Cordilleran Batholiths Research Articles

Contributing to understand how Cordilleran batholiths build from detailed geochronology of the Carboniferous Tabaquito batholith, Frontal Cordillera (Argentina)

Cordilleran Granitic batholiths or Andean/Cordilleran batholiths serve as plutonic expressions of continental arcs, offering valuable insights into the processes that operate in large silicic magmatic systems at subduction environments. This work presents a comprehensive geochronological study of the Carboniferous Tabaquito batholith in the Frontal Cordillera of western Argentina, which is the best exponent of the Carboniferous arc-related magmatism in this region. Using new data obtained through LA-MC-ICP-MS and comparing it with previously published data (LA-MC-ICP-MS and SHRIMP techniques), we identify five distinct magmatic events in the Tabaquito batholith: 325 ± 2 Ma; 332 ± 2 Ma; 337 ± 2 Ma; 346 ± 1 Ma, and 362 ± 2 Ma. The 325 Ma event is considered as the best estimate of the crystallization age during the emplacement. The 332 Ma, 337 Ma, 346 Ma and 362 Ma events suggest the presence of zircon antecrysts, implying a complex magmatic system that worked for a protracted time lapse of ca. 40 Myr, or ca. 15 Myr if we consider the ages of 346 and 362 Ma as xenocrysts from a source like the Potrerillos pluton (346 ± 3 Ma to 356 ± 3 Ma), which was interpreted in previous studies as possible source of the parental magmas of the Tabaquito batholith. Furthermore, it has been also identified inherited zircons with ages older than ca. 370 Ma that are consistent with the detrital zircon pattern obtained for the country rock. Inherited zircon ages from the referred country rock yield a maximum deposition age estimated at 387 Ma that is consistent with the fossil ammonoids record, indicating that the granitic rocks were emplaced in Devonian rocks.

Generation of magmatism under active continental margins: A thermodynamic study of subduction and translithospheric diapirs

A recent model of continental arc magmatism states that subducted sediments mix physically with the oceanic slab basalts in the subduction channel and produce diapirs that ascend through the mantle wedge, undergo melting and relaminate to the base of the continental crust. From here, melt or magma batches ascend through the crust, producing the Cordilleran batholiths and associated volcanism ranging in composition from 0 to 25 wt% maficity (FeO t + MgO) and 44–79 wt% SiO 2 , with a gap around 11.5 wt% maficity and 57–60 wt% SiO 2 . This model has been predicted by thermomechanical numerical studies and later supported by phase equilibrium experiments; however, thermodynamic modelling to verify whether the composition of the Cordilleran batholiths is reproduced by this model has not yet been carried out to complement the experimental approach. In this article, the evolution of the mélange along the subduction path and during the ascent of the diapirs is investigated, with focus on the conditions of generation of the diapirs, the composition and proportion of melt at the P-T conditions of relamination of the diapirs, the relationship between the basalt:sediment proportion (the composition) of the diapir to this melt composition and proportion and whether the composition of the melts match the Cordilleran batholiths. The key point to test is the process or processes that produce the compositional variability observed in these granitoids. Our findings indicate that (1) parental melts and/or magmas range in maficity from ∼1 to ∼11.5 wt%, with the compositional gap of the Cordilleran trend representing the most mafic composition possible of the parent, (2) the diapirs are restricted to having 50–60% of sediment component: diapirs with lower sediment contents may be too dense to detach from the slab and mélanges with higher sediment contents may be too less dense to be subducted to the depth of formation of diapirs, (3) melts at 1100 °C and 1.5 GPa from these diapirs have the bulk granodioritic composition of the batholiths, (4) restite unmixing occurs when magma batches segregate from the relaminated diapirs, (5) the lower the temperature of the region of the diapir from which the batch segregates, the higher the restite content, (6) the temperature, by controlling the restite content, is the ultimate factor controlling the compositional trends of Cordilleran batholiths, and (7) fractional crystallization or cotectic evolution is still needed to account for the full compositional range of these batholiths, from the parental 1–11.5 wt% to the observed 0–25 wt% in maficity. • Cordilleran granitoids can be formed by melting of subducted basalt and sediment. • Parental magmas of Cordilleran granitoids derive from diapirs relaminated to the crust. • The main process controlling the composition of granitoids is restite entrainment. • The higher the temperature of the diapir, the lower the amount of restites entrained. • Composition of diapirs is restricted to having 50–60% sediment component.

