Apatite fission-track thermochronology of the Sierras Pampeanas, central western Argentina: Implications for the mechanism of plateau uplift in the Andes

Coughlin, Timothy J.; O’Sullivan, Paul B.; Kohn, Barry P.; Holcombe, R. J.

doi:10.1130/0091-7613(1998)026<0999:afttot>2.3.co;2

Cited by 106 publications

(88 citation statements)

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“…Late Miocene tectonic activity in the Sierras Pampeanas was dominated by broad regions of basement uplifts forming kilometer-scale blocks (Jordan and Allmendinger 1986;Allmendinger 1986). Uplift occurred as a result of regional compression since the late Miocene (Coughlin et al 1998). Associated reverse faults subdivided the FNVC into several zones with various degrees of uplift, exposing volcanic and subvolcanic intrusive rocks at variable levels (Llambı´as 1970;Halter et al 2004a).…”

Section: Geological Settingmentioning

confidence: 99%

Magma evolution and the formation of porphyry Cu?Au ore fluids: evidence from silicate and sulfide melt inclusions

2005

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Silicate and sulfide melt inclusions from the andesitic Farallo´n Negro Volcanic Complex in NW Argentina were analyzed by laser ablation ICPMS to track the behavior of Cu and Au during magma evolution, and to identify the processes in the source of fluids responsible for porphyry-Cu-Au mineralization at the 600 Mt Bajo de la Alumbrera deposit. The combination of silicate and sulfide melt inclusion data with previously published geological and geochemical information indicates that the source of ore metals and water was a mantle-derived mafic magma that contained approximately 6 wt.% H 2 O and 200 ppm Cu. This magma and a rhyodacitic magma mixed in an upper-crustal magma chamber, feeding the volcanic systems and associated subvolcanic intrusions over 2.6 million years. Generation of the ore fluid from this magma occurred towards the end of this protracted evolution and probably involved six important steps: (1) Generation of a sulfide melt upon magma mixing in some parts of the magma chamber. (2) Partitioning of Cu and Au into the sulfide melt (enrichment factor of 10,000 for Cu) leading to Cu and Au concentrations of several wt.% or ppm, respectively. (3) A change in the tectonic regime from local extension to compression at the end of protracted volcanism. (4) Intrusion of a dacitic magma stock from the upper part of the layered magma chamber. (5) Volatile exsolution and resorption of the sulfide melt from the lower and more mafic parts of the magma chamber, generating a fluid with a Cu/Au ratio equal to that of the precursor sulfide. (6) Focused fluid transport and precipitation of the two metals in the porphyry, yielding an ore body containing Au and Cu in the proportions dictated by the magmatic fluid source. The Cu/S ratio in the sulfide melt inclusions requires that approximately 4,000 ppm sulfur is extracted from the andesitic magma upon mixing. This exceeds the solubility of sulfide or sulfate in either of the silicate melts and implies an additional source for S. The extra sulfur could be added in the form of anhydrite phenocrysts present in the rhyodacitic magma. It appears, thus, that unusually sulfur-rich, not Cu-rich magmas are the key to the formation of porphyry-type ore deposits. Our observations imply that dacitic intrusions hosting the porphyryCu-Au mineralization are not representative of the magma from which the ore-fluid exsolved. The source of the ore fluid is the underlying more mafic magma, and unaltered andesitic dikes emplaced immediately after ore formation are more likely to represent the magma from which the fluids were generated. At Alumbrera, these andesitic dikes carry relicts of the sulfide melt as inclusions in amphibole. Sulfide inclusions in similar dykes of other, less explored magmatic complexes may be used to predict the Au/Cu ratio of potential ore-forming fluids and the expected metal ratio in any undiscovered porphyry deposit.

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Section: Geological Settingmentioning

confidence: 99%

Magma evolution and the formation of porphyry Cu?Au ore fluids: evidence from silicate and sulfide melt inclusions

2005

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“…Likewise, most terrestrial sediments are deposited in association with active mountain belts, creating the additional challenge of deconvolving regional climatic signals from local tectonic-orographic signals. These issues are especially challenging in the south-central Andes where a regionally extensive climate transition in the latest Miocene (14, 27, 28) appears synchronous with significant tectonic uplift at this time (29)(30)(31)(32). It thus remains unclear whether the timing and extent of latest Miocene landscape ecologic changes in the south-central Andes were primarily controlled by tectonic-orographic effects or by globalscale changes in climate (e.g., refs.…”

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confidence: 99%

Mio-Pliocene aridity in the south-central Andes associated with Southern Hemisphere cold periods

Amidon

Fisher

Burbank

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Although Earth’s climate history is best known through marine records, the corresponding continental climatic conditions drive the evolution of terrestrial life. Continental conditions during the latest Miocene are of particular interest because global faunal turnover is roughly synchronous with a period of global glaciation from ∼6.2–5.5 Ma and with the Messinian Salinity Crisis from ∼6.0–5.3 Ma. Despite the climatic and ecological significance of this period, the continental climatic conditions associated with it remain unclear. We address this question using erosion rates of ancient watersheds to constrain Mio-Pliocene climatic conditions in the south-central Andes near 30° S. Our results show two slowdowns in erosion rate, one from ∼6.1–5.2 Ma and another from 3.6 to 3.3 Ma, which we attribute to periods of continental aridity. This view is supported by synchrony with other regional proxies for aridity and with the timing of glacial ‟cold” periods as recorded by marine proxies, such as the M2 isotope excursion. We thus conclude that aridity in the south-central Andes is associated with cold periods at high southern latitudes, perhaps due to a northward migration of the Southern Hemisphere westerlies, which disrupted the South American Low Level Jet that delivers moisture to southeastern South America. Colder glacial periods, and possibly associated reductions in atmospheric CO2, thus seem to be an important driver of Mio-Pliocene ecological transitions in the central Andes. Finally, this study demonstrates that paleo-erosion rates can be a powerful proxy for ancient continental climates that lie beyond the reach of most lacustrine and glacial archives.

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“…Our hypothesis predicts the largest changes south of the region from the Beni basin where rainfall rates decrease from > 3500 mm per year at 16 ° S, to the Pilcomayo basin at 20 ° S with ca 1000 mm per year, as seen by the shift towards low channel concavities and gradients 14 . Th is is indeed supported by northsouth reversal trends of apatite fi ssion track ages 26,44,52,53 , and a series of left -lateral transtensional faults ( Fig. 1a ).…”

Section: Discussionmentioning

confidence: 60%

Migrating deformation in the Central Andes from enhanced orographic rainfall

Norton

Schlunegger

2011

Nat Commun

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Apatite fission-track thermochronology of the Sierras Pampeanas, central western Argentina: Implications for the mechanism of plateau uplift in the Andes

Cited by 106 publications

References 0 publications

Magma evolution and the formation of porphyry Cu?Au ore fluids: evidence from silicate and sulfide melt inclusions

Magma evolution and the formation of porphyry Cu?Au ore fluids: evidence from silicate and sulfide melt inclusions

Mio-Pliocene aridity in the south-central Andes associated with Southern Hemisphere cold periods

Migrating deformation in the Central Andes from enhanced orographic rainfall

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