Episodic construction of the early Andean Cordillera unravelled by zircon petrochronology

The subduction of oceanic plates beneath continental lithosphere is responsible for continental growth and recycling of oceanic crust, promoting the formation of Cordilleran arcs. However, the processes that control the evolution of these Cordilleran orogenic belts, particularly during their early stages of formation, have not been fully investigated. Here we use a multi-proxy geochemical approach, based on zircon petrochronology and whole-rock analyses, to assess the early evolution of the Andes, one of the most remarkable continental arcs in the world. Our results show that magmatism in the early Andean Cordillera occurred over a period of ~120 million years with six distinct plutonic episodes between 215 and 94 Ma. Each episode is the result of a complex interplay between mantle, crust, slab and sediment contributions that can be traced using zircon chemistry. Overall, the magmatism evolved in response to changes in the tectonic configuration, from transtensional/extensional conditions (215–145 Ma) to a transtensional regime (138–94 Ma). We conclude that an external (tectonic) forcing model with mantle-derived inputs is responsible for the episodic plutonism in this extensional continental arc. This study highlights the use of zircon petrochronology in assessing the multimillion-year crustal scale evolution of Cordilleran arcs.

Las Cabras volcano, Michoacán-Guanajuato Volcanic Field, México: Topographic, climatic, and shallow magmatic controls on scoria cone eruptions

Scoria cones are abundant in most volcanic fields on Earth, such as the Michoacán-Guanajuato Volcanic Field, in the central-western sector of the Trans-Mexican Volcanic Belt. However, there are few in-depth studies on their eruptive style and controlling factors, despite of their diversity in shape and composition which implies a wide range of hazards. Here, we present results of morphologic, stratigraphic, sedimentary, petrographic, and geochemical studies of the prominent Las Cabras scoria cone located west of the Zacapu lacustrine basin in the center of the Michoacán-Guanajuato Volcanic Field. This basaltic andesitic to andesitic volcano formed between 27 and 26 kyrs BP on the steep slopes (>10º) of the lava shield of El Tule volcano. Over time, its dominant eruptive style changed from Strombolian to effusive. Initial explosive activity built a 170-m-high scoria cone and deposited thick tephra fallout on the surrounding sloping terrain. Structures in the deposits indicate that early friable fine-grained tephra underwent significant erosion due to syn-eruptive heavy rain coupled with the sloping nature of the underlying ground. This erosion generated lahars that very likely reached the Zacapu lake based on the pre-eruptive topography. As the explosivity dropped, lava was emitted from the base of the cone first to the S and SE, forming a thick, viscous lobe that filled a pre-existing E-W valley. The flow direction then deviated to the N and NE, to form thinner, less-viscous lobes fed from the vent by an open-channel. The lavas are covered by hummocks made of agglutinates and bombs that indicate that the eruption terminated by catastrophic collapse of the SE sector of the cone, possibly triggered by the intrusion of magma within the cone, which destabilized its downslope segment. The sudden flank failure was potentially associated with a late effusive event and the hummocks may have been carried away by the lava surge. Whole-rock chemical variations and crystal disequilibrium textures point toward a complex magma feeding system, involving mixing and mingling between different magma batches. This study shows that the formation of scoria cones on a terrain with a marked slope (>10°) has profound impacts on the eruption dynamics and related hazards due to its effect on cone stability and ash erosion. It also evidences the erosive effect of syn-eruptive rain on fine-grained tephra, especially when deposited on a slope. Finally, it reveals the complex magmatic processes that may occur in the shallow plumbing system of monogenetic andesitic volcanoes, which could be particularly important in inland areas of continental arcs.

Global chemical weathering dominated by continental arcs since the mid-Paleozoic

Earth’s plate tectonic activity regulates the carbon cycle, and hence, climate, via volcanic outgassing and silicate-rock weathering1,2,3. Mountain building, arc-continent collisions, and clustering of continents in the tropics have all been invoked as controlling the weathering flux4,5,6, with arcs also acting as a major contributor of carbon dioxide (CO2) to the atmosphere7. However, these processes have largely been considered in isolation when in reality they are all tightly coupled. To properly account for the interactions between these processes, and the inherent multi-million-year time lags at play in the Earth system, we need to characterise their complex interdependencies. Here we analyse these interdependencies over the past 400 million years, using a Bayesian network to identify primary relationships, time lags and drivers of the global chemical weathering signal. We find that the spatial extent of continental volcanic arcs — the fastest-eroding surface features on Earth — exerts the strongest control on global chemical weathering fluxes. We find that the rapid drawdown of CO2 tied to arc weathering stabilises surface temperatures over geological time, contrary to the widely held view that this stability8 is achieved mainly by a delicate balance between weathering of the seafloor and the continental interiors.

