Hamish A. Sandeman

Revised age of the Salla beds, Bolivia, and its bearing on the age of the Deseadan South American Land Mammal “Age”

ABSTRACT The Salla beds of Bolivia contain a mammalian faunal assemblage assigned to the Deseadan South American Land Mammal “Age” (SALMA), known elsewhere principally in Patagonia. The earliest platyrrhine monkey Branisella comes from a single stratigraphie level in these beds. The age of the Salla beds is debated; new radiometrie dates constrain the age of the Deseadan SALMA in Bolivia and clarify the timing of the first record of platyrrhines in South America. The oldest vertebrate fossils from the Salla beds are contained within the magnetic polarity interval ClOr, between 28.8 and 29.4 Ma. The youngest fossils occur above the 275 m ash in Chron C7Ar, between 25.65 to 25.82 Ma. The best known faunas, including those of the Branisella level, come from several localities within Chron C8 (25.82 to 27.02 Ma). If the recent suggestion by Flynn and S wisher (1995) that redating indicates that the Deseadan in Patagonia spans only about 27 to 29 Ma, then the bulk of the Salla fauna is younger than that of Pat...

An Integrated Tectono-Magmatic Model for the Evolution of the Southern Peruvian Andes (13-20°S) since 55 Ma

The definition and characterization of the Oligocene-Miocene Crucero Supergroup of the southern Peru Inner Arc domain provide a previously unavailable lithostratigraphic framework for the clarification of the geodynamic context of magmatism across the southern Peruvian transect. We integrate new lithostratigraphic, petrologic, and 40Ar/39Ar geochronologic data for these volcanic and hypabyssal rocks with published information to establish a geotectonic model for the transect since 55 Ma. The early Eocene to Early Miocene tectono-magmatic history of the region was controlled by the evolution of a flat-subduction regime, which was initiated at ∼52 Ma with the onset of rapid convergence, terminating the magmatic and hydrothermal activity of the Toquepala superunit of the Coastal Batholith. Collision of the flat slab with the leading edge of the Brazilian Shield lithospheric mantle at 39-40 Ma resulted in catastrophic failure of the overriding plate and crustal-scale, NE-directed ramping, generating the “Inca...

Siegenian generation of the Lizard ophiolite: U-Pb zircon age data for plagiogranite, Porthkerris, Cornwall

The long-controversial age of the Lizard ophiolite is resolved by U-Pb dating of three single magmatic zircons from a plagiogranite dyke near Porthkerris Point. This demonstrates that the axial oceanic crust was undergoing extensional ductile shearing, mylonitization, amphibolite facies metamorphism and, probably, anatexis at 397 ± 2 (2σ) Ma. In conjunction with published K–Ar, 40Ar–39Ar and Rb–Sr data for the complex, this Early Devonian age predicates a protracted interval between intra-oceanic magmatism and deformation, on the one hand, and obduction during Late Devonian, northwestward displacement of the Normannian High, on the other.

Volcano-stratigraphy and 40Ar/39Ar geochronology of the Macusani ignimbrite field: monitor of the Miocene geodynamic evolution of the Andes of southeast Peru

Abstract A total of 1400 m of ignimbritic ash-flow tuffs distributed in five cross-sections and extending from the pre-Neogene base to the uppermost flows were studied in the Miocene Macusani ignimbrite basin (southeast Peru). Volcano-stratigraphic interpolation and precise 40Ar/39Ar dating of sanidine, biotite, and rhyolitic glass from the ash-flow tuffs give rise to a southwest-northeast correlation chart subdivided into six volcanic cycles or fundamental cooling units i.e., (1) 10.0 ± 0.5; (2) 7.8–8.0 ± 0.1; (3) 7.5 ± 0.1; (4) 7.3 ± 0.1; (5) 6.8-7.0 ± O.1; and (6) 6.7 ± O.1 Ma. These delimit two brief e 10 ± 1 and 7 ± 1 Ma, which are sensibly synchronous with the Quechua 2 and Quechua 3 compressional events characterizing the tectonic regime in the Central Andes. This close relationship between tectonic pulses and felsic magmatic activity adds a supplementary constraint to models of the Miocene tectono-magmatic evolution of the Cordillera Oriental. The stratigraphic relationships of the uranium mineralization of the Macusani field are defined for the first time: the stratiform-stratabound occurrences are restricted to three main cooling units dated at 7.8, 7.5 and 6.9-6.8 Ma which constitute a maximum age for uranium deposition.

