James K.W. Lee

Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids

Collision tectonics and the associated transformation of continental crust to high-pressure rocks (eclogites) are generally well-understood processes, but important contradictions remain between tectonothermal models and petrological–isotopic data obtained from such rocks. Here we use 40Ar–39Ar data coupled with a thermal model to constrain the time-integrated duration of an orogenic cycle (the burial and exhumation of a particular segment of the crust) to be less than 13 Myr. We also determine the total duration of associated metamorphic events to be ∼20 kyr, and of individual heat pulses experienced by the rocks to be as short as 10 years. Such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events (earthquakes). Such events triggered transient heat advection by hot fluid along deformation (shear) zones, which cut relatively cool and dry subducted crust. In contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust, our non-steady-state cold-crust model satisfactorily explains several otherwise contradictory geological observations.

The Cenozoic History of Volcanism and Hydrothermal Alteration in the Central Andean Flat-Slab Region: New 40Ar-39Ar Constraints from the El Indio–Pascua Au (-Ag, Cu) Belt, 29°20′–30°30′ S

Ninety-three new 40Ar-39Ar laser step-heating plateau dates for igneous rocks and alteration minerals from the El Indio-Pascua Au-Ag belt permit significant refinement of the Tertiary volcanic stratigraphy and the definition of a succession of alteration events in this major mineralized district. Eight distinct Tertiary stratigraphic units are now recognized (two newly defined in this research): (1) the 30-36 Ma Bocatoma Unit, comprising dioritic and granodioritic shallow-level intrusions; (2) the voluminous 23-26 Ma Tilito Formation, consisting predominantly of dacitic tuffs; (3) the 17.5-21 Ma Escabroso Group made up of extensive successions of andesitic flows and coeval hypabyssal intrusions; (4) the 14-17 Ma, andesitic, Cerro de las Tórtolas Formation and its intrusive lithodeme, the Infiernillo Unit; (5) the dacitic, 11.0-12.7 Ma, Vacas Heladas Formation; (6) the rhyodacitic 7.5-8 Ma Pascua Formation, defined in this study; (7) the 5.5-6.2 Ma rhyolitic Vallecito Formation; and (8) the recently defined rhyolitic 2 Ma Cerro de Vidrio Formation. Magmatic activity decreased markedly following the eruption of the Cerro de las Tórtolas Formation. Hydrothermal activity occurred at least from the late Eocene to the Late Miocene, but economic Au-Ag-Cu mineralization was confined to the 6-9.5 Ma interval, the only observed contemporaneous igneous unit being the restricted Pascua Formation. Epithermal Au-Ag-Cu deposits and major prospects emplaced in this period include, from north to south, Pascua-Lama, Veladero, Sancarron, Rio del Medio, El Indio, Tambo, and Vacas Heladas. The widespread, albeit barren, alteration associated with the Bocatoma, Escabroso, Infiernillo, and Vacas Heladas magmatism indicates that the availability of hydrothermal fluid was not the controlling factor for ore formation, emphasising instead the role of the metal content of the magmas associated with epithermal mineralization, and/or the requirement for favorable physiographic conditions at the site of ore deposition.

An intercalibration study of the Fish Canyon sanidine and biotite 40Ar/39Ar standards and some comments on the age of the Fish Canyon Tuff

