Donald C. Noble

Episodic Cenozoic volcanism and tectonism in the Andes of Peru

Abstract Radiometric and geologic information indicate a complex history of Cenozoic volcanism and tectonism in the central Andes. K-Ar ages on silicic pyroclastic rocks demonstrate major volcanic activity in central and southern Peru, northern Chile, and adjacent areas during the Early and Middle Miocene, and provide additional evidence for volcanism during the Late Eocene. A provisional outline of tectonic and volcanic events in the Peruvian Andes during the Cenozoic includes: one or more pulses of igneous activity and intense deformation during the Paleocene and Eocene; a period of quiescence, lasting most of Oligocene time; reinception of tectonism and volcanism at the beginning of the Miocene; and a major pulse of deformation in the Middle Miocene accompanied and followed through the Pliocene by intense volcanism and plutonism. Reinception of igneous activity and tectonism at about the Oligocene-Miocene boundary, a feature recognized in other circum-Pacific regions, may reflect an increase in the rate of rotation of the Pacific plate relative to fixed or quasifixed mantle coordinates. Middle Miocene tectonism and latest Tertiary volcanism correlates with and probably is genetically related to the beginning of very rapid spreading at the East Pacific Rise.

Cenozoic stratigraphy, magmatic activity, compressive deformation, and uplift in northern Peru

Integrated field study and radiometric dating in the Llama-Bambamarca area of the Western Cordillera of northern Peru have resulted in major revisions in the stratigraphic and structural relations and ages of thick sequences of Tertiary volcanic rocks and constrain the timing of compressive deformation, magmatic activity, and uplift. Rocks of the upper part of the Llama Formation yield K-Ar mineral dates of 54.8 ± 1.8 and 44.2 ± 1.2 Ma; altered volcanic rocks composing the lower part of the formation in the western part of the area may be older. Eocene K-Ar dates have been reported for several granitic plutons in the region, and other plutons of probable similar age intrude the Llama Formation and are overlain by upper Eocene strata. The pervasive alteration that affects much of the Llama Formation may be the result of large hydrothermal systems driven by a coeval, and presumably cogenetic, batholith. The Chota Formation, which is mostly volcaniclastic, has yielded ages of about 50 and 44 Ma. The Chota Formation thus does not predate the Llama Formation but rather is an eastern distal-facies equivalent of rhyolitic ash-flow sheets and dacitic volcanic rocks of the Llama Formation. The major unconformity at the base of the Llama Formation reflects deformation during Late Cretaceous (Peruvian tectonic phase) and/or Paleocene (Incaic I) time. The time of Incaic II deformation is bracketed between the 44.2 ± 1.2 Ma date on the Llama Formation and the age of about 39 Ma of a thick ash-flow sheet that comprises the lower part of the unconformably overlying Huambos Formation. Dates on rocks postdating Incaic II tectonism in central Peru suggest that deformation had ceased by about 41 Ma. Incaic II tectonism in northern and central Peru appears to have been a short but intense compressive event that peaked about 43 m.y. ago. The Incaic II event is coeval with formation of the Hawaiian-Emperor bend and with a period of rapid convergence of the Nazca and South American plates and therefore may reflect a major plate change in lithospheric plate movement patterns. A largely volcaniclastic sequence of early Miocene age shows that the early Neogene pulse of volcanic activity recognized throughout the Central Andes is represented in northern Peru. Beds of conglomerate within this sequence are probably the result of Quechua I tectonism, although the 23.2 ± 1.5 Ma date obtained on tuff from the unit appears somewhat older than the time of about 19 Ma recognized for Quechua I tectonism in central and southern Peru. Units of partly welded and unaltered ash-flow tuff that fill deeply incised paleovalleys have ages of about 8.2 and 11.4 Ma, showing that the Western Cordillera of northern Peru was uplifted before late Miocene time.

Multiple pulses of Neogene compressive deformation in the Ayacucho intermontane basin, Andes of central Peru