Lithostratigraphy, petrology and 40Ar-39Ar geochronology of the Crucero Supergroup, Puno department, SE Peru

Abstract The Cordillera de Carabaya region of the southern Peruvian Andes preserves a collage of Tertiary igneous suites, including extrusive and hypabyssal units, that are herein assigned to the Crucero Supergroup and Crucero Intrusive Supersuite, respectively. The supergroup consists of two petrologically and temporally distinct assemblages, the Picotani and the Quenamari Groups, both with silicic hypabyssal representatives. Stratigraphic, petrologic and 40 Ar- 39 Ar geochronologic studies of these rocks demonstrate that they are exposed in four localities, the Quenamari, Antauta, Cayconi and Picotani volcanic fields. The Picotani Group incorporates a diverse assemblage of lavas and pyroclastic rocks, including lamprophyres (minette), medium- to high-K calc-alkaline basalts, shoshonites, and S -type rhyodacites and rhyolites, as well as commingled and mixed associations of these, constituting ten mappable formations and twelve delimited hypabyssal units. These were emplaced over a brief interval from ca . 22 -to- 26 Ma. The Quenamari Group, in contrast, comprises entirely silicic, strongly peraluminous suites including, biotite + sillimanite ± muscovite ± andalusite ± tourmaline-bearing pyroclastic units and cogenetic epizonal intrusions. These constitute three distinct volcanic formations and at least 8 intrusive bodies. Rocks of the Quenamari Group span a broad temporal interval, from ca . 6.5 to 17 Ma. The rock associations of the Crucero Supergroup are clearly distinct from time-equivalent igneous units of the calc-alkaline, Main Andean arc in southern Peru, thereby implying differing geodynamic conditions during their genesis.

A critical appraisal of the Cayconi Formation, Crucero Basin, southeastern Peru

Abstract The Cayconi Formation of the Crucero Basin, Puno Department, southeastern Peru, has been described as a 800–1000 m sequence of Oligocene and Miocene fanglomerate and lacustrine sedimentary rocks, interlayered with mafic and silicic volcanic rocks, and unconformably overlying Paleozoic and Cretaceous successions. On the basis of new field and petrological investigations, key aspects of the stratigraphic relationships of the rocks comprising this formation, and hence the viability of this lithostratigraphic name, are questioned. Thus, several sedimentary units previously assigned to the Cayconi Formation are reinterpreted as Cretaceous or older. We further argue that the formational terminology fails to accomodate the great variety of volcanic rocks, which are, moreover, disposed in isolated eruptive fields. We therefore propose establishment of the Crucero Supergroup as a broad, yet flexible framework for lithostratigraphic subdivision of the Tertiary sequences of the Cordillera Oriental of southeastern Peru. This new nomenclature accomodates the voluminous two-mica ash flow tuffs (Macusani Volcanics) and associated rocks of the Quenamari Meseta, a succession excluded from the existing lithostratigraphic classification scheme. It also permits distinction between the petrogenetically-contrasted upper Oligocene — Lower Miocene and Middle Miocene — Upper Miocene volcanic suites, which dominate, respectively, the Picotani and Quenamari Groups comprising the proposed Crucero Supergroup. Finally, the economically important granitic/rhyolitic intrusive centers cogenetic with the volcanic rocks are readily assignable to intrusive lithodemes in each group.