Alfredo Camacho

U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin

Abstract New SHRIMP zircon and monazite 206Pb/238U and 208Pb/232Th ages on structurally controlled units and 40Ar-39Ar step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by c. 530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at c. 480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450–460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian.

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.

Large volumes of anhydrous pseudotachylyte in the Woodroffe Thrust, eastern Musgrave Ranges, Australia

A mylonitic thrust zone, at least 1.5 km thick, forms a sharp contact between granulite and amphibolite facies gneisses in the eastern Musgrave Ranges, central Australia. The thrust dips gently to the south and is interpreted as an extension of the Woodroffe Thrust, which was formed about 550 Ma ago. Mylonites at the base of the thrust grade upwards into ultramylonites, which pass abruptly into a pseudotachylyte-bearing zone approximately 1 km thick, containing approximately 4% of pseudotachylyte veining. The orientation of the veins appears to be random. Pseudotachylytes occur only in the granulite facies rocks, and their precursors are felsic pyroxene and/or garnet granofelses. Rotated blocks of ultramylonite are present in some of the pseudotachylytes, and some pseudotachylyte veins have been plastically deformed, suggesting nearly contemporaneous semiductile and brittle behaviour. The matrix of the pseudotachylyte shows spectacular examples of igneous quench microstructures, especially skeletal and dendritic crystals of plagioclase and feathery pyroxene dendrites. Also present are glass devitrification microstructures (spherulites), evidence of liquid flow, and partly melted residual grains with former glassy rims showing different optical properties from those of the surrounding isotropic material. These features confirm that the pseudotachylyte formed by melting in anhydrous conditions. The matrix of the pseudotachylyte veins is less siliceous than the host rocks, owing to non-equilibrium melting of pyroxene, garnet and plagioclase. The igneous assemblages of the melt, notably the crystallization of pigeonite, are consistent with rapid cooling from very high-temperature (>1000°C). Melting and quenching is probably due to very local, short-lived rises in temperature accompanied by dilation.

Some isotopic constraints on the evolution of the granulite and upper amphibolite facies terranes in the eastern Musgrave Block, central Australia

Abstract The central to eastern Musgrave Block, central Australia, consists of the Meso- to Neoproterozoic Fregon and Mulga Park subdomains that are separated by a zone of mylonites and pseudotachylytes known as the Woodroffe Thrust. The subdomains were previously considered to have undergone different geological histories, but the differences relate mainly to metamorphic grade. The Fregon Subdomain attained at least transitional granulite facies metamorphic conditions and the Mulga Park Subdomain reached upper amphibolite facies. Felsic gneisses from both the Fregon and Mulga Park subdomains contain zircons that are structured with euhedral centres overgrown by round elongate rims. Ion-microprobe UPb analyses of the cores yield ages of 1557±24 and 1554±28 Ma, respectively, and are interpreted to represent the time of igneous crystallization. These ages place a maximum age constraint on the deposition or emplacement of the gneiss precursor. High-grade metamorphism, grading from upper amphibolite to granulite facies, took place between 1200 and 1150 Ma and prior to the intrusion of several granitoid suites at around 1150 Ma. The Kulgera Adamellite and Ayers Ranges Adamellite granitoid suites of the Fregon Subdomain yield ion-microprobe UPb crystallization ages of 1150±12 and 1152±11 Ma, respectively. A concordant UPb isotope dilution analysis on the Ayers Ranges Adamellite yields a more precise age of 1152±3 Ma. The porphyritic suite intruding the Mulga Park Subdomain yield an ion-microprobe UPb magmatic age of 1159±20 Ma. These emplacement ages indicate extensive Grenvillian felsic magmatism not previously recognized in Australia. The Woodroffe Thrust formed during a period of substantial crustal shortening, the Petermann Orogeny (∼550 Ma), when the Fregon Subdomain was thrust onto the Mulga Park Subdomain. Synkinematic muscovite and biotite record KAr and RbSr ages between 560 and 520 Ma. This Petermann event is not recorded in the Fregon Subdomain due to confinement of the strain and associated mineral recrystallization to the underlying Mulga Park Subdomain. We suggest that the granulites were not exposed prior to the Petermann Orogeny, as the basal quartzites of the Amadeus Basin occur only on the amphibolite facies Mulga Park Subdomain.