The age of the Fish Canyon Tuff (FCT) has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the K/Ar and U/Pb isotopic systems. Minerals from the FCT (notably sanidine and biotite) are in widespread use as 40Ar/39Ar fluence monitors (“standards”) and have been employed as both primary and intercalibrated standards. In attempting to constrain the true age of FCT, the distinction between primary vs. intercalibrated ages is important, as only the primary age determinations are valid. An investigation of both Fish Canyon sanidine (FCs) and Fish Canyon biotite (FCT-3) was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain-to-grain scale. Electron microprobe results show that FCs is chemically homogeneous, verifying previous studies, although there are some very small zones of minor K depletion; FCT-3 biotite commonly contains numerous and small mineral inclusions. 40Ar/39Ar experiments further show that FCs is remarkably homogeneous, confirming previous observations, whereas FCT-3 shows much greater isotopic variability due to contamination from the mineral inclusions. By setting the age of FCs equal to 27.98±0.15 Ma (2σ), as intercalibrated to the U/Pb system by Villeneuve et al. (2000), the resultant intercalibration factor between FCs and FCT-3 is 1.005±0.017 yielding an age of 28.13±0.47 Ma (2σ) for FCT-3, although the significant intragrain chemical and isotopic variability precludes its use as an effective 40Ar/39Ar fluence monitor. The true age of the FCT is problematic; a survey of all primary ages for FCT shows that mean K/Ar ages for plagioclase, hornblende, biotite, and sanidine are consistently younger than mean U/Pb ages from titanite and zircon. In conjunction with recently published petrological results which indicate that the FCT contains material from multiple crystallization events, this significant discrepancy between the K/Ar and U/Pb data sets demonstrates that the Fish Canyon Tuff cannot serve as a golden standard between the K/Ar and U/Pb systems.

Argon release mechanisms of biotite in vacuo and the role of short-circuit diffusion and recoil

Abstract Understanding argon release mechanisms in K-bearing minerals is essential in interpreting the 40 Ar / 39 Ar data and their application to geological studies. The release mechanisms of argon in vacuo have been examined in a series of 40 Ar / 39 Ar isothermal heating experiments on two biotite specimens with Fe/(Fe+Mg) (Fe # )=0.50 and 0.87 respectively. The crystal structure of the biotite was also monitored during in vacuo heating by an in–situ high temperature X-ray diffractometer (HTXRD), and also examined by scanning electron microscopy (SEM). At temperatures greater than 600°C, argon release is mainly controlled by the structural decomposition of the biotite crystal arising from oxidation and dehydroxylation, whereas at temperatures less than 600°C, argon release appears to be controlled by a multipath-diffusion mechanism, with effective D / a 2 values about 2–4 orders of magnitude higher than those extrapolated from hydrothermal data. Both the argon diffusivity and Ar release patterns are strongly related to biotite composition, in which the Fe-rich biotite has a higher argon diffusivity and degasses at lower temperatures than the Mg-rich biotite. Unless contaminated by other phases, biotites will tend to yield flat age spectra for temperature steps higher than 600°C, regardless of the initial distribution of argon isotopes in the crystal structure, since the argon released at T >600°C is strongly correlated with the decomposition process. At temperature steps lower than 600°C, however, biotite age spectra can exhibit discordant dates since the gas release is controlled mainly by defect-enhanced (short-circuit) diffusion mechanisms. Consequently, models using such low- T steps with the intent of extracting information on the spatial distribution of Ar will not lead to accurate interpretations of geologic histories, unless the potential effects of short-circuit diffusion are well-constrained.

Response of Supergene Processes to Episodic Cenozoic Uplift, Pediment Erosion, and Ignimbrite Eruption in the Porphyry Copper Province of Southern Perú

