ALBERT

Description: Global compilations indicate that the geochemistry of arc magmatism is sensitive to Moho depth. Magmatic products are prevalent throughout the history of Cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes. We investigate temporal variations in crustal thickness in the United States Cordillera using Sr/Y from intermediate continental arc magmas. Our results suggest that crustal thickening began during the Late Jurassic to Early Cretaceous and culminated with 55–65-km-thick crust at 85–95 Ma. Crustal thicknesses remained elevated until the mid-Eocene to Oligocene, after which time crustal thicknesses decreased to 30–40 km in the Miocene. The results are consistent with independent geologic constraints and suggest that Sr/Y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems.

Description: Temporal and spatial patterns in the architecture of the Franciscan Complex provide valuable insights into the subduction processes through which such patterns arise. The Nacimiento Franciscan belt is an allochthonous sliver of subduction assemblages in the central California Coast Ranges displaced either: (1) from southern California by 〉300 km of Neogene dextral slip along the San Andreas fault system or (2) from central California to southern California and back again, by 〉500 km of Late Cretaceous–Paleocene sinistral slip along the Sur-Nacimiento fault followed by San Andreas–related motion. New U-Pb detrital zircon data from 20 (meta)clastic samples indicate that the Nacimiento Franciscan section was assembled between ca. 95 and 80 Ma. Abundant Cretaceous (particularly Late Cretaceous) and diminishing amounts of Jurassic and Proterozoic zircon grains point to a southern California origin for Nacimiento Franciscan protoliths, precluding significant sinistral strike-slip along the Sur-Nacimiento fault. Furthermore, the suite of detrital zircon ages reported here bears a strong resemblance to new and existing data from subduction complexes in southern California that were emplaced during Laramide shallow subduction (i.e., Sierra de Salinas, Portal Ridge, Quartz Hill, Rand, San Emigdio, and Tehachapi schists). Hence, the Nacimiento Franciscan is distinct from Franciscan rocks in central and northern California and more likely represents an outboard element of the Late Cretaceous southern California low-angle subduction system. Upon restoring the Nacimiento block to its Late Cretaceous position, an inboard-younging trend is apparent in the composite Nacimiento–southern California schist belt, suggesting that progressively younger accretionary materials were underplated farther inboard by tectonic erosion. We posit that arc and forearc elements absent from southern California were removed by a combination of physical and tectonic erosion attending shallow subduction, interleaved in the subduction complex, and recycled into the mantle. Steepening of the Laramide slab was marked by a phase of crustal extension in the overriding plate. During this phase, the Sur-Nacimiento fault likely functioned as a segment of a low-angle normal fault system spanning the southern Sierra Nevada batholith to the Nacimiento accretionary system.

Description: Newly discovered xenoliths within Pliocene and Quaternary intermediate volcanic rocks from southern Peru permit examination of lithospheric processes by which thick crust (60–70 km) and high average elevations (3–4 km) resulted within the Altiplano, the second most extensive orogenic plateau on Earth. The most common petrographic groups of xenoliths studied here are igneous or meta-igneous rocks with radiogenic isotopic ratios consistent with recent derivation from asthenospheric mantle ( 87 Sr/ 86 Sr = 0.704–0.709, 143 Nd/ 144 Nd = 0.5126–0.5129). A second group, consisting of felsic granulite xenoliths exhibiting more radiogenic compositions ( 87 Sr/ 86 Sr = 0.711–0.782, 143 Nd/ 144 Nd = 0.5121–0.5126), is interpreted as supracrustal rocks that underwent metamorphism at ~9 kbar (~30–35 km paleodepth, assuming a mean crustal density of 2.8 g/cm 3 ) and ~750 °C. These rocks are correlated with nonmetamorphosed rocks of the Mitu Group and assigned a Mesozoic (Upper Triassic or younger) age based on detrital zircon U-Pb ages. A felsic granulite Sm-Nd garnet whole-rock isochron of 42 ± 2 Ma demonstrates that garnet growth took place in Eocene time. Monazite grains associated with quenched anatectic melt networks in the same rocks yield ion microprobe U-Pb ages ranging from 3.2 ± 0.2 to 4.4 ± 0.3 Ma (2). These disparate geochronologic data sets are reconciled by a model wherein Mesozoic cover rocks were transferred to 〉30 km depth beneath the plateau in the Eocene and progressively heated until at least Pliocene time. Isothermal decompression and partial melting ensued as these rocks were entrained as xenoliths in volcanic host magmas and transported toward the surface. Mafic granulites and peridotites from the same xenolith suite comprise the basement of the metasedimentary sequence, exhibiting isotopic characteristics of Central Andean crust. Calculated equilibrium pressures for these basement rocks are 〉11 kbar, suggesting that the basement-cover interface lies beneath the northernmost Altiplano at ~30–40 km below the surface. Together, these results indicate that crustal thickening under the northernmost Altiplano started earlier than major latest Oligocene and Miocene uplift episodes affecting the region and was coeval with a flat slab–related regional episode of deformation. Total shortening must have been at least 20% more than previous estimates in order to satisfy the basement to cover depth constraints provided by the xenolith data. Sedimentary rocks at 〉30 km paleodepth require that Andean basement thrusts decapitated earlier Triassic normal faults, trapping Paleozoic and Mesozoic rocks below the main décollement. Magma loading from intense Cenozoic plutonism within the plateau probably played an additional role in transporting Mesozoic cover rocks to 〉30 km and thickening the crust beneath the northern Altiplano.