Strata of the Neogene Ayacucho intermontane basin, central Peru, can be subdivided into three tectonostratigraphic units: the Larampuquio volcanics, consisting largely of intermediate lavas and coarse conglomerates of similar lithology; the unconformably overlying Huanta Formation, consisting of a lower part formed predominantly of lacustrine and volcaniclastic sedimentary rocks and an upper part composed largely of intermediate lavas, tuffs, and conglomerates; and the unconformably overlying Ayacucho Formation, consisting of complexly interfingering volcaniclastic sedimentary rocks, silicic tuffs, and lavas of intermediate to silicic composition. Radiometric ages of 17.3 ± 0.2 m.y. and 18.3 ± 0.6 m.y. have been obtained on tuffs from the upper and lower parts of the Larampuquio volcanics. Determinations on 2 rocks from the lower member of the Huanta Formation indicate an age of ∼11.4 ± 0.5 m.y. and a rock from the upper member is dated at 9.3 ± 0.3 m.y. B.P. Units of the Ayacucho Formation yield ages showing that it was deposited between ∼7.7 and 6 m.y. ago. The intervening Molinoyocc and Puchcas volcanics were erupted between ∼9.9 and 7.4 m.y. ago; these units may postdate the Huanta Formation or may be associated in part with the Huanta and in part with the Ayacucho Formation. Coarse conglomerates of the Larampuquio volcanics represent debris eroded from highlands uplifted during the first, and most intense, pulse of the late Cenozoic “Quechuan” phase of Andean compressive tectonism that began in the early Miocene. Folding of the Larampuquio volcanics may have taken place during this, or a subsequent, pulse of compression. A marked unconformity between the Huanta and Ayacucho Formations demonstrates a period of deformation between ∼8.5 and 9.5 m.y. ago. Compression also was active during at least part of the deposition of the Ayacucho Formation, as shown by inter-fingering wedges of coarse clastic debris along the northeastern margin of the basin and by angular unconformities within the formation. Folded rocks of the Ayacucho Formation and its equivalents to the west are overlain by undeformed volcanic units dated at 3.8 ± 0.4 m.y. B.P. and ∼5.5 m.y. B.P., bracketing a well-defined pulse of deformation slightly younger than 6 m.y. These subsequent episodes of compressive deformation represent recurrent pulses of the “Quechuan” phase. Folding related to these late Quechuan pulses is restricted in the Andes to northwest-trending belts corresponding to reactivated fault zones in the pre-Neogene substratum. A system of north-trending dikes in the Huachocolpa district 70 km west of Ayacucho suggests relaxation of compression in the Pliocene, and compression of Quaternary age has been recognized in the Huancayo area 170 km northwest of Ayacucho. Taken together with the first pulse of Quechuan deformation in the early and middle Miocene, these data indicate at least four pulses of crustal compression during the late Cenozoic separated by periods during which compressional stresses were at least partly relaxed and at times possibly replaced by extension.

Some observations on the cenozoic volcano-tectonic evolution of the Great Basin, western United States.

Abstract The onset of calc-alkalic intermediate volcanism over much of the central and southern parts of the western United States about 37 my ago cannot be adequately explained by a steady-state subduction model, and may reflect an abrupt change in the relative movements of the North American and Farallon plates and the underlying asthenosphere. Late Oligocene (33-29 my) quartz latitic and rhyolitic ash-flow volcanism in the east-central and central Great Basin may represent the later stages of this early phase of calc-alkalic volcanic activity. Very highly differentiated rhyolitic ash-flow tuffs and subordinate lavas erupted during the early Miocene along an arcuate belt concave to the northeast extending from eastern Oregon to southwesternmost Utah represent a second pulse of igneous activity. The near absence of mafic and intermediate lavas of this age reflects trapping of the dense primary mafic and derivative intermediate magmas beneath an unbroken crust. Middle Miocene and younger lavas, mainly andesite and dacite with minor quartz latitic pyroclastic rocks lying to the west and southwest of the axis of early Miocene silicic volcanism comprise a southern extension of the western Cascade belt of subduction-related volcanism reaching into southeastern Nevada. In the southern and western Great Basin generally more potassic volcanics of late Miocene and Pliocene age were erupted southwest of the middle Miocene calc-alkalic belt. The parallelism of these belts and the progressive shift of volcanism towards the west and southwest from Oligocene to Pliocene time favor the interpretation that these rocks are related to subduction. The outward shift of volcanism suggests that subduction became steeper with time. Crustal extension commenced virtually simultaneously over the Great Basin and adjacent areas during the middle Miocene 16 to 17 my ago. In the southern and west-central Great Basin the early phases of extension were superimposed on the latter stages of subduction-related volcanism. The wide range of rock types erupted in this region during the late Miocene and Pliocene probably reflects this overlap which resulted in derivation of primary magmas from various source materials over a wide range of pressure. In contrast, inception of bimodal basalt-rhyolite volcanism in the northern Great Basin 16 to 17 my ago was the direct result of the onset of crustal extension. Late Miocene and Pliocene rocks of this suite interfinger complexly with subduction-related calc-alkalic rocks erupted further to the west. The fact that faulting began simultaneously within, in front of, and behind the belt of contemporaneous andesitic volcanism supports the hypothesis that crustal extension in the western United States is the result of oblique divergence of the Pacific and North American plates and that it was initiated by an abrupt change in their direction of relative movement. True extension-related basaltic volcanism did not predominate in the southern and west-central Great Basin until the latter part of the Pliocene.