The Jurassic to middle Eocene porphyry copper deposits and prospects exposed on the Pacific slopes of the central Andean Cordillera Occidental of southern Peru between latitudes 16°309 and 18° S record a protracted, ca. 30-m.y. history of supergene processes that were fundamentally controlled by the evolving local geomorphologic environment, itself a response to successive regional tectonic events, including the late Eocene Incaic, the late Oligocene to earliest Miocene Aymara, and the middle to late Miocene Quechuan events. Weathering of the porphyry centers also overlapped temporally with the local resumption of arc volcanism in southern Peru at 25.5 Ma following a 27-m.y. amagmatic interval, and supergene processes were variously interrupted or terminated by ignimbrite blanketing, although in several locations supergene profiles were preserved by such cover. The landform chronology for the area surrounding the Cuajone, Quellaveco, and Toquepala deposits (ca. 17° S) is revised and extended northwestward through field mapping to the Cerro Verde-Santa Rosa district (ca. 16° 309 S). The 40 Ar- 39 Ar incremental-heating dates of supergene alunite group minerals from the Angostura (38.1 and 38.8 Ma) and Posco (38.8 Ma) prospects and the Cerro Verde deposit (36.1–38.8 Ma) demonstrate that supergene processes were underway in the late Eocene beneath a subplanar topography resulting from uplift and erosion during the Incaic orogeny, now represented by a regional unconformity in the Cenozoic volcanic-sedimentary rock succession. Broadly contemporaneous supergene processes were probably active in the Cuajone-Quellaveco-Toquepala district. Slow erosion and the accumulation of clastic sediments through the tectonically quiescent early to mid-Oligocene are envisaged to have caused a rise in the water table and the widespread preservation of the Incaic supergene profiles. Aymara uplift subsequently led to the incision of the 23.8 to 24 Ma Altos de Camilaca and the 18.8 to 19.1 Ma Pampa Lagunas pediplains and their regional correlatives. The ensuing water-table lowering was associated with intense leaching and sulfide enrichment from the late Oligocene (24.4–28 Ma natroalunite at Cerro Verde, 26–27 Ma natroalunite at Santa Rosa, and 28.6 Ma jarosite at La Llave) to the early Miocene (23 Ma alunite and 21 Ma natroalunite at Cerro Verde, and 19.2 Ma jarosite at La Llave) and was plausibly responsible for much of the upgrading of the Cuajone and Toquepala deposits and thr Quellaveco prospect, which are intersected by both the Altos de Camilaca pediplain and erosional features representing upslope extensions of the Pampa Lagunas pediplain. The younger supergene profiles were widely superimposed on the remnants of those generated during the Incaic orogeny. Middle Miocene (<14.2 Ma biotite age) Chuntacala Formation flows protected the Cuajone supergene profile from destruction by erosion, but at 13.0 Ma interrupted supergene processes at Quellaveco. Revision of volcano-stratigraphic relationships in the latter area reveals that subsequent erosion of the Chuntacala Formation ignimbrites and part of the supergene profile took place prior to the deposition of a 10.1 Ma ash-flow tuff of the Asana Formation. Elsewhere, supergene activity persisted at the Cachuyito prospect through 11.4 Ma, and minor jarosite development occurred at least until 4.9 Ma both there and at Cerro Verde during and following the Multiple Pediment landform stage (ca. 7.9–15.0 Ma). The occurrence of relics of late Eocene alunite group minerals within considerably younger late Oligocene to late Miocene supergene alteration profiles suggests that the overall physiographic configuration of the Pacific piedmont of southern Peru remained remarkably consistent from the late Eocene to the middle Miocene. Moreover, the new age data confirm that, as in northern Chile, semiarid climatic conditions prevailed along much of the plate boundary from the mid-Eocene until the late Miocene or early Pliocene onset of hyperaridity. The local geomorphologic and volcanic conditions in southern Peru, however, conspired to generate more complex supergene profiles with lower aggregate enrichment factors relative to the strongly enriched profiles in the late Eocene to early Oligocene porphyry copper belt of northern Chile, which underwent supergene upgrading over relatively brief periods.

Geology and geochronology of Paleozoic rocks in western Acatlán Complex, southern Mexico: Evidence for contiguity across an extruded high-pressure belt and constraints on Paleozoic reconstructions