New insights into the petrogenesis of the Puerto Vallarta Batholith, Mexico: Evidence from petrology, zircon petrochronology, and phase equilibrium modeling

At subduction zones, continental magmatic arcs are produced by differentiation processes. The Puerto Vallarta Batholith, at the Mexican Pacific margin, is the final product of the subduction of oceanic Farallon plate beneath the North American plate during the Cretaceous. Although this Cordilleran Batholith has been subject to different studies, its petrogenetic evolution remains unclear. To better constrain the magmatic processes that gave origin to the granitic rocks of the Puerto Vallarta Batholith, we combined detailed fieldwork, zircon petrochronology, and conventional thermobarometry coupled with phase equilibrium modeling. We present the crystallization and emplacement evolution, and its relationship to magmatic processes of the Puerto Vallarta Batholith. In the granitic rocks, we observed calc-alkaline and metaluminous signatures, as well as enrichment in light-rare earth elements (LREE) relative to the heavy-rare earth elements (HREE). The early crystallization stages of the Puerto Vallarta Batholith started at ~729–776 °C and ~3.5–5.3 kbar, with the last melt portion crystallized at ~675–685 °C. Fieldwork observations and Zr saturation thermometry reveal mixing of melts during the last crystallization stages. Geochemical signatures of zircons from granitic rocks suggest crystallization after minor plagioclase fractionation with likely assimilation of crustal components. Our results from geochemical whole-rock data further indicate partial melting of a heterogenous source in the mantle wedge and the differentiation of those primary melts as the principal processes that gave origin to the Puerto Vallarta Batholith. We show that the magmatic processes that originated the granitic rocks of this Cordilleran Batholith occurred in the middle crust during the Late Cretaceous. Our results provide new evidence to better constrain the origin of this magmatic system.

Stable and transient isotopic trends in the crustal evolution of Zealandia Cordillera

Abstract We present >500 zircon δ18O and Lu-Hf isotope analyses on previously dated zircons to explore the interplay between spatial and temporal magmatic signals in Zealandia Cordillera. Our data cover ~8500 km2 of middle and lower crust in the Median Batholith (Fiordland segment of Zealandia Cordillera) where Mesozoic arc magmatism along the paleo-Pacific margin of Gondwana was focused along an ~100 km wide, arc-parallel zone. Our data reveal three spatially distinct isotope domains that we term the eastern, central, and western isotope domains. These domains parallel the Mesozoic arc-axis, and their boundaries are defined by major crustal-scale faults that were reactivated as ductile shear zones during the Early Cretaceous. The western isotope domain has homogenous, mantle-like δ 18O (Zrn) values of 5.8 ± 0.3‰ (2 St.dev.) and initial εHf (Zrn) values of +4.2 ± 1.0 (2 St.dev.). The eastern isotope domain is defined by isotopically low and homogenous δ18O (Zrn) values of 3.9 ± 0.2‰ and initial εHf values of +7.8 ± 0.6. The central isotope domain is characterized by transitional isotope values that display a strong E-W gradient with δ18O (Zrn) values rising from 4.6 to 5.9‰ and initial εHf values decreasing from +5.5 to +3.7. We find that the isotope architecture of the Median Batholith was in place before the initiation of Mesozoic arc magmatism and pre-dates Early Cretaceous contractional deformation and transpression. Our data show that Mesozoic pluton chemistry was controlled in part by long-lived, spatially distinct isotope domains that extend from the crust through to the upper mantle. Isotope differences between these domains are the result of the crustal architecture (an underthrusted low-δ18O source terrane) and a transient event beginning at ca. 129 Ma that primarily involved a depleted-mantle component contaminated by recycled trench sediments (10–20%). When data showing the temporal and spatial patterns of magmatism are integrated, we observe a pattern of decreasing crustal recycling of the low-δ18O source over time, which ultimately culminated in a mantle-controlled flare-up. Our data demonstrate that spatial and temporal signals are intimately linked, and when evaluated together they provide important insights into the crustal architecture and the role of both stable and transient arc magmatic trends in Cordilleran batholiths.

Critical role of water in the formation of continental crust

Continental arcs are the sites of production of continental crust, but the origin of these magmatic systems is not well understood. Although a number of processes have been suggested to be important, the role of water migrating from slab to surface during subduction has been underappreciated. Directly below the Moho, hot (approximately 1,100 °C), hydrous basaltic magmas fractionate as they cool to the regional geotherm at 750 to 800 °C, ultimately solidifying as mafic underplates. Cooling and fractionation cause water to exsolve and ascend, triggering fluid-fluxed melting of overlying mafic underplates and other crust. Melting of prior mafic underplates buffers temperatures and generates the voluminous, juvenile low-K magmas of Cordilleran batholiths. These granitoid magmas comprise a low-temperature slurry of melt and residue, and recrystallize into silicic mush during adiabatic ascent. Such hydrous mushes are intermittently infused by hotter, more mafic magmas, which hybridize and facilitate ascent and, potentially, eruption. Fluid-fluxed melting overcomes many of the general petrological and geochemical problems associated with models dominated by fractional crystallization. The role of water during repeated episodes of mafic underplating is critical to generate the juvenile granitoid infrastructure of the continents. Migration of water from the slab to the surface during subduction is highlighted as a key process in the formation of continental crust.

