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Devonian magmatism in the accretionary complex of southern Chile

Profile image of Francisco HerveFrancisco Herve

2016, Journal of the Geological Society

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Abstract

Supposed or potential Devonian igneous rocks in the accretionary complex of southern Chile were investigated using sensitive high-resolution ion microprobe U-Pb dating of zircon, with Hf-and O-isotope analyses of selected grains. Ages of 384 ± 3 and 382 ± 2 Ma are confirmed for two igneous bodies (another having been previously dated at 397 ± 1 Ma). Detrital zircon ages in the host rocks, some associated with Devonian marine fossils, indicate maximum possible sedimentation ages of c. 330 -385 Ma. Devonian ages of 391 ± 10 and 374 ± 3 Ma for plutonic rocks at the western edge of the North Patagonian Massif are somewhat older than those of orthogneisses in the western flank of the Andes near Chaitén (361 ± 7 and 364 ± 2 Ma). O and Hf isotopes indicate that the Devonian intrusions in the accretionary complex crystallized from mantle-derived magmas, whereas those in the North Patagonian Massif show a strong crustal influence, corresponding to oceanic and continental margin subduction environments of magma genesis, respectively. Devonian zircon provenance in the accretionary complex was from the North Patagonian Massif and not from the mantle-derived intrusions, suggesting that the accretionary complex formed an integral part of the Gondwana margin during Devonian-Carboniferous times.

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Provenance variations in the Late Paleozoic accretionary complex of central Chile as indicated by detrital zircons
Mauricio Calderon

Gondwana Research, 2013

We present detrital zircon U-Pb SHRIMP age patterns for the central segment 3 (34-42ºS) of an extensive accretionary complex along coastal Chile together 4 with ages for some relevant igneous rocks. The complex consists of a basally 5 accreted high pressure/low temperature Western Series outboard of a frontally 6 accreted Eastern Series that was overprinted by high temperature/low pressure 7 metamorphism. Eleven new SHRIMP detrital zircon age patterns have been 8 obtained for metaturbidites from the central (34-42ºS) segment of the 9 accretionary complex, four from previously undated metamorphic complexes 10 and associated intrusive rocks from the main Andean cordillera, and three from 11 igneous rocks in Argentina that were considered as possible sediment source 12 areas. There are no Mesozoic detrital zircons in the accretionary rocks. Early 13 Paleozoic zircons are an essential component of the provenance, and Grenville-14 age zircons and isolated grains as old as 3 Ga occur in most rocks, although 15 much less commonly in the Western Series of the southern sector. In the 16 northernmost sector (34-38º30"S) Proterozoic zircon grains constitute more 17 than 50% of the detrital spectra, in contrast with less than 10% in the southern 18 sector (39-42ºS). The youngest igneous detrital zircons in both the northern 19 Western (307 Ma) and Eastern Series (345 Ma) are considered to closely date 20 sedimentation of the protoliths. Both oxygen and Lu-Hf isotopic analyses of a 21 selection of Permian to Neoproterozoic detrital zircon grains indicate that the 22 respective igneous source rocks had significant crustal contributions. The 23 results suggest that Early Paleozoic orogenic belts (Pampean and Famatinian)

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The country rocks of Devonian magmatism in the North Patagonian Massif and Chaitenia
F. Hervé

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Previous work has shown that Devonian magmatism in the southern Andes occurred in two contemporaneous belts: one emplaced in the continental crust of the North Patagonian Massif and the other in an oceanic island arc terrane to the west, Chaitenia, which was later accreted to Patagonia. The country rocks of the plutonic rocks consist of metasedimentary complexes which crop out sporadically in the Andes on both sides of the Argentina-Chile border, and additionally of pillow metabasalts for Chaitenia. Detrital zircon SHRIMP U-Pb age determinations in 13 samples of these rocks indicate maximum possible depositional ages from ca. 370 to 900 Ma, and the case is argued for mostly Devonian sedimentation as for the fossiliferous Buill slates. Ordovician, Cambrian-late Neoproterozoic and “Grenville-age” provenance is seen throughout, except for the most westerly outcrops where Devonian detrital zircons predominate. Besides a difference in the Precambrian zircon grains, 76% versus 25% respect...