The Acatlan Complex straddles a high-pressure belt previously interpreted as either: (1) a suture zone within the Iapetus or the Rheic oceans, which would have a contrasting geological record across the suture; or (2) a tectonic slice extruded into the upper plate, which would imply contiguity across the complex. Distinguishing between these hypotheses is critical to paleogeographic reconstructions. Examination of the western Acatlan Complex reveals the following: (1) deposition of clastic rocks between 654 and 464 Ma; (2) intrusion of bimodal Ordovician bodies at ca. 464 Ma; (3) high-grade deformation with cooling through 400 °C by ~360–335 Ma; (4) deposition of clastic rocks and pillow lavas after ~350–400 Ma; (5) deformation accompanied by greenschist facies metamorphism at ca. 335 Ma; (6) deposition of clastic and bimodal volcanic rocks at ca. 327 Ma; (7) ~320–270 Ma subgreenschist deformation; (8) deposition of the Middle-Upper Permian sedimentary rocks; and (9) intrusion of a 61 ± 1 Ma diorite followed by early Cenozoic (Laramide) ENE folding and faulting. Zircon ages (~350–400, 570–505, 827–890 Ma, 0.9–1.3 Ga) suggest both local and Amazonian sources with deposition above a local Mesoproterozoic (Oaxacan) basement on the southern margin of the Rheic Ocean. This geological record is very similar to that of the eastern Acatlan Complex, which supports the extrusion hypothesis, a model that may be applicable to other orogens.

Counterclockwise Rotation of the Michoacan Block: Implications for the Tectonics of Western Mexico

Subduction of the Farallon plate beneath North America resulted in formation of the Rivera and Cocos oceanic plates, the extensive magmatic arcs of the Sierra Madre Occidental (SMO), and the Trans-Mexican Volcanic Belt (TMVB). Southern Mexico consists of crustal blocks separated by a regional extensional structural system; the latter, called the Guadalajara triple junction, is defined by the Tepic-Zacoalco (TZR), Colima (CR), and Chapala (CHR) rifts. TZR and CHR separate the SMO from the Jalisco and Michoacan blocks, whereas CR is the boundary between the Jalisco and Michoacan blocks. In this study, we carried out combined radiometric and paleomagnetic analyses in the Michoacan block. Radiometric dates of 31.60 to 8.39 Ma confirm both the southern extension of the Sierra Madre Occidental and the early mafic TMVB succession into the Michoacan block. The Oligocene age agrees well with the radiometric dating reported for the southern SMO and the Tertiary volcanic fields of the Sierra Madre del Sur. Paleomagnetic data indicate a counterclockwise rotation of ∼24° about a vertical axis for the Michoacan block. Several plate models suggest either dextral or sinistral oblique convergence of the Cocos plate relative to North America. Our new results help to constrain these different models. These data demostrate that deformation in the Michoacan block is as old as late Miocene, and is related to sinistral oblique convergence of the Cocos plate relative to North America—inducing the southeast relative motion of the Michoacan block. The structural trends along both CHR and CR are thereby explained. On the other hand, right-lateral transtension along the TZR is related to the westward motion of the Jalisco block because of oblique convergence of the Rivera plate.

Insights into the tectonomagmatic evolution of NW Mexico: Geochronology and geochemistry of the Miocene volcanic rocks from the Pinacate area, Sonora

Miocene volcanic rocks in the Pinacate area, Sonora, record a progressive change in the source of magmatism induced by asthenospheric upwelling and lithospheric thinning. 40 Ar/ 39 Ar age data, mineral chemistry, and major- and trace-element contents allow the identifi cation of two volcanic sequences: an oldest basaltic episode (ca. 20 Ma), and a middle Miocene (12‐15.5 Ma) sequence that consists of mesa basalts with transitional alkali character, calc-alkaline dacites, and high-silica rhyolites evolving toward peralkaline liquids. Sr, Nd, and Pb isotope ratios reveal different sources for the Miocene basalts. The easternmost basalts have signatures indicating a Precambrian lithospheric mantle source, while the westernmost tholeiitic to transitional basalts are related to mixing of lithospheric and asthenospheric mantle. Rhyolites are the result of fractional crystallization of transitional basalt magmas with slight contamination by Precambrian crust. Chemical modeling shows that peralkaline rhyolites are related to slightly higher assimilation during their residence in the upper crust but also to a change in the mantle source of the parent basalt. The evolution of the isotopic signatures in space and time indicates that: (1) the volcanic activity is located over a major lithospheric boundary, i.e., the western limit of the North American Craton; (2) the lithosphere was progressively thinned so that huge volumes of alkalic basalts could access the surface during the Quaternary, building the Pinacate Volcanic Field. Correlation between geochemical signatures and the tectonic evolution of the western margin of the North American Craton shows that a progressive change in the source of magmatism can be related to the development of a slab window during the Miocene.