The dual origin of I-type granites: the contribution from experiments

Abstract New laboratory experiments using granulite xenoliths support a dual origin for I-type granites as primary and secondary. Primary I-type granites represent fractionated liquids from intermediate magma systems of broadly andesitic composition. Fluid-fluxed melting of igneous rocks that resided in the continental crust generates secondary I-type granites. The former are directly related to subduction, with Cordilleran batholiths as the most characteristic examples. Experiments with lower crust granulite sources, in the presence of water, show that amphibole is formed by a water-fluxed peritectic rehydration melting reaction . Entrainment of only 10% of restites composed of amphibole, pyroxene, plagioclase and magnetite, is sufficient to account for discrepancies in aluminium saturation index and maficity in secondary I-type granites. Lower crust granulite xenoliths, attached to a sanukitoid containing 6 wt% water, have been used in two-layer capsules to test fluid-fluxed melting reactions as the origin of secondary I-type granites. It is proposed that sanukitoid magmas act as water donors that trigger extensive melting of the lower crust, giving rise to granodioritic liquids. Because primary granites are related to coeval subduction, and secondary ones are crustal melts from older subduction-related rocks, the distinction between both I-types is essential in tectonic reconstructions of ancient orogenic belts.

Periodic activity in continental magmatic arcs

Cordilleran orogenic belts are formed primarily by processes associated with the subduction of oceanic plates beneath the edges of continental plates. On Earth today, the American Cordilleras, spanning one-third the circumference of the globe, represent the range of variations seen in older, less coherent cordilleran belts. The Alpine-Himalayan-New Guinean orogenic belt embodies Earth’s other great orogenic system and has formed by both collisional and cordilleran-style processes. Because of the abundance of oceanic lithosphere on this planet, it stands to reason that cordilleran-style orogenesis has been important throughout the portion of geological time during which plate tectonics has operated. Inasmuch as continental magmatic arcs lie at the heart of most cordilleran orogenic belts, their long-term (107–108 yr) behavior must be intimately tied to the diverse processes that collaborate with magmatism to build these great orogenic belts. Thus, changes in magma production through time, as recorded by cordilleran batholiths and volcanic piles, have implications for the broad web of tectonic, magmatic, metamorphic, mantle dynamic, and surface processes operating in cordilleran orogenic systems, as well as potential global climatic effects (McKenzie et al. 2016). For this reason sundry geoscientists, including petrologists, geophysicists, basin analysts, structural geologists, geomorphologists, geochronologists, and even climatologists, are paying unprecedented attention to the histories of cordilleran …

Neoproterozoic active continental margin of the Cathaysia block: Evidence from geochronology, geochemistry, and Nd–Hf isotopes of igneous complexes

Previous studies have suggested that northwest Cathaysia was a passive continental margin during Neoproterozoic time, given the absence of Neoproterozoic magmatism. However, this view has been challenged by recent discoveries of mafic and felsic magmatic rocks with geochemical affinities to continental arc or back-arc volcanic rocks from the 1.0–0.9Ga Wuyi–Yunkai belt, indicating the development of an early Neoproterozoic continental margin arc and back-arc system in the Cathaysia block. In this paper we present new age data for the Miaohou and Shanhou arc complexes, demonstrating that the active continental margin of the Cathaysia block lasted until ca. 830Ma. The Miaohou complex is composed of gabbro, diorite, and granite, the Shanhou complex of diorite and granite, and both complexes are bound by the Jiangshao Fault. Twelve samples from the Miaohou and Shanhou complexes give zircon U–Pb ages of ca. 830Ma, indicating that the magmatism of the Miaohou and Shanhou complexes were coeval. The Miaohou gabbro has high zircon ɛHf(t) (+2.8 to +9.6) and whole-rock ɛNd(t) (+3.9) values together with the geochemical features of volcanic arc basalts, while the Miaohou and Shanhou diorites and granites show large variations and high values of zircon ɛHf(t) and ɛNd(t) [ɛNd(t) (+1.1 to +5.5) and ɛHf(t) (−6.1 to +12.3) for the diorites, and ɛNd(t) (+0.3 to +3.1) and ɛHf(t) (−3.8 to +11.3) for the granites] together with a similar geochemistry to continental arc felsic rocks (e.g., Cordilleran batholiths). The geochemical and isotopic data suggest that the southeastwards subduction of an oceanic plate under the Cathaysia block caused the dehydration of sediments in the subducted slab, leading to the metasomatism of the mantle wedge by subduction-related fluids, partial melting of the mantle wedge, followed by varying degrees of assimilation and fractional crystallization (AFC) as the magmas ascended. These processes provide a reasonable explanation for the Neoproterozoic arc magmatism on the northwestern margin of the Cathaysia block, represented by the Miaohou and Shanhou complexes.

