Patagonian Batholith Research Articles

Sedimentary record of a Late Cretaceous volcanic arc in central Patagonia: petrography, geochemistry and provenance of fluvial volcaniclastic deposits of the Bajo Barreal Formation, San Jorge Basin, Argentina

The Upper Cretaceous Bajo Barreal Formation (San Jorge Basin, Argentina) is a fluvial succession mostly composed of channel sandstones interbedded with thicker floodplain deposits dominated by tuffaceous strata. The goal of this contribution is to infer the provenance of the unit through combined petrographical and geochemical data of channel sandstones, primary tuffs and tuffo-psammites (reworked, no-mixed tuffs). Channel sandstones are dominantly litharenites and feldspathic litharenites with abundant participation of volcanic lithic fragments with different textures (porphyritic, eutaxitic, felsitic, pilotaxitic, trachytic and vitric-vitrophyric), pumice and plagioclase. K-feldspar, quartz and sedimentary rock fragments are scarce. QFLu diagram and several provenance indicators including dominance and textural types of rock fragments, K-feldspar/plagioclase ratio and inclusion-free quartz grains indicate a provenance from felsic to intermediate, pyroclastic-rich, arc-related volcanic rocks. This interpretation agrees with the values of various elemental ratios of trace elements such as La/Sc, Th/Sc, Cr/Th, Eu/Eu ∗ and (La/Lu) N and discriminant functions based on major elements. Pyroclastic floodplain strata are commonly pumice-rich, occasionally glass shard-rich, vitric tuffs suggesting an origin from plinian-like eruptions. Subordinate components include volcanic lithic fragments, quartz, orthoclase and plagioclase. Zr/Ti versus Nb/Y and SiO 2 versus K 2O diagrams, as well as a multi-element diagram of selected trace elements shows that the pyroclastic rocks were originated from medium-K content, intermediate, arc related magmatic source. Considering the palaeoflow direction towards the east and south-east and presence of subduction processes along the western margin of Patagonia during the Late Cretaceous, it is interpreted that Bajo Barreal Formation constitutes the sedimentary record (distal facies of volcano-flanking fan or apron) of this coeval volcanic arc. Channel sandstones probably were mostly derived from the Divisadero Formation volcanic rocks because of similar petrographical features and REE pattern. In contrast, pyroclastic tuffaceous floodplain deposits would be derived from vents located over the actual outcrops of the Patagonian Batholith.

The metamorphic complexes of the Patagonian and Fuegian Andes

The Patagonian and Fuegian Andes are made up in part by late Paleozoic to Mesozoic metamorphic complexes. The mostly low grade late Paleozoic Eastern Andes Metamorphic Complex (EAMC) crops out to the East of the Meso-Cenozoic South Patagonian batholith (SPB), which intruded the metamorphic complexes. The protholit of the EAMC was likely deposited in a passive margin setting and at the Puerto Eden area underwent Late Jurassic sillimanite grade and migmatite local metamorphic conditions. It is suspected, but not proven, that the Cordillera Darwin Metamorphic Complex is a higher grade metamorphic complex equivalent of the EAMC. West of the SPB, paleo subduction complexes occur and are represented by the allochtonous Madre de Dios terrane. This terrane is composed of the ocean floor lithologies of the Denaro Complex topped by the Tarlton limestones that represent a guyot assemblage. The low grade continent derived Duque de York complex was deposited down top of the ocean floor lithologies. Further west, the blueschist bearing Middle Jurassic Diego de Almagro Complex, with psammopelitic, mafic and siliceous volcanic rock protoliths, evolved deep in a subduction zone during the Cretaceous. The possibility that the Antarctic Peninsula was located west of the present margin of South America is discussed.

Cooling histories and deformation of plutonic rocks along the Liquiñe-Ofqui Fault Zone, Southern Chile (41°-42°15'S)

Structural and microstructural observations combined with apatite and zircon fi ssion-track thermochronology within two sectors of the Main Andean Range in the Los Lagos Region of Chile reveal an episodic history of intrusion and deformation in the North Patagonian Batholith (NPB). A dextral displacement of ~30 km along the Liquine-Ofqui fault zone (LOFZ) is inferred from the correlation of corresponding Cretaceous intrusions of the NPB across the fault zone at Reloncavi. Relative uplift of the western block in the late Miocene is indicated by apatite and zircon fi ssion- track cooling histories. Microstructures in samples from Miocene and Cretaceous plutons along the fault zone generally indicate deformation at temperatures below ~300°C, with the exception of some samples from Cretaceous intrusions showing deformation at higher temperatures. In the Hornopiren area, signifi cant relative uplift of the eastern block is indicated by 1. the different styles of deformation observed across the fault zone, 2. kinematic analysis of a shear zone in tonalite and 3. geobarometry of contact metamorphic mineral assemblages. For the plutonic rocks in the Hornopiren area, extremely rapid cooling is indicated by thermochronometry, suggesting the activity of hydrothermal systems or thermal effects of late shallow intrusions.