Description: The Sierra Valle Fértil Complex of west–central Argentina represents a section of the Ordovician (~470 Ma) Famatinian arc and exposes a continuous, tilted crustal arc section ranging in depth from ~12 to 32 km (~4–8 kbar pressure). This arc section exposes the complete compositional architecture from ultramafic and mafic rocks to upper crustal granodiorites. Field and compositional data are presented to document the deep (~6–8 kbar) mafic complex of the Sierra Valle Fértil. The mafic complex is composed of many tens to hundreds of plutonic cumulate bodies in a complex and non-regular arrangement. There is no simple compositional, kinematic or age relationship between neighboring plutons throughout the section, as expressed by cumulate compositions, emplacement horizon, size, composition, texture or style of contact. Amphibole gabbronorites and mafic tonalites dominate, but norites, amphibole websterites, troctolites and minor anorthosites are present. Amphibole is common but always as a replacement phase, and is never observed undergoing subsequent dehydration melting. Hence there is no evidence that voluminous tonalites were produced by dehydration melting of mafic precursors. A field-based, cumulate-removal fractionation model is presented that produces the observed compositional variations in five steps. Isotopic compositions of Sr and Nd deviate significantly from primitive mantle values, indicating a crustal contribution; however, this hybridization appears to have played a minor role in the major element evolution of the mafic complex. We interpret this isotopic and elemental decoupling as a byproduct of prolonged, punctuated MASH (melting, assimilation, storage, homogenization) processes in the lower crust. Isotopes may be the only residual evidence of assimilation within the mafic zone. This requires that melt removal from the cumulates was extraordinarily efficient.

Description: The petrogenesis of calc-alkaline magmatism in the Famatinian arc is investigated in the central Sierra Valle Fértil, a major, lower to middle crustal section of the Early Ordovician active margin of West Gondwana. Large-scale field relationships show a gradual and continuous compositional variation of the plutonic sequence, ranging from olivine-bearing gabbronorites to hornblende- and biotite-bearing granodiorites. Distinctive lithostratigraphic units are, however, discernible as one compositional type of plutonic rock dominates over mappable areas. These results allow us to identify a continuous plutonic arc stratigraphy that progressively exposes shallower paleo-depths towards the east. At all the exposed levels, calc-alkaline plutonic rocks are volumetrically dominant, interrupted only by granulite-facies migmatites and leucogranites. The migmatites are interpreted to be refractory remnants of supracrustal sedimentary successions, whereas the peraluminous leucogranites have field relationships and chemical and isotopic compositions suggesting that they were produced via anatexis of metasedimentary packages. Mass-balance calculations predict that a parental gabbroic magma after progressive closed-system fractionation would crystallize about 80% of the original mass to yield a granodioritic daughter. Because the crystallizing mineral assemblage comprises hornblende and plagioclase, mass balance suggests a volume of residual amphibole-rich gabbroic rocks much larger than that observed, suggesting that differentiation is significantly driven by open-system processes. Indeed, the combination of field and petrographic observations with bulk-rock geochemistry and petrogenetic modeling demonstrates that most dioritic and tonalitic rocks are hybrids formed by either (1) bulk assimilation of metasedimentary materials into gabbroic magmas, or (2) multi-stage and complex interactions between gabbroic rocks and metasedimentary-derived leucogranitic melts. The source region of the granodioritic magmas is located at the transition zone between a tonalite-dominated intermediate unit and a granodiorite-dominated silicic unit. Typical granodiorites have a hornblende-bearing mineralogy, metaluminous chemical signature and isotopic compositions [ 87 Sr/ 86 Sr(T) = 0·7075–0·7100 and Nd (T) ~ –5·0] broadly overlapping those of the tonalites of the intermediate rock unit. These major compositional features of the granodiorites can be best explained if three end-member components contribute to their generation. As field observational data suggest, primitive gabbroic rocks, metaluminous intermediate magmas and anatectic leucogranitic melts mixed to produce the calc-alkaline granodiorites; however, the exact petrological process generating the granodioritic magmas is unclear because the mafic end-member may have been incorporated as mafic inclusions in the intermediate magmas or as syn-magmatic dikes, or both. The polygenetic nature of the intermediate to silicic plutonic rocks, along with the preponderance of parental gabbroic rocks at the inferred base of the plutonic column, suggests an upward growth of the intermediate to silicic crust that involved the complete reconstitution of the pre-existing crustal configuration. The main implication of this study is that intermediate and silicic plutonic rocks in the Valle Fértil section formed within a crustal column in which the mass transfer and heat input of mantle-derived magmas promoted fusion of fertile metasedimentary rocks and favored mixing of gabbroic or dioritic magmas with crustal granitic melts. Our results lend support to models asserting that the thermal and material budget of arc magmatism is primarily governed by the rate at which mafic magmas ascend from their mantle sources and intrude repeatedly into the crust.