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California

Time scales and processes of Cordilleran batholith construction and high-Sr/Y magmatic pulses: Evidence from the Bald Mountain batholith, northeastern Oregon

Cordilleran granitic batholiths (sensu lato) preserve information about time scales and processes of upper crustal magmatic arc construction during Mesozoic subduction and mountain building. The Bald Mountain batholith in northeastern Oregon (USA) is a classic example of a composite, incrementally constructed batholith that formed during terrane amalgamation outboard of the western U.S. Cordillera. Whole-rock geochemistry and zircon trace element, U-Pb, Lu-Hf, and O isotopic data reveal that batholith construction occurred over ~15 Ma, commencing with the syncollisional emplacement of small, low-Sr/Y ( 40) tonalite-granodiorite mag matism that produced the main mass of the batholith, including the granodiorite of Anthony Lake (147 Ma) and the tonalite of Bald Mountain (145‐141 Ma). Zircons from the norite-granite suite display a narrow range in initial eHf of 7.2‐7.7 and elevated δ 18 O (Zrn) ranging from 8.2‰ to 10.0‰ (excluding one outlier). Zircons from the later granodiorite-tonalite suite show a similar range of initial eHf values (6.3‐8.9) and δ 18 O (7.1‰‐10.0‰), indicating a similar history of interaction with evolved crustal material. Modeling of whole-rock and zircon geochemistry indicates that both the low- and high-Sr/Y magmas composing the main phase of the batholith were generated by dehydration‐partial melting of mafi c arc crust (e.g., amphibolite), leaving behind a plagioclase-poor restite, which was garnet granulite in the case of the high-Sr/Y magmas . Final magma compositions in both suites were affected by assimilation of supracrustal material either at depth or during ascent. We suggest that high-Sr/Y magmas in the Bald Mountain batholith were generated by partial melting of thickened arc crust ~10 m.y. after arc-arc collision began at 159‐154 Ma. Heat to drive lower crustal melting was conveyed by an increase in mantle power input as a result of renewed subduction-related magmatism. Mixing and homogenization in the lower crust involving mantle-derived basalts and crustally derived partial melts can account for the geochemical variation we observe in tonalites and granodiorites in the Bald Mountain batholith.

Arc and Slab-Failure Magmatism in Cordilleran Batholiths I – The Cretaceous Coastal Batholith of Peru and its Role in South American Orogenesis and Hemispheric Subduction Flip

Arc and Slab-Failure Magmatism in Cordilleran Batholiths I – The Cretaceous Coastal Batholith of Peru and its Role in South American Orogenesis and Hemispheric Subduction Flip

Timing and significance of gabbro emplacement within two distinct plutonic domains of the Peninsular Ranges batholith, southern and Baja California