Age of the Cardenas Lavas, Grand Canyon, Arizona

Six whole-rock specimens of basalt from the Cardenas Lavas of the younger Precambrian Unkar Group yield a Rb-Sr isochron of 1.09 ± 0.07 b.y. This age is believed to approximate the time of extrusion of the lava. Potassium-argon age determinations of the lava are considerably younger and may reflect either diffusive loss of 40Ar or a period of heating about 800 m.y. ago.

Rare-element–enriched, S-type ash-flow tuffs containing phenocrysts of muscovite, andalusite, and sillimanite, southeastern Peru

Ash-flow tuffs of Neogene age exposed over 2,500 km 2 in the Macusani region of southeastern Peru are the volcanic equivalent of S-type granites. The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high 87 Sr/ 86 Sr i (0.7258 and 0.7226) and δ 18 O (+11‰). Elevated concentrations of Li, Cs, Be, Sn, B, and other minor elements compare with those in “tin granites.” Mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a HF/ a H 2 O. The chemical, isotopic, and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock.

Timing of late Tertiary deformation in the Andes of Peru

Radiometric (K-Ar) dating of a reconnaissance nature conservatively brackets late Tertiary deformation throughout Peru between about 20 and 5 m.y. ago. Folding appears to have begun about 15 to 17 m.y. ago. If the timing for Huancavelica Department, central Peru, is applicable to other parts of the country, then deformation and considerable uplift and erosion had taken place by 10.5 m.y. ago. Intense deformation and uplift in Peru is one of many major igneous and tectonic events that affected the western parts of North, Central, and South America and other parts of the globe during middle Miocene time. These events appear to be associated with first-order changes in the movement patterns of lithospheric plates and thus may reflect a major perturbation of the lithosphere-asthenosphere system.

Miocene volcanism and deformation in the western Cordillera and high plateaus of south-central Peru.

New radiometric ages on tuffs from south-central Peru support the postulated flare-up of volcanic activity during early Miocene time. In the region of Huancavelica, Julcani, and Lircay, lower Miocene rocks lie on folded strata of pre-Cenozoic age; the absence of units of Eocene and early Oligocene age indicates that this area remained positive after Incaic deformation in Eocene time. Conglomerate beds reflecting erosion attendant on the first pulse of late Cenozoic (Quechuan) compressive deformation do not appear in the stratigraphic record until after 21.5 m.y. B.P. In one section, beds of coarse conglomerate are underlain by tuff dated at 18.3 ± 0.6 m.y. and overlain by tuff dated at 17.3 ± 0.2 m.y. At another locality, beds of conglomerate conformably overlie tuff dated at 19.6 ± 0.8 m.y. If we incorporate published data from other areas in central and southern Peru, it appears that Quechuan deformation began in early Miocene time between 19.5 and 17 m.y. ago. A firm age for the end of the first pulse of Quechuan deformation in this region is provided by an ash-flow sheet dated at 12 to 12.5 m.y. that unconformably overlies strata of Eocene to early Miocene age.

Eureka Valley Tuff, East-Central California and Adjacent Nevada

The Eureka Valley Tuff consists of two major ash-flow sheets and a local over-lying sequence of ash-flow tuff erupted from vents within the Little Walker caldera 11 mi west-northwest of Bridgeport, in east-central California. The lower of the two major ash-flow sheets, here named the Tollhouse Flat Member, is the “biotite-augite-latite” of Ransome (1898). The overlying By-Day Member can readily be identified by the absence of phenocrystic biotite and by paleomagnetic and other petrographic criteria. The recognition of the distinctive By-Day Member above the Tollhouse Flat Member both in the Bridgeport area and west of the Sierra crest unequivocally demonstrates the generally accepted correlation of the latitic ash-flow tuffs of the two areas. K-Ar age determinations indicate that the three members of the Eureka Valley Tuff were erupted within a very short interval of time about 9.5 m.y. ago.

Initial gold contents of silicic volcanic rocks: Bearing on the behavior of gold in magmatic systems

Many fresh silicic volcanic rocks have markedly lower initial gold contents than previously recognized. Of 129 carefully selected glassy silicic volcanic rocks analyzed, 113 contain f (O 2 ) more readily accommodate gold. The mean of 23 relatively silicic intermediate rocks is 0.54 ppb Au, with tholeiitic andesites (icelandites) generally higher in gold than calc-alkalic types. There is little evidence that particular geologic regions are intrinsically richer in gold than others. Bulk composition, melt structure, and the amount and timing of separation of vapor, mineral, and sulfide and/or metal melt phases would appear to largely determine the gold contents of silicic magmas. The various lines of evidence for the removal of gold from magma by the separation of vapor, crystalline, and immiscible melt phases indicate that fresh volcanic rocks provide only minimum limits on magmatic gold concentrations. In most cases, magmatic values will be much higher. Elevated gold contents (0.3 to >1.0 ppm) of some porphyry deposits suggest the existence of salic magmas containing gold in concentrations appreciably greater than the 1-2+ ppb concentrations of basalts.