The Sierra Madre del Sur mountain range is an uplifted forearc associated with the subduction of the Cocos plate along the Acapulco trench beneath mainland southern Mexico. The shallow subduction ...angle, the truncation of geologic features along the modern Acapulco trench, and direct seismic and drill hole observations in the trench through deep sea drilling data suggest that subduction erosion is an important process during the evolution of this margin. Turbidites derived from the uplifted forearc are the predominant sedimentary input into this trench, while pelagic sediments are subordinate. Apatite (U‐Th)/He ages were obtained on 23 samples from two transects across the Sierra Madre del Sur (Acapulco and Puerto Escondido) and reveal slow cooling during the Miocene. (U‐Th)/He ages range between ∼25 and 8 Ma and correlate inversely with elevation. Long‐term erosional exhumation rates inferred from these ages range from 0.11 to 0.33 km/m.y., with higher rates in the range core, and suggest that the Sierra Madre del Sur has been a slowly decaying mountain range, since at least the early Miocene. Apparent Miocene‐Pliocene sedimentation (“preservation”) rates in the Acapulco trench derived from Deep Sea Drilling Project data are about an order of magnitude smaller than the Miocene forearc erosion rates estimated from (U‐Th)/He ages, suggesting that the terrigenous input to the trench was almost entirely recycled via subduction erosion, at least during the Miocene. The Miocene subducted flux per unit length of the margin is about 30 km3/(km m.y.), or a subducted volume per unit time of 44 × 103 km3/m.y., when integrated over the length of the trench.
Reactivation of inherited nappe contacts is a common process in orogenic areas affected by back-arc extension. The amount of back-arc extension is often variable along the orogenic strike, owing to ...the evolution of arcuated mountain chains during stages of rapid slab retreat. This evolution creates low rates of extension near rotation poles, where kinematics and interplay with the pre-existing orogenic structure are less understood. The amount of Miocene extension recorded by the Pannonian Basin of Central Europe decreases SE-wards along the inherited Cretaceous – Paleogene contact between the Dinarides and Carpathian Mountains. Our study combines kinematic data obtained from field and micro-structural observations assisted with fission track thermochronological analysis and U-Pb zircon dating to demonstrate a complex poly-phase evolution in the key area of the Jastrebac Mountains of Serbia. A first event of Late Cretaceous exhumation was followed by latest Cretaceous – Eocene thrusting and magmatism related to a continental collision that sutured the accretionary wedge containing contractional trench turbidites. The suture zone was subsequently reactivated and exhumed by a newly observed Miocene extensional detachment that lasted longer in the Jastrebac Mountains when compared with similar structures situated elsewhere in the same structural position. Such extensional zones situated near the pole of extensional-driven rotation favour late stage truncations and migration of extension in a hanging-wall direction, while directions of tectonic transport show significant differences in short distances across the strike of major structures.
•Evolution characterized by Late Cretaceous exhumation and latest Cretaceous – Eocene thrusting and magmatism.•Miocene extensional reactivation of inherited Cretaceous – Eocene nappe stack.•Late stage truncations and migration of extension in a hanging-wall direction.•Extensional detachments depict consistent transport directions in zones near pole of rotations.
The source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the Famatinian arc (Sierras Pampeanas of western Argentina). The source zones of intermediate ...igneous rocks are located at the transition between a gabbro-dominated mafic unit and a tonalite-dominated intermediate unit. In the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall-rocks, crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments. In turn, the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks. The gabbroic rocks became mafic migmatites comprising amphibole-rich pyroxene-bearing mesosomes and leucotonalitic veins. Significantly, most of the mafic migmatites have isotopic compositions 87Sr/86Sr(T) < 0·7063 and εNd(T) = –0·94 to +2·24 similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins 87Sr/86Sr(T) = 0·7075–0·7126 and εNd(T) < –2·65, providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts 87Sr/86Sr(T) = 0·715 and εNd(T) = –6·21. Mass-balance calculations show that the model reaction plagioclase + amphibole + leucogranitic melt → leucotonalitic melt + clinopyroxene ± orthopyroxene can better explain the partial melting of the gabbroic rocks. Based on field observations, we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt-dominated leucotonalitic pools. However, the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically (elemental and isotopic) more evolved than the dioritic and tonalitic rocks. We envisage that once detached from their source region the leucotonalitic magmas were able to react, commingle and mix with entrained fragments of both mafic and metasedimentary rocks. This process gave rise to melts that became tonalitic and dioritic magmas. This study concludes that the generation of intermediate magmas is a multistage process with three critical steps: (1) influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks; (2) physically and chemically coupled melting of mafic and metasedimentary rocks, leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies; (3) retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors. The dimensions of the source zone seem to be insufficient to generate crustal-scale volumes of intermediate igneous rocks. However, the Famatinian paleo-arc crust would expose only those magma source zones that were still active during the tectonic closure of the arc. Ultimately, a time-integrated perspective indicates that early active source zones were cannibalized during the downward expansion of the plutonic bodies already dominated by intermediate plutonic rocks.