Intracratonic, strike‐slip partitioned transpression and the formation and exhumation of eclogite facies rocks: An example from the Musgrave Block, central Australia

Mountain belts developed in an intracratonic setting during the Neoproterozoic in the center of Australia. Mesoproterozoic granulite and amphibolite facies gneisses of the Musgrave Block were overprinted during the Petermann Orogeny (∼550 Ma) with zones of high strain concentrated along broadly east-west trending shear zones. These zones formed under eclogite facies (T ∼650°C and P ∼12 kbars) to greenschist facies conditions and may form part of a strike-slip-related, crustal-scale flower-type structure. Granulite facies gneisses preserve ages older than 700 Ma, suggesting that these rocks did not experience temperatures greater than ∼350°C at ∼550 Ma. For these rocks not to have been thermally equilibrated to temperatures of >350°C at a depth of ∼12 kbars requires either an extraordinarily low geothermal gradient (<9°C km−1) or, if the geothermal gradient was more typical (∼20°C km−1), then transport to that depth and subsequent exhumation occurring in a short time interval (≤40 Myr).

Evidence for shear heating, Musgrave Block, central Australia

The phenomenon of shear-heating is generally difficult to recognise from petrologic evidence alone. Establishing that shear zones attain higher temperatures than the surrounding country rocks requires independent evidence for temperature gradients. In the Musgrave Block, central Australia, there is a clear spatial association between shear zones and interpreted elevated temperatures. Eclogite facies shear zones that formed at ∼550 Ma record temperatures of ∼650–700°C. Outside the high-pressure shear zones, minerals with low closure temperatures such as biotite (∼450°C in the 40Ar–39Ar and Rb–Sr systems), preserve ages >800 Ma, suggesting that these rocks did not experience temperatures greater than about 450°C at ∼550 Ma for any extended period. Thus, the shear zones record temperatures that are ∼200°C higher than the surrounding country rocks. Simple calculations show that the combination of relatively high shear stresses (∼100 MPa) and high strain rates (∼10−11 s−1) for short durations (<1 Ma) can account for the observed apparent temperature variations. The evidence indicates that shear heating is the dominant mechanism for localised temperature increases in the shear zones, while the country rock remained at relatively lower temperatures.

Isotopic test of a thermally driven intraplate orogenic model, Australia

A recently proposed model for intraplate orogenesis couples long-term self-heating of basement rocks by radioactive decay with thermal blanketing by overlying sedimentary deposits. This model has been tested in one of the type areas in central Australia, the Proterozoic Musgrave Complex, which was reworked in an Early Cambrian orogeny. We have determined the source of the pre orogenic and postorogenic sediments in the Amadeus basin immediately to the north of the reworked basement, including the fan deposits associated with uplift, comprising Uluru (Ayers Rock) and Kata Tjuta (Olgas). Detrital-zircon age populations indicate that all basin sediments were derived from the Musgrave Complex, which was therefore emergent rather than covered by sediments as required by the model. In the basement, the preservation of Mesoproterozoic mica ages during transpressive burial to depths of ∼40 km ca. 550 Ma indicates that the associated thermal pulse was short-lived, not long-lived as envisaged in the model. We conclude therefore that the thermal-blanketing model is inconsistent with the isotopic data and that the localization of deformation in intracratonic settings is associated with regions of contrasting strengths; in central Australia, these are along the margins of the Amadeus basin with the Musgrave Complex and Arunta inlier.