Evaluating the crucial role postulated for mantle-derived mafic magmas in the formation of silicic batholiths requires well-determined igneous crystallization ages from mafic rocks as well as petrologic data reflecting possible mantle source heterogeneities and/or variations in subcrustal processes. The abundant mafic plutons that crop out within the mid-Cretaceous Peninsular Ranges batholith (PRB, southern and Baja California) provide an important opportunity to investigate the relationship of mafic and silicic magmatism in Cordilleran batholiths. Zircon U-Pb laser ablation inductively coupled plasma mass spectrometry ages are reported for 24 samples of PRB gabbro collected throughout its 1400-km-long extent. The samples span the entire compositional spectrum of PRB gabbro from cumulate olivine gabbro with <10 ppm whole rock Zr content, to gabbronorite, to more fractionated hornblende gabbro. Eighteen of the samples are from the western zone “gabbro belt” of the batholith including the type locality for San Marcos Gabbro, while six are from the eastern zone where gabbro is rare, composing <1% of the outcrop area. The morphology of zircon in PRB gabbro ranges from anhedral fragments to euhedral grains, and this zircon has distinctly lower uranium concentrations and higher Th/U ratios than zircon from PRB granitoids. Most samples exhibit simple U-Pb systematics, but several contain inherited or entrained zircons, and two eastern gabbro samples contain distinctly bimodal age populations. Weighted mean 206Pb/238U zircon ages are interpreted as crystallization ages and range from 130.5 ± 1.5 Ma to 100.2 ± 3.0 Ma for western zone samples. The ages demonstrate that gabbros were emplaced throughout the ∼30 m.y. interval during which the bulk of the western zone was constructed, and conflict with the earlier interpretation of western gabbros as mafic forerunners that preceded emplacement of granitoid plutons. Eastern zone gabbros, in contrast, yield U-Pb ages that range mostly from 109.4 ± 2.9 Ma to 97.8 ± 2.3 Ma, narrowly predating the ca. 92–98 Ma eastern zone tonalite-trondhjemite-granodiorite batholith flareup, and thus may have played a role in crustal thickening and triggering of the flareup. Despite their partially cumulate character, whole rock major and trace element compositions of PRB gabbro closely match high-alumina olivine tholeiite and arc basalt and basaltic andesite from modern arcs. Gabbros from the west and east sides of the batholith, transverse to regional strike, have similar nearly flat rare earth element patterns that reflect derivation from a common partially contaminated depleted mantle source. The apparent across-strike petrologic uniformity of the gabbros contrasts strongly with PRB granitoids, which are well known for across-strike asymmetries in trace elements, initial 87Sr/86Sr, and δ18O, reflecting progressively deeper partial melting of mafic source rocks from west to east with additional input from a high-18O, high-87Sr end member on the east side of the batholith. The western PRB gabbros represent a plausible deep crustal source composition for derivation of western granitoids by partial melting or fractional crystallization. For the eastern batholith, the mafic source region requires modification by input of accretionary complex supracrustal rocks delivered via forearc subduction erosion underthrusting.

The inception of a Paleotethyan magmatic arc in Iberia

This paper presents a compilation of recent U-Pb (zircon) ages of late Carboniferous–early Permian (LC–EP) calc-alkaline batholiths from Iberia, together with a petrogenetic interpretation of magma generation based on comparisons with Mesozoic and Tertiary Cordilleran batholiths and experimental melts. Zircon U-Pb ages distributed over the range ca. 315–280 Ma, indicate a linkage between calc-alkaline magmatism, Iberian orocline generation and Paleotethys subduction. It is also shown that Iberian LC–EP calc-alkaline batholiths present unequivocal subduction-related features comparable with typical Cordilleran batholiths of the Pacific Americas active margin, although geochemical features were partially obscured by local modifications of magmas at the level of emplacement by country rock assimilation. When and how LC–EP calc-alkaline batholiths formed in Iberia is then discussed, and a new and somewhat controversial interpretation for their sources and tectonic setting (plume-assisted relamination) is suggested. The batholiths are proposed to have formed during the subduction of the Paleotethys oceanic plate (Pangaea self-subduction) and, consequently, they are unrelated to Variscan collision. The origin of the Iberian batholiths is related to the Eurasian active margin and probably represents the inception of a Paleotethyan arc in the core of Pangaea.

Magma emplacement process zone preserved in the roof of a large Cordilleran batholith, Wallowa Mountains, northeastern Oregon

The crestal portion of the upper-crustal Wallowa batholith, northeastern Oregon, provides an exceptional three-dimensional section through a flat batholith roof which rolls over into a steep wall at elevations of about 1600–2800m. The roof–batholith margin is interpreted as representing the ‛frozen-in’ process zone of granodiorite magma which was arrested during its ascent, thus providing a direct view into processes that operate during construction of large, shallow-level Cordilleran-type batholiths. The batholith host rock records two principal and presumably coeval emplacement processes: ductile flow largely accommodated by rheologically weak and thus severely deformed marbles in the steep wall, and voluminous stoping of rheologically stronger, bedded siliciclastic rocks along the flat roof. Structures preserved along the roof suggest that the stoping was a multi-stage process involving emplacement of up to hundred meters long sills along bedding planes and formation of short connecting dikes cutting across bedding to produce sill–dike networks along the batholith–roof contact. Portions of the sill–dike networks and enclosed large rectangular roof blocks were then stoped into and continued to be mechanically disintegrated in the magma chamber. This inferred mechanism departs from the commonly assumed mode of stoped block formation solely by thermal shattering. In our view, host rocks in the process zone are fractured by dike propagation. It follows that such process of roof disruption and subsequent block stoping could be extremely rapid. In combination with other processes (e.g., roof uplift, ductile flow in weak lithologies), such rapid stoping could contribute to emplacement of voluminous and mostly non-sheeted Cordilleran-type batholiths at upper crustal levels.