Some insights into the origin of cratonic mantle can be gained from “eclogite” (loosely defined here as an assemblage containing garnet and any pyroxene) xenoliths hosted in kimberlites erupted ...through Archean (~
2.53.5 Gy) cratons. One subset of Archean eclogite xenoliths, the low MgO Archean xenoliths, is presently believed to represent metamorphosed fragments of ancient altered oceanic crust, leading to the suggestion that Archean cratons were built, at least in part, by the accretion of oceanic lithospheric segments. However, another Archean subset, the high MgO Archean eclogite xenoliths, have major and compatible trace-element (Ni and Cr) systematics similar to high MgO arc-eclogite xenoliths originating from the lithospheric root underlying the Sierra Nevada batholith in California, an example of a Phanerozoic arc. The Sierran high MgO arc-eclogites represent cumulates from hydrous basaltic magmas beneath a thick continental arc. The compositional similarities between the Archean and Sierran high MgO eclogites suggest that not only might the Archean high MgO eclogites have a cumulate origin, as has previously been suggested, but they may be arc-related. If so, Archean high MgO eclogites provide evidence from within the mantle roots of cratons that some form of arc magmatism contributed to the formation and evolution of Archean continents.
Here we test for peridotite versus pyroxenite input in Mongolian Mesozoic and Cenozoic magmatism. A combination of new 40Ar/39Ar radiometric dating results, whole-rock major- and trace-element, SrNd ...isotope, and mineral phenocryst geochemical data is used to decipher the petrogenesis of Cretaceous lavas (Tsagaan Nuur and Khukh Tolgoi) and dykes (Samaan Damba) from the Argalant Range, Gobi-Altai (Mongolia). This magmatism is compared to Cretaceous asthenospheric mantle-derived basalts from Tsost Magmatic Field and Cenozoic volcanism from the Gobi-Altai, Khangai Range Watershed, Tariat and Togo to assess changing source conditions. We also compare this magmatism to Cenozoic magmatism from the North China Craton. The Argalant Range magmatism has geochemical signatures consistent with the involvement of both peridotite and pyroxenite-like components, and we suggest that this pyroxenite-like component was obtained through the melting of metasomatized subcontinental lithospheric mantle (SCLM). Mineral-liquid thermobarometer results for samples from Khukh Tolgoi and Samaan Damba indicate that upwelling magma stalled at ~30 km depth, before finally traversing further to surface. A model to explain Mesozoic magmatic genesis is presented here, whereby piecemeal delamination and convective erosion of a metasomatized SCLM drives magmatism. The Cenozoic volcanism also has geochemical signatures consistent with the melting of non-peridotite components, and the presence of samples with >9 wt% MgO from Khangai Range Watershed, Tariat and Togo enabled assessments on the relative contribution of non-peridotite melt input. We suggest that magmatism from Togo contains the greatest amount of non-peridotite melt input, followed by Tariat and Khangai Range Watershed localities. We hypothesize that intermittent Cenozoic magmatism is the result of a slab graveyard under East Asia foundering into the upper mantle.
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•Mesozoic magmatism from a peridotite source with minor mafic components.•Khukh Tolgoi and Samaan Damba magma fractionated at ~30 km depth.•Piecemeal delamination and convective erosion triggered Mesozoic magmatism.•Cenozoic volcanism was derived from an olivine-rich pyroxenite/mafic source.•Foundering slabs might have caused intermittent Cenozoic East Asian magmatism.
