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The lithium cycling in the supra-subduction mantle wedge is crucial for understanding the generation of Li-rich magmas that may potentially source ore deposition in continental arcs. Here, we look from the mantle source perspective at the geological processes controlling the Li mobility in convergent margins, by characterizing a set of sub-arc mantle xenoliths from the southern Andes (Coyhaique, western Patagonia). The mineral trace element signatures and oxygen fugacity estimates (FMQ > + 3) in some of these peridotite xenoliths record the interaction with arc magmas enriched in fluid-mobile elements originally scavenged by slab dehydration. This subduction-related metasomatism was poorly effective on enhancing the Li inventory of the sub-arc lithospheric mantle, underpinning the inefficiency of slab-derived fluids on mobilizing Li through the mantle wedge. However, major and trace element compositions of mantle minerals in other xenoliths also record transient thermal and chemical anomalies associated with the percolation of slab window-related magmas, which exhibit an "adakite"-type geochemical fingerprint inherited by slab-derived melts produced during ridge subduction and slab window opening event. As these melts percolated through the shallow (7.2–16.8 kbar) and hot (952–1054 °C) lithospheric mantle wedge, they promoted the crystallization of metasomatic clinopyroxene having exceptionally high Li abundances (6–15 ppm). Numerical modeling shows that low degrees (< 10%) of partial melting of this Li-rich and fertile sub-arc lithospheric mantle generates primitive melts having two-fold Li enrichment (~13 ppm) compared with average subduction-zone basalts. Prolonged fractional crystallization of these melts produces extremely Li-enriched silicic rocks, which may stoke the Li inventory of mineralizing fluids in the shallow crust. [ABSTRACT FROM AUTHOR]

Complex zoning in crystals including repeated resorption and overgrowth is characteristic for arc magmas and occurs in response to closed-system changes in magmatic P–T– f O2 conditions and open system processes such as magma mixing and degassing or regassing. However, over which time frame do such changes occur? Do zoning patterns record changes occurring during the polybaric ascent of magmas that carry crystals or glomerocrysts sourced from variably mushy magma reservoirs, or alternatively indicate the uptake of antecrysts that experienced long periods of cold storage in plutonic precursors? A priori , these scenarios are endmember models, with the former transcrustal magmatic systems, where the crystals record changing conditions during magma ascent or changing interstitial melt compositions, traditionally preferred over the latter, which we here term transcrustal plutonic systems. In subsolidus plutonic systems, aphyric parental melts would acquire their entirely antecrystic crystal cargo during ascent from plutonic protoliths, and only crystal rims may be related to the host magma. We discuss the evidence for dominantly plutonic antecrystic cargo in some continental arc magmas, identified by considering mineral phase proportions, hydration of crystal rims that indicate hydrothermally altered cargo picked up by fresh melts, and uranium isotope disequilibria between crystals and matrix. We then turn to two-pyroxene thermobarometry and review the evidence for plutonic antecryst dominance revealed by this method in southwest Japan and the southern Taupo Volcanic Zone. We provide additional two-pyroxene data from the Andes, the Cascades, and the Tatun Volcano Group in northern Taiwan, corroborating that the uptake of crystals by aphyric to scarcely phyric melts is prevalent in continental arc magmatic systems. Thus, in many cases transcrustal plutonic systems seem to dominate, implying that a significant proportion of parental melts of continental arc magmas are variably enriched in silica, too hot to carry crystals, and typically too hot and not hydrous enough to be generated by differentiation in frequently postulated lower crustal hot zones, as we will demonstrate here. Our data indicate that in continental subduction zones, the mantle wedge is the source of a diversity of melt compositions (low- to high-silica), irrespective of the age and temperature of the subducting slab. Before discussing some of the implications of the prevalence of non-canonical transcrustal plutonic systems for the thermal structure of the crust, magma ascent processes, volcano monitoring, economic geology, as well as the evolution of continental crustal growth and recycling through deep time, we critically evaluate this novel perspective in terms of published data that might favour more traditional supersolidus transcrustal magmatic systems. This contribution provides the community with the opportunity to consider significantly colder crustal environments than typically accepted, and outlines avenues of future research. [ABSTRACT FROM AUTHOR]