Lithium systematics in global arc magmas and the importance of crustal thickening for lithium enrichment

Much of the world’s Li deposits occurs as basinal brines in magmatic orogens, particularly in continental volcanic arcs. However, the exact origin of Li enrichment in arc magmatic systems is not clear. Here, we show that, globally, primitive arc magmas have Li contents and Li/Y ratios similar to mid-ocean ridge basalts, indicating that the subducting slab has limited contribution to Li enrichment in arc magmas. Instead, we find that Li enrichment is enhanced by lower degrees of sub-arc mantle melting and higher extents of intracrustal differentiation. These enrichment effects are favored in arcs with thick crust, which explains why magmatism and differentiation in continental arcs, like the Andes, reach greater Li contents than their island arc counterparts. Weathering of these enriched source rocks mobilizes and transports such Li into the hydrologic system, ultimately developing Li brines with the combination of arid climate and the presence of landlocked extensional basins in thickened orogenic settings.

Eruptive history and 40Ar/39Ar geochronology of the Milos volcanic field, Greece

High-resolution geochronology is essential to determine the growth-rate of volcanoes, which is one of the key factors to establish the periodicity of explosive volcanic eruptions. However, there are less high-resolution eruptive histories (> 106 years) determined for long-lived submarine arc volcanic complexes than for subaerial complexes, since the submarine volcanoes are far more difficult to observe than subaerial ones. In this study, high-resolution geochronology and major element data are presented for Milos Volcanic Field (VF) in the South Aegean Volcanic Arc, Greece. The Milos VF has been active for over 3 Myrs, and the first two million years of its eruptive history occurred in a submarine setting that has emerged above sea level nowadays. The long submarine volcanic history of the Milos VF makes it an excellent natural laboratory to study the growth-rate of a long-lived submarine arc volcanic complex. This study reports twenty-one new high-precision 40Ar/39Ar ages and major element compositions for eleven volcanic units of the Milos VF. This allows us to refine the volcanic evolution of Milos into nine phases and five volcanic quiescence periods of longer than 200 kyrs, on the basis of age, composition, volcano type and location. Phase 1–5 (~ 3.34–1.60 Ma) contributed ~ 85 % by volume to the Milos VF, whereas the volcanoes of Phase 6–9 only erupted small volumes (2–6 km3 in DRE) rhyolitic magmas. Although there are exceptions of the felsic cone volcanoes of Phase 1–2, in general the Milos VF becomes more rhyolitic in composition from Phase 1 to Phase 9. In particular, the last three phases (Phase 7–9) only contain rhyolites. Moreover, the high-resolution geochronology suggests that there are at least three periods of different long term volumetric volcanic output rate (Qe). In the Milos VF, the Qe varies between 0.2 and 6.6 × 10−5 km3 yr−1, 2–3 orders of magnitude lower than the average for rhyolitic systems and continental arcs.

Interplay of Cretaceous transpressional deformation and continental arc magmatism in a long-lived crustal boundary, central Fiordland, New Zealand

Recovering the time-evolving relationship between arc magmatism and deformation, and the influence of anisotropies (inherited foliations, crustal-scale features, and thermal gradients), is critical for interpreting the location, timing, and geometry of transpressional structures in continental arcs. We investigated these themes of magma-deformation interactions and preexisting anisotropies within a middle- and lower-crustal section of Cretaceous arc crust coinciding with a Paleozoic boundary in central Fiordland, New Zealand. We present new structural mapping and results of Zr-in-titanite thermometry and U-Pb zircon and titanite geochronology from an Early Cretaceous batholith and its host rock. The data reveal how the expression of transpression in the middle and lower crust of a continental magmatic arc evolved during emplacement and crystallization of the ∼2300 km2 lower-crustal Western Fiordland Orthogneiss (WFO) batholith. Two structures within Fiordland’s architecture of transpressional shear zones are identified. The gently dipping Misty shear zone records syn-magmatic oblique-sinistral thrust motion between ca. 123 and ca. 118 Ma, along the lower-crustal WFO Misty Pluton margin. The subhorizontal South Adams Burn thrust records mid-crustal arc-normal shortening between ca. 114 and ca. 111 Ma. Both structures are localized within and reactivate a recently described >10 km-wide Paleozoic crustal boundary, and show that deformation migrated upwards between ca. 118 and ca. 114 Ma. WFO emplacement and crystallization (mainly 118–115 Ma) coincided with elevated (>750 °C) middle- and lower-crustal Zr-in-titanite temperatures and the onset of mid-crustal cooling at 5.9 ± 2.0 °C Ma−1 between ca. 118 and ca. 95 Ma. We suggest that reduced strength contrasts across lower-crustal pluton margins during crystallization caused deformation to migrate upwards into thermally weakened rocks of the mid-crust. The migration was accompanied by partitioning of deformation into domains of arc-normal shortening in Paleozoic metasedimentary rocks and domains that combined shortening and strike-slip deformation in crustal-scale subvertical, transpressional shear zones previously documented in Fiordland. U-Pb titanite dates indicate Carboniferous–Cretaceous (re)crystallization, consistent with reactivation of the inherited boundary. Our results show that spatio-temporal patterns of transpression are influenced by magma emplacement and crystallization and by the thermal structure of a reactivated boundary.