We performed a detrital zircon (DZ) U‐Pb geochronologic survey of the lower parts of the Danube River approaching its Danube delta, Black Sea sink, and a few large tributaries (Tisza, Jiu, Olt, and ...Siret) originating in the nearby Carpathian Mountains. Samples are modern sediments. DZ age spectra reflect the geology and specifically the crustal age formation of the source area, which in this case is primarily the Romanian Carpathians and their foreland with contributions from the Balkan Mountains to the south of Danube and the East European Craton. The zircon cargo of these rivers suggests a source area that formed during the latest Proterozoic and mostly into the Cambrian and Ordovician as island arcs and back‐arc basins in a Peri‐Gondwanan subduction setting (~600–440 Ma). The Inner Carpathian units are dominated by a U‐Pb DZ peak in the Ordovician (460–470 Ma) and little inheritance from the nearby continental masses, whereas the Outer Carpathian units and the foreland have two main peaks, one Ediacaran (570–610 Ma) and one in the earliest Permian (290–300 Ma), corresponding to granitic rocks known regionally. A prominent igneous Variscan peak (320–350 Ma) in the Danube's and tributaries DZ zircon record is difficult to explain and points out to either an extra Carpathian source or major unknown gaps in our understanding of Carpathian geology. Younger peaks corresponding to arc magmatism during the Alpine period make up as much as about 10% of the DZ archive, consistent with the magnitude and surface exposure of Mesozoic and Cenozoic arcs.
Key Points
A detrital zircon U‐Pb study of modern sands from the lower Danube and its tributaries documents the main magmatic events that led to the continental crustal formation of the nearby Carpathians
The great majority of basement was formed in latest Proterozoic‐Ordovician island arcs, a finding that is consistent with previous studies
An unexpected and prominent Carboniferous magmatic peak in the detrital record has no known source in the nearby Carpathians
Though continental magmatic arcs are factories for new continental crust, a significant proportion of continental arc magmas are recycled from supracrustal material. To evaluate the relative ...contributions of retroarc underthrusting and trench side partial sediment subduction for introducing supracrustal rocks to the middle and lower crust of continental magmatic arcs, we present results from the deeply exposed country rocks of the Coast Mountains batholith of western British Columbia. Prior work demonstrates that these rocks underwent widespread partial melting that contributed to the Coast Mountains batholith. We utilize U‐Pb zircon geochronology, Sm‐Nd thermochronology, and field‐based studies to document the protoliths and early burial history of amphibolite and granulite‐facies metasedimentary rocks in the Central Gneiss Complex. U‐Pb detrital zircon data from the structurally highest sample localities yielded ~190 Ma unimodal age peaks and suggest that retroarc rocks of the Stikine terrane constitute a substantial portion of the Central Gneiss Complex. These supracrustal rocks underwent thrust‐related burial and metamorphism at >25 km depths prior to ~80 Ma. These rocks may also be underlain at the deepest exposed structural levels by Upper Cretaceous metasedimentary rocks, which may have been emplaced as a result of trench side underplating or intraarc burial. These results further our understanding of the mechanisms of material transport within the continental lithosphere along Cordilleran subduction margins.
Key Points
Metasedimentary rocks forming the framework of a substantial proportion of the Coast Mountains batholith correlate to Stikinia
Underthrusting to midcrustal levels occurred from the retroarc side during Cretaceous time
Retroarc underthrusting is important for introducing supracrustal rocks to batholithic roots
Major element, trace element and Nd-Sr isotopic data are presented for 82 plutonic rocks from the southern Coast Mountains Batholith (CMB) in British Columbia, Canada, ranging in emplacement age from ...210 to 50 Ma. The rocks are part of a large composite magmatic arc batholith, which the major element data show to be of calc-alkaline affinity. The majority of CMB samples lack the depletion in Eu that would be consistent with equilibration of magmas and plagioclase-bearing crystalline residues or fractionates, suggesting that equilibration took place deeper than the pressure limit of plagioclase stability at 35-40 km depth. The CMB samples show a wide variation in the slope of normalized rare earth element (REE) patterns, with chondrite-normalized La/Yb ratios above 10 being mostly confined to periods of high magmatic flux in the arc at 160-140, 120-80, and 60-50 Ma. The clearest relationships between major and trace elements are negative correlations between SiO sub(2) and each of Sc, Y, and the heavier REE Gd to Lu. Nd and Sr isotopes mostly document juvenile origins for the granitoids, but show variations to higher super(87)Sr/ super(86)Sr and lower epsilon sub(Nd) during high-flux periods. The results are interpreted to indicate a deep origin for most CMB magmas, below similar to 40 km where mafic to intermediate rock assemblages previously added to the arc crust by mantle melting were transformed to an (amphibole-bearing)-eclogite facies cumulate or restite, such that melting residues consisted mainly of two pyroxenes, garnet and variable proportions of amphibole. Thickened orogenic crust, for which there is clear geological evidence during the period 100-80 Ma, promoted this process. During high-flux periods, larger amounts of older rocks, mostly mafic rocks and some metasediments added to the base of the arc during orogenic shortening, became involved in magma genesis.
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