error. Thermal modelling shows that this cooling history can be explained by impingement of the base of the Sierran lithosphere against a cold subducting slab at similar to 90 km depth, precluding cooling by shallowing subduction. Rather, the coincidence of the radiometric ages with the magmatic flare-up (120-80 Ma) suggests that the hot mantle wedge above the subducting slab may have been pinched out by magmatic (+/- tectonic) thickening of the upper plate, eventually terminating mantle melting. Magmatic flare-ups in continental arcs are thus self-limiting, which explains why continental arc magmatism occurs in narrow time intervals. Convective removal of the deep arc lithosphere can initiate another magmatic cycle.