40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province

The Lipez region of southwest Bolivia is the locus of a major Neogene ignimbrite flare-up, and yet it is the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. Recent mapping and laser-fusion 40 Ar/ 39 Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refine the timing and volumes of ignimbrite emplacement in Bolivia and northern Chile to reveal that monotonous intermediate volcanism was prodigious and episodic throughout the complex. The new results unravel the eruptive history of the Pastos Grandes and Guacha calderas, two large multicyclic caldera complexes located in Bolivia. These two calderas, together with the Vilama and La Pacana caldera complexes and smaller ignimbrite shields, were the dominant sources of the ignimbrite-producing eruptions during the ∼10 m.y. history of the Altiplano-Puna volcanic complex. The oldest ignimbrites erupted between 11 and 10 Ma represent relatively small volumes (approximately hundreds of km 3 ) of magma from sources distributed throughout the volcanic complex. The first major pulse was manifest at 8.41 Ma and 8.33 Ma as the Vilama and Sifon ignimbrites, respectively. During pulse 1, at least 2400 km 3 of dacitic magma was erupted over 0.08 m.y. Pulse 2 involved near-coincident eruptions from three of the major calderas resulting in the 5.60 Ma Pujsa, 5.65 Ma Guacha, and 5.45 Ma Chuhuilla ignimbrites, for a total minimum volume of 3000 km 3 of magma. Pulse 3, the largest, produced at least 3100 km 3 of magma during a 0.1 m.y. period centered at 4 Ma, with the eruption of the 4.09 Ma Puripicar, 4.00 Ma Chaxas, and 3.96 Ma Atana ignimbrites. This third pulse was followed by two more volcanic explosivity index (VEI) 8 eruptions, producing the 3.49 Ma Tara (800 km 3 DRE) and 2.89 Ma Pastos Grandes (1500 km 3 DRE) ignimbrites. In addition to these large caldera-related eruptions, new age determinations refine the timing of two little-known ignimbrite shields, the 5.23 Ma Alota and 1.98 Ma Laguna Colorada centers. Moreover, 40 Ar/ 39 Ar age determinations of 13 ignimbrites from northern Chile previously dated by the K-Ar method improve the overall temporal resolution of Altiplano-Puna volcanic complex development. Together with the updated volume estimates, the new age determinations demonstrate a distinct onset of Altiplano-Puna volcanic complex ignimbrite volcanism with modest output rates, an episodic middle phase with the highest eruption rates, followed by a decline in volcanic output. The cyclic nature of individual caldera complexes and the spatiotemporal pattern of the volcanic field as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith.

Melting Relations of MORB-Sediment Melanges in Underplated Mantle Wedge Plumes; Implications for the Origin of Cordilleran-type Batholiths

This paper gives the results of a set of laboratory experiments designed to analyse the petrological implications of mantle wedge plumes—large buoyant structures predicted by thermomechanical numerical modelling of subduction zones. A particular design of layered capsule was used to simulate the complex multilayer formed by intense flow within the mantle wedge as predicted by numerical models. A basaltic [mid-ocean ridge basalt (MORB)-derived amphibolite] component was sandwiched between two adjacent layers of a sedimentary (Bt-rich metagreywacke) component. Conditions were fixed at temperatures of 1000–1200°C at pressures of 1·5–2·0 GPa. Our results suggest that significant volumes of hybrid, Cordilleran-type granodioritic magmas can be generated by sub-lithospheric partial melting of a mechanically mixed source. Partial melting of the end-members is not buffered, forming granitic (melting of metasediment) and trondhjemitic (melting of MORB) melts in high-variance assemblages Melt + Grt + Pl and Melt + Grt + Cpx, respectively. However, the composition of melts formed from partial melting of metasediment–MORB melanges is buffered for sediment-to-MORB ratios ranging from 3:1 to 1:3, producing liquids of granodiorite to tonalite composition along a cotectic with the lower-variance phase assemblage Melt + Grt + Cpx + Pl. Our model explains the geochemical and isotopic characteristics of Cordilleran batholiths. In particular, it accounts for the observed decoupling between major element and isotopic compositions. Large variations in isotopic ratios can be inherited from a compositionally heterogeneous source; however, major element compositions are more strongly dependent on the temperature of melting rather than on the composition of the source.