Significance of 40Ar–39Ar encapsulation ages of metapelites from late Palaeozoic metamorphic complexes of Aysén, Chile

AbstractThe ages obtained by the 40Ar–39Ar encapsulation technique (retention and total gas ages) on <2 μm fractions of five metapelites from the Eastern Andean Metamorphic Complex and two from the Chonos Metamorphic Complex allow discussion of the latest recorded metamorphic event in each zone. The Kübler Index (KI) of illite/muscovite (principal component of the metapelites) varies between 0.15° and 0.45° Δ°2θ, indicating regional variation from diagenetic to epizonal metamorphic grade. The 40Ar–39Ar encapsulation analyses reveal 39Ar loss varying between 21 and 25%, which shows a limited positive correlation with KI values. The obtained retention and total gas metapelite ages reflect distinct metamorphic conditions. Retention ages most probably indicate burial or regional metamorphic events without plutonic influence in the southern Eastern Andean Metamorphic Complex. Total gas ages reflect contact ages for metapelites close to intrusions in the northern and southern Eastern Andean Metamorphic Complex and in the Chonos Metamorphic Complex. The thermal overprinting of metapelites occurred in Early Cretaceous times at 130 Ma and 145 Ma and is related to the contact metamorphism of an emplacement pulse of the North Patagonian Batholith. Total gas metapelite ages obtained from the western belt of the Chonos Metamorphic Complex suggest a thermal event related to a distinct pulse of the North Patagonian Batholith.

The South Patagonian batholith: 150 my of granite magmatism on a plate margin

A new database of 70 U–Pb zircon ages (mostly determined by SHRIMP) indicates that the South Patagonian batholith resulted from the amalgamation of subduction-related plutons from the Late Jurassic to the Neogene. Construction of the batholith began with a voluminous, previously undetected, Late Jurassic bimodal body mainly composed of leucogranite with some gabbro, emplaced along its present eastern margin within a restricted time span (157 to 145 Ma). This episode is, at least in part, coeval with voluminous rhyolitic ignimbrites of the Tobífera Formation, deposited in the deep Rocas Verdes Basin east of the batholith; this was the last of several southwestward-migrating silicic volcanic episodes in Patagonia that commenced in an Early Jurassic extensional tectonic regime. The quasi-oceanic mafic floor of the basin was also contemporaneous with this Late Jurassic batholithic event, as indicated by mutually cross-cutting field relationships. Changes in subduction parameters then triggered the generation of earliest Cretaceous plutons (Cretaceous 1: 144–137 Ma) west of the Late Jurassic ones, a westward shift that culminated at 136–127 Ma (Cretaceous 2) along the present western margin of the batholith. Most mid- to Late Cretaceous (Cretaceous 3: 126–75 Ma) and Paleogene (67–40 Ma) granitoids are represented by geographically restricted plutons, mainly emplaced between the previously established margins of the batholith, and mostly in the far south; no associated volcanic rocks of similar age are known at present in this area. During the final Neogene stage of plutonism (25–15 Ma) a recurrence of coeval volcanism is recognized within and east of the batholith. Typical εNdt values for the granitoids vary from strongly negative (− 5) in the Late Jurassic, to progressively higher values for Cretaceous 1 (− 4), Cretaceous 2 (− 0.7), Cretaceous 3 (+ 2) and the Paleogene (+ 5), followed by lower and more variable ones in the Neogene (− 1 to + 5). These variations may reflect different modes of pluton emplacement: large crustal magma chambers developed in the early stages (Late Jurassic to Cretaceous 1), leading to widespread emplacement of plutons with a crustal signature, whereas the Cretaceous 2, Cretaceous 3 and Palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher εNdt. This does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed.

Constraints on the timing and isotopic evolution for the 150 my emplacement history of the South Patagonian batholith, Southern Chile

Constraints on the timing and isotopic evolution for the 150 my emplacement history of the South Patagonian batholith, Southern Chile

Petrogenesis of the Puerto Edén Igneous and Metamorphic Complex, Magallanes, Chile: Late Jurassic syn-deformational anatexis of metapelites and granitoid magma genesis

A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial 87Sr/ 86Sr ratios of up to 0.7134 and εNd 150 values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial 87Sr/ 86Sr ratios between 0.7073 and 0.7089, and εNd 150 values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial 87Sr/ 86Sr ratios of 0.7048 and 0.7061, and εNd 100 values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.