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Zircon U-Pb ages and Hf isotopes for the Diablillos Intrusive Complex, Southern Puna, Argentina: Crustal evolution of the Lower Paleozoic Orogen, Southwestern Gondwana margin
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The evolution of the rocks of the Lower Paleozoic Orogen in Puna, at the Southwestern Gondwana margin, has been widely debated. In particular, the scarce amount of geological and geochemical data available for the Diablillos Intrusive Complex, Eastern Magmatic Belt, Southern Puna, require a further study for new evidence towards the understanding of sources, magmatic processes and emplacement of magmas, in order to better comprehend the crustal evolution in this setting. We present new combined UePb and Hf isotope analyses on zircon by LA-MC-ICP-MS from monzogranite, granodiorite and diorite rocks of the Diablillos Intrusive Complex. We obtained 206 Pb/ 238 U concordant weighted average ages of 517 ± 3 Ma and 515 ± 6 Ma for the monzogranite and diorite, respectively, and a concordant age of 521 ± 4 Ma for the granodiorite. These ages permit to constrain the climax of magmatic activity in the Diablillos Complex around ~515e520 Ma, while the emplacement of the complex took place between ~540 Ma and 490 Ma (representing a ca. 50 Ma magmatic event). Major and trace element data, initial 87 Sr/ 86 Sr values varying from 0.70446 to 0.71278, positive and negative 3Nd(t) values between þ2.5 and À4, as well as 3Hf(t) for zircon data between þ 3 and À3 indicate that the analyzed samples represent contaminated magmas. The 3Hf(t) and the 3Nd(t) values for this complex specify that these rocks are derived from interaction of a dominant Mesoproterozoic crystalline and/or a metasedimentary source and juvenile mantle-derived magmas, with a T DM model age range of ~1.2e1.5 Ga, with later reworking during lower Paleozoic times. The combined data obtained in this contribution together with previous data, allow us to suggest that the formation of the Eastern Magmatic Belt of the Puna was part of a long-lived magmatic event during Early Paleozoic times. Whereby the granitoids of the Eastern Magmatic Belt formed through intra-crustal recycling at an active continental margin, with minor contributions from juvenile material in the back-arc setting.

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Geochronology and geochemistry of Devonian magmatism in the Frontal cordillera (Argentina): geodynamic implications for the pre-Andean SW Gondwana margin
Juan Dahlquist

International Geology Review, 2020

Whereas north of 33°S Devonian calcalkaline magmatism is notably absent, a Devonian arc is well developed to the south of 33°S. The Carrizalito, Pampa de los Avestruces, and Papagayos plutons are located in the Frontal Cordillera between 34° and 35°S. U-Pb zircon SHRIMP and LA-MC-ICP-MS data from the plutons yield Early Devonian ages of 389 ± 3 and 409 ± 3 for Carrizalito, 413 ± 2 Ma for Pampa de los Avestruces, and 346 ± 3, 393 ± 5 and 412 ± 3 Ma for Papagayos. The younger zircon age from the Papagayos pluton suggests that the Devo nian magmatism continued during the Early Carboniferous. Carrizalito and Pampa de los Avestruces plutons display metaluminous to slightly peraluminous granodiorite composition, with SiO 2 ranging from 65.1 to 67.8 wt.% and from 67.9 to 69.7 wt.%, respectively, and define a magnesian calc-alkalic trend. The Papagayos pluton is metaluminous to slightly peraluminous and slightly more evolved in composition (felsic granodiorite to monzogranite), with a SiO 2 content that varies between 68.4 to 70.5 wt.%, defining a magnesian with ferroan enrichment and calc-alkalic to alkali-calcic trend. These geochemical characteristics are similar to those reported for the Carboniferous arc granitoids in the Frontal Cordillera and Western Sierras Pampeanas at 27°-30°S and the archetypical calc-alkaline granitoids of the Peruvian Coastal batholith, indicating an Early Devonian magmatic arc. Rare earth element patterns ([La/Yb] N = 10-20) with mostly slight to moderate negative Eu anomalies (Eu/ Eu* = 0.62-0.80) and geochemical 'spider-diagrams' are similar to arc granitoids. Combined wholerock Nd and Sr isotope data from the largest Carrizalito and Pampa de los Avestruces plutons (εNd t = −4.3 and −4.6; 87 Sr/ 86 Sr t = 0.70704 and 0.70762; T DM = ca. 1.5 Ga) together with in situ U-Pb and Hf isotope data from magmatic zircon (εHf t ranging from −16.5 to −5.4; T DM = ca. 1.7 Ga), suggest a common older continental lithosphere source. The presented data indicate tectonic segmentation in the pre-Andean margin controlling the presence or absence of Devonian subductionrelated magmatism. This segmentation is best explained by changes in the tectonic plate configuration occurred in a long-lived convergent margin.

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Detrital zircon age patterns and provenance of the metamorphic complexes of southern Chile
Francisco Herve

Journal of South American Earth Sciences, 2003

Zircon SHRIMP U-Pb age patterns are reported for 13 metasedimentary rocks from the low grade metamorphic complexes of the Patagonian Andes. Combined with four recently published patterns, these provide the first detailed survey of the provenance of these complexes. The youngest dated zircons, corresponding to maximum sedimentation ages, are Devonian-Late Triassic in the eastern Andes metamorphic complex, Carboniferous in the main range metamorphic complex, Permian in the Duque de York complex, and Late Triassic in the Chonos metamorphic complex. In the last two cases, these ages are in agreement with their respective fossil ages. Older components in the eastern Andes metamorphic complex include a large proportion of Proterozoic (predominantly 1000-1200 Ma) zircons, which may indicate distribution, probably by rivers, of detrital material from regions currently in northern South America, Africa, or east Antarctica. The abundance of Proterozoic zircons is very much less in the Duque de York complex, possibly because of the rise of an inferred Permian magmatic arc related to the Gondwanan orogeny and consequent westward migration of the watershed. A Late Triassic magmatic episode is registered in the Chonos metamorphic complex, where reappearance of significant Proterozoic zircons indicates exhumation of the cratonic areas or of recycled sedimentary material.

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