Tectonomagmatic evolution of Western Amazonia: Geochemical characterization and zircon U-Pb geochronologic constraints from the Peruvian Eastern Cordilleran granitoids

The results of a coupled, in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb study on zircon and geochemical characterization of the Eastern Cordilleran intrusives of Peru reveal 1.15 Ga of intermittent magmatism along central Western Amazonia, the Earth's oldest active open continental margin. The eastern Peruvian batholiths are volumetrically dominated by plutonism related to the assembly and breakup of Pangea during the Paleozoic-Mesozoic transition. A Carboniferous-Permian (340-285 Ma) continental arc is identified along the regional orogenic strike from the Ecuadorian border (6 degrees S) to the inferred inboard extension of the Arequipa-Antofalla terrane in southern Peru (14 degrees S). Widespread crustal extension and thinning, which affected western Gondwana throughout the Permian and Triassic resulted in the intrusion of the late- to post-tectonic La Merced-San Ramon-type anatectites dated between 275 and 220 Ma, while the emplacement of the southern Cordillera de Carabaya peraluminous granitoids in the Late Triassic to Early Jurassic (220-190 Ma) represents, temporally and regionally, a separate tectonomagmatic event likely related to resuturing of the Arequipa-Antofalla block. Volcano-plutonic complexes and stocks associated with the onset of the present Andean cycle define a compositionally bimodal alkaline suite and cluster between 180 and 170 Ma. A volumetrically minor intrusive pulse of Oligocene age (ca. 30 Ma) is detected near the southwestern Cordilleran border with the Altiplano. Both post-Gondwanide (30-170 Ma), and Precambrian plutonism (691-1123 Ma) are restricted to isolated occurrences spatially comprising less than 15% of the Eastern Cordillera intrusives. Only one remnant of a Late Ordovician intrusive belt is recognized in the Cuzco batholith (446.5 +/- 9.7 Ma) indicating that the Famatinian arc system previously identified in Peru along the north-central Eastern Cordillera and the coastal Arequipa-Antofalla terrane also existed inboard of this parautochthonous crustal fragment. Hitherto unknown occurrences of late Mesoproterozoic and middle Neoproterozoic granitoids from the south-central cordilleran segment define magmatic events at 691 +/- 13 Ma, 751 +/- 8 Ma, 985 +/- 14 Ma, and 1071-1123 +/- 23 Ma that are broadly coeval with the Braziliano and Grenville-Sunsas orogenies, respectively. Our data suggest the existence of a continuous orogenic belt in excess of 3500 km along Western Amazonia during the formation of Rodinia, its "early" fragmentation prior to 690 Ma, and support a model of reaccretion of the Paracas-Arequipa-Antofalla terrane to western Gondwana in the Early Ordovician with subsequent detachment of the Paracas segment in form of the Mexican Oaxaquia microcontinent in Middle Ordovician. A tectonomagmatic model involving slab detachment, followed by underplating of cratonic margin by asthenospheric mantle is proposed for the genesis of the volumetrically dominant Late Paleozoic to early Mesozoic Peruvian Cordilleran batholiths

U-Pb geochronology of mid-Paleozoic plutonism in western New Zealand: Implications for S-type granite generation and growth of the east Gondwana margin

Research Article| September 01, 2009 U-Pb geochronology of mid-Paleozoic plutonism in western New Zealand: Implications for S-type granite generation and growth of the east Gondwana margin A.J. Tulloch; A.J. Tulloch † 1GNS Science, Private Bag 1930, Dunedin 9054, New Zealand †E-mail: a.tulloch@gns.cri.nz Search for other works by this author on: GSW Google Scholar J. Ramezani; J. Ramezani 2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar D.L. Kimbrough; D.L. Kimbrough 3Department of Geological Sciences, San Diego State University, San Diego, California 92182, USA Search for other works by this author on: GSW Google Scholar K. Faure; K. Faure 4GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar A.H. Allibone A.H. Allibone 5Rodinian Pty Ltd., P.O. Box 1970, Queanbeyan, NSW 2620, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2009) 121 (9-10): 1236–1261. https://doi.org/10.1130/B26272.1 Article history received: 16 Jun 2007 rev-recd: 07 Aug 2008 accepted: 12 Aug 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.J. Tulloch, J. Ramezani, D.L. Kimbrough, K. Faure, A.H. Allibone; U-Pb geochronology of mid-Paleozoic plutonism in western New Zealand: Implications for S-type granite generation and growth of the east Gondwana margin. GSA Bulletin 2009;; 121 (9-10): 1236–1261. doi: https://doi.org/10.1130/B26272.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract New U-Pb isotope-dilution–thermal ionization mass spectrometry (ID-TIMS) ages (371–305 Ma) for 30 granitic plutons along the New Zealand sector of the East Gondwanan active margin reveal a highly episodic emplacement history and crustal growth pattern. The Late Devonian–late Carboniferous ages also establish specific links with both the mostly older, Lachlan and the mostly younger, New England fold belts of eastern Australia. Dated plutons are representative of two S- and I-type suite pairs, the volumetrically predominant Karamea-Paringa (371–360 Ma) and minor Ridge-Tobin (355–342 Ma) pulses, as well as sporadic Foulwind Suite A-type granites (350–305 Ma). Emplacement of the bulk of the dominant ~3400 km2 Karamea Suite S-type granite-granodiorite plutons within a 2.1 Ma interval is explained by major and intimate intrusion of mantle-derived magma into largely metasedimentary crust during intra-arc extension of previously overthickened crust. Transient emplacement rates were thus of similar magnitude as some young ignimbrite flare-ups and an order of magnitude greater than long-term averages for Mesozoic-Cenozoic cordilleran batholiths of the western Americas. Extension likely was terminated abruptly by resumption of convergence, possibly associated with amalgamation of the Buller and Takaka terranes, between 368 and 355 Ma. Significant crustal growth occurred during generation of the two S-type suites, where mantle basalt contributed mass, and heat for rapid melting, during transient intra- or backarc extensional episodes. In contrast, the I-type suites were dominated by partial melting of meta-igneous crust, and they are relatively small in volume. The Karamea S-type suite shares striking similarities in terms of age, composition, and extensional tectonic setting with S-type granites of the Melbourne terrane of the Lachlan fold belt. Both regions may have formed in a backarc position with respect to the Late Devonian–early Carboniferous subduction zone in the New England fold belt. Foulwind Suite A-type magmatism in New Zealand overlaps in age with the widespread 320–285 Ma A- and I-type magmatism in the northern New England fold belt. The likely continuation of the New England subduction system must have subsequently been removed from outboard of the New Zealand region after 320–285 Ma magmatism, and prior to Triassic accretion of a Permian oceanic arc terrane to the New Zealand margin. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Crustal growth along a non-collisional cratonic margin: A Lu–Hf isotopic survey of the Eastern Cordilleran granitoids of Peru