Description: The world's biggest Phanerozoic magmatic arcs formed above subduction zones and comprise the products of continuous magma emplacement into the crust over periods of up to 500 My. However, the intensity of magmatic activity can vary significantly. Punctuated magmatic events lasting from 5 to 20 My can dwarf the volume of magmas generated through the remainder of an arc's history: these high-volume events are called "flare-ups" and can completely rebuild an arc's crust. In arcs formed on continental lithosphere, flare-ups typically correlate with regional structural events that shorten and/or thicken the crust. Geochemical and isotopic signatures show that these high magmatic addition rate events involve ~50% recycled upper-plate crust and mantle lithosphere; the remaining ~50% comes from the mantle wedge.

Description: In this issue of Elements we explore the characteristics, potential causes, and implications of episodic magmatism in arcs. A comparison of U–Pb bedrock and detrital zircon ages in arcs with independent calculations of volumetric magma addition rates (MARs) indicates that the former nicely track the episodic temporal histories of arc magmatism but not MARs. MAR estimates indicate that 100–1000 times more magmatism is added to continental arcs during flare-ups than during lulls and result in plutonic/volcanic ratios of 〉30/1. Episodic arc magmatism may result from external forcing on arc systems caused by events outside the arc and/or from internal cyclic processes driven by feedback between linked tectonic and magmatic processes within the arc. Along and across arc strike, changes and asymmetries in magmatic, tectonic, and geochemical histories provide important constraints for evaluating these poorly understood driving mechanisms.

Description: The ~151 Ma Guadalupe Igneous Complex (GIC) is a tilted, bi-modal intrusion that provides a rare view into the deeper, mantle-derived portions of a granitic pluton. Major oxide relationships show that GIC granitic rocks formed by in situ differentiation. Assimilation of sedimentary country rock is precluded, as GIC alumina saturation indices (ASI) are too low by comparison, while TiO 2 and P 2 O 5 contents disallow partial melting of metavolcanic lower/middle crust. In contrast, Rb-Sr systematics support in situ magmatic differentiation, as unaltered GIC whole rock samples fall on a single 151 Ma isochron (initial 87 Sr/ 86 Sr = 0.7036) matching zircon age dates ( Saleeby et al. 1989 ). Crystal/liquid segregation, though, was not continuous: mafic and felsic samples form discordant compositional trends, with a gap between 60–66% SiO 2 . We posit that crystal/liquid segregation is continuous between 50–60% SiO 2 , and leads to the genesis of intermediate composition liquids that are then too viscous to allow further continuous liquid segregation. Further crystal/liquid separation thereafter occurs discontinuously (at F 45–50%), to yield a mafic crystalline (52–59% SiO 2 ) residue and a silicic (70–75% SiO 2 ) liquid ( Bachmann and Bergantz 2004 ), which are, respectively, preserved in the Meladiorite and Granite/Granophyre units of the GIC. Outcrops in the gabbroic section support this view, where mafic crystalline layers feed directly into granitic dikes, and intermediate compositions are absent; mass balance calculations at the outcrop scale also support this model. It is unclear, though, to what extent this model applies to larger Sierran plutons; the smaller GIC may represent an end-member process, where rapid cooling limits mixing, due to rapid increases in mafic/felsic melt viscosity contrasts.