UPb zircon dating of tectonomagmatic events in the northern Arunta Inlier, central Australia

Abstract The Mount Doreen area of the northern Arunta Inlier has a Proterozoic magmatic and tectonic history that extends over 300 Ma. The oldest unit, the turbiditic Lander Rock beds (LRB), was folded and metamorphosed from greenschist to granulite grade during the Yuendumu Tectonic Event. A UPb zircon age of 1880±5 Ma for the late-tectonic Ngadarunga Granite provides a minimum age for this event and supports its correlation with the widespread Barramundi Orogeny of northern Australia. Two subsequent major deformations, the Hardy and the Wabudali Tectonic Phases, considered to represent discrete phases of the Strangways Orogeny, are tentatively placed at around 1770 Ma. The Hardy Tectonic Phase locally retrogressed the LRB and produced strong foliations in some granites, including one of the Carrington Suite dated at 1779±6 Ma. The Wabudali Tectonic Phase formed high-strain zones, in places more than 2 km wide, within which the sedimentary Reynolds Range Group was tightly folded and the LRB were refolded and retrogressed. Felsic volcanics and arenites of the fault-bound Nicker beds and Patmungala beds, dated at 1772±5 Ma and 1799±10 Ma, respectively, were also tightly folded during the Strangways Orogeny. Juxtaposition of low-grade and high-grade LRB took place during faulting that occurred after the 1770 Ma tectonism, but prior to post-tectonic granites which may be indirectly related to the ∼ 1600 Ma Chewings Orogeny of the southern Arunta Inlier. One post-tectonic granite (1635±9 Ma) is associated with a mafic to intermediate intrusive complex and another (1567±11 Ma) is part of an extensive megacrystic suite that intrudes folded Reynolds Range Group. Zircon inheritance was found in five of the six analysed samples, with ages ranging from 1.85 to 3.45 Ga.

Constraints from diffusion profiles on the duration of high-strain deformation in thickened crust

Diffusion profiles in garnet are used to determine the duration of movement on shear zones. In the Musgrave Block, central Australia, Grenville age granulites were overprinted at depths of ~40 km by high-strain zones during the intracratonic Petermann orogeny (ca. 550 Ma ago). In these zones, quartzofeldspathic mylonites contain garnet from the early granulite facies event that developed compositional gradients during the high-pressure overprint. The diffusion of Ca into garnet is linked with the release of Ca in plagioclase (the only calcic mineral in the rock) during neocrystallization of sodic feldspar during high-strain deformation. Available diffusion data on garnet directly define the duration of high-strain deformation at eclogite facies. Our results indicate that the integrated time scales of movement in individual high-strain zones were short lived (between 0.07 Ma and 1.4 Ma), and that wide shear zones were active longer than narrow ones. In addition, we find that relict K-feldspar in one of the narrow eclogite facies shear zones yields a 40 Ar- 39 Ar age that is partially reset, indicating that the thermal event was also short lived. These short time scales support the hypothesis that shear heating was the dominant source of heat for the eclogite facies overprint.

Structural and biological control of the Cenozoic epithermal uranium concentrations from the Sierra Peña Blanca, Mexico

Epithermal uranium deposits of the Sierra Peña Blanca are classic examples of volcanic-hosted deposits and have been used as natural analogs for radionuclide migration in volcanic settings. We present a new genetic model that incorporates both geochemical and tectonic features of these deposits, including one of the few documented cases of a geochemical signature of biogenic reducing conditions favoring uranium mineralization in an epithermal deposit. Four tectono-magmatic faulting events affected the volcanic pile. Uranium occurrences are associated with breccia zones at the intersection of fault systems. Periodic reactivation of these structures associated with Basin and Range and Rio Grande tectonic events resulted in the mobilization of U and other elements by meteoric fluids heated by geothermal activity. Focused along breccia zones, these fluids precipitated under reducing conditions several generations of pyrite and uraninite together with kaolinite. Oxygen isotopic data indicate a low formation temperature of uraninite, 45–55°C for the uraninite from the ore body and ∼20°C for late uraninite hosted by the underlying conglomerate. There is geochemical evidence for biological activity being at the origin of these reducing conditions, as shown by low δ34S values (∼−24.5‰) in pyrites and the presence of low δ13C (∼−24‰) values in microbial patches intimately associated with uraninite. These data show that tectonic activity coupled with microbial activity can play a major role in the formation of epithermal uranium deposits in unusual near-surface environments.