An extensive in situ Lu–Hf isotopic study of zircon from the Eastern Cordilleran batholiths of Peru using laser ablation multi-collector inductively coupled mass spectrometry (LA-MC-ICPMS) reveals a systematic covariance between granitoid magma sources and tectonic regimes which shaped the proto-Andean margin of central Western Amazonia since > 1.15 Ga. The Hf isotope systematics are characterised by a range in initial 176Hf/ 177Hf compositions for a given intrusive event suggesting mixing of material derived from the Paleoproterozoic crustal substrate and variable addition of juvenile sources from Neoproterozoic to Cenozoic time. Intrusives associated with phases of regional compressive tectonism correspond to mean initial εHf values of − 6.73, − 2.43, − 1.57 for the Ordovician (Famatinian), Carboniferous–Permian and Late Triassic respectively, suggesting a minimum crustal contribution between 40% and 74% by mass. The average initial Hf systematics from granitoids associated with periods of regional extension such as the middle Neoproterozoic, Permian–Triassic and Cenozoic Andean back arc plutonism are consistently shifted towards positive values (mean initial εHf = − 0.7 to + 8.0) indicating systematically larger inputs of juvenile magma (25% to 38% of ancient crust by mass). In the absence of evidence for significant lateral accretion of exotic crust, the time integrated Hf record from the central proto-Andean margin of western Amazonia suggests crustal reworking was the dominant process during episodes of arc magmatism, implying that most crustal growth took place vertically via crustal underplating of isotopically juvenile, mantle derived melts during intervals of crustal extension.

A Geochemical Classification for Feldspathic Igneous Rocks

In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeO þMgO) ratio or Fe-index, the modified alkali^lime index, the aluminum-saturation index, the alkalinity index, and the feldspathoid silica-saturation index.The Fe-index distinguishes between melts that have undergone extensive iron enrichment during differentiation from those that have not. The transition from tholeiite to ferrobasalt allows us to extend this boundary to silica values as low as 48 wt %. We introduce the feldspathoid silica-saturation index, which, coupled with the alkalinity index, allows us to extend the geochemical classification to alkaline rocks. We show that most alkaline rocks are ferroan and that this probably reflects extensive fractional crystallization of olivine and pyroxene with minimal participation of Fe^Ti oxides. The expanded classification allows us to illustrate the geochemical and petrogenetic relationship of the plutonic rocks from ferroan granites to nepheline syenites that commonly occur in intracratonic environments. It also allows us to distinguish four families of feldspathic rocks: (1) magnesian rocks, which are exemplified by Caledonian and Cordilleran batholiths and are characterized by differentiation under oxidizing and relatively hydrous conditions; (2) ferroan rocks, which include fayalite granites, alkali granites, and nepheline syenites and are characterized by differentiation under reducing and relatively dry conditions; (3) leucogranites, which commonly form by crustal melting; (4) potassic and ultrapotassic rocks, which originate from mantle that has been enriched in K2O.