The Nazca-South America subduction zone in Ecuador is characterized by a complicated along-strike geometry as the slab transitions from flat slab subduction in the south, with the Peruvian flat slab, to what has been characterized as 'normal' dipping subduction beneath central Ecuador. Plate convergence additionally changes south to north as the trench takes on a convex shape. Highly heterogeneous bathymetry at the trench, including the aseismic oceanic Carnegie Ridge (CR), and sparse intermediate-depth seismicity has led many to speculate about the behaviour of the downgoing plate at depth. In this study, we present a finite-frequency teleseismic P -wave tomography model of the northern Andes beneath Ecuador and Colombia from 90 to 1200 km depth. Our model builds on prior tomography models in South America by adding relative traveltime residuals recorded at stations in Ecuador. The complete data set is comprised of 114 096 relative traveltime residuals from 1133 stations across South America, with the added data serving to refine the morphology of the Nazca slab in the mantle beneath the northern Andes. Our tomography model shows a Nazca slab with a fragmented along-strike geometry and the first teleseismic images of several proposed slab tears in this region. At the northern edge of the Peruvian flat slab in southern Ecuador, we image a shallow tear at 95–200 km depth that appears to connect mantle flow from beneath the flat slab to the Ecuadorian Arc. Beneath central Ecuador at the latitudes of the CR, the Nazca slab is continuous into the lower mantle. Beneath southern Colombia, the Malpelo Tear breaks the Nazca slab below ∼200 km depth. [ABSTRACT FROM AUTHOR]

The style of overriding plate deformation at subduction zones varies from backarc spreading and basin formation, as found in the Scotia Sea, to shortening and cordilleran orogeny, as observed in the Andes. Why this difference exists and why overriding plate extension occurs more frequently than shortening remains unexplained. In this contribution, various conceptual mechanisms of overriding plate deformation are reviewed and assessed. Time-evolving numerical models of buoyancy-driven subduction in three-dimensional space are presented to investigate the influence of subduction zone size and time on the style and rate of overriding plate deformation, and, more generally, on the geometry, kinematics and dynamics of subduction. The models demonstrate how time (temporal evolution, and its related parameter subduction depth) and subduction zone size (width, trench-parallel extent) control deformation style. Cordilleran orogeny occurs only for wide, mature subduction zones during subduction into the lower mantle, while narrow and intermediate-width subduction zones, and wide but young subduction zones, are characterized by overriding plate extension, even during lower mantle slab penetration for narrow and intermediate-width subduction zones and for subduction zone edges of wide subduction zones during early stage lower mantle penetration. This difference in deformation style is explained by the fact that subduction zone size, time and slab depth also control subduction kinematics, slab geometries, slab sinking directions, and mantle flow patterns. Narrow slabs, intermediate slabs, slab edges and short wide slabs generally have an important, slab-pull-driven, backward (oceanward) sinking component resulting in trench retreat and slab rollback, inducing strong local return flow patterns in the upper mantle close to the slab (toroidal for the narrow slabs, intermediate-width slabs and slab edges). The combination of strong local return flow near the slab and slab-pull-driven trench retreat induces deviatoric tension in the overriding plate through trenchward-increasing basal drag and subduction interface deviatoric tension, causing extension. In contrast, a wide slab, in particular that of an old subduction zone, generally sinks forward and subduction occurs mostly through trenchward subducting plate motion, inducing a strong whole-mantle asymmetric poloidal flow cell. The combination of strong poloidal flow-induced basal tractions at the base of the overriding plate that decrease trenchward and subduction interface deviatoric compression due to trenchward overriding plate motion and a relatively immobile subduction hinge induces deviatoric compression in the overriding plate, causing shortening, which is most pronounced in the centre of the wide subduction zone. Comparison between a global review of active subduction zones and the numerical modelling results shows consistent positive correlations for subducting plate velocity and negative correlations for trench velocity, overriding plate deformation rate and slab dip as a function of subduction zone size. Ultimately, the geodynamic models and subduction zone review provide a new explanation for the diversity of overriding plate deformation as exemplified by two endmembers, the small Scotia subduction zone (with backarc spreading) and large South American subduction zone (with shortening), for the non-uniform spatial and temporal occurrence of different deformation styles, and for the more frequent occurrence of active overriding plate extension compared to active overriding plate shortening on Earth. [ABSTRACT FROM AUTHOR]