Late Cretaceous subduction of the continental basement of the Maya block (Rabinal Granite, central Guatemala): Tectonic implications for the geodynamic evolution of Central America

The Rabinal Granite is a peraluminous S-type composite pluton formed upon partial melting of a metasedimentary source region that fringes the southernmost North America plate in central Guatemala. It is therefore considered, together with the intruded metasedimentary sequences, to be part of the continental basement of the Maya block. This leucocratic K-feldspar–plagioclase–quartz–muscovite ± biotite granite shows increasing deformation along its southern margin, where it is cut across by the dextral, Late Cretaceous, top-to-the-NE Baja Verapaz shear zone. Although it has been recently dated at 562–453 Ma (isotope dilution–thermal ionization mass spectrometry), the new data presented here, including laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb and 40 Ar- 39 Ar geochronology and electron-probe mineral chemistry, allow us to more precisely establish the timing of intrusion and metamorphic overprinting of the Rabinal Granite. The zircons dated by LA-ICP-MS indicate a crystallization age of 471 +3 / −5 Ma (Early Ordovician), as well as abundant inherited cores with Pan-African and Mesoproterozoic dates. Laser total fusion Ar-Ar analyses of magmatic low-silica muscovite (Si = 6.2–6.4 atoms per formula unit) indicate cooling following magmatic crystallization during the mid-late Paleozoic and variable extents of resetting of Ordovician micas during Cretaceous metamorphism and deformation. The pressure-temperature ( P - T ) conditions of the inferred Ordovician metamorphism that produced partial melting of the metasedimentary source of the Rabinal Granite and the ascent and crystallization of the granitic melt are uncertain, but a clockwise P - T -time path with maximum P and T of P and T of ∼8.5 kbar and ∼300 °C, respectively. This second event, dated at 70.1 ± 0.6 Ma by means of laser total fusion 40 Ar- 39 Ar analyses on high-Si muscovite grains, is interpreted to be the result of subduction and accretion of the basement of the Maya block during the latest Cretaceous, likely in a transpressional tectonic regime related to the lateral collision of the Maya block with the Pacific (Farallon)–derived Caribbean arc. This finding represents the first direct evidence for latest Cretaceous subduction of the metamorphic Paleozoic basement of the Maya block, north of the Baja Verapaz shear zone.

Arc plutonism in a transtensional regime: the late Palaeozoic Totoltepec pluton, Acatlán Complex, southern Mexico

The ENE-trending, ca. 306–287 Ma, Totoltepec pluton is part of a Carboniferous–Permian continental magmatic arc on the western Pangaean margin. The 15 km × 5 km pluton is bounded by two N–S Permian dextral faults, an E–W thrust to the south, and an E–W normal fault to the north. Thermobarometric data indicate that the main, ca. 289–287 Ma, part of the pluton was emplaced at ≤20 km depth and ≥700°C and was exhumed to 11 km and 400°C in 4 ± 2 million years. We have documented the following intrusive sequence: (1) the 306 Ma northern marginal mafic phase; (2) the 287 Ma main trondhjemitic phase; and (3) ca. 289–283 Ma sub-vertical dikes that vary from (a) N39E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50–73E, foliated and folded with sinistral shear indicators, to (c) N73–140E and boudinaged. The obliquity of the boundary between the folded and stretched dikes relative to the N–S dextral faults suggests sequential emplacement in a transtensional regime (with 20% E–W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289–287 Ma intrusion also contains a steep ENE-striking foliation and hornblende lineations varying from sub-horizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 Ma and 283 Ma. The distinctive characteristics of the Totoltepec pluton should prove useful in identifying similar tectonic settings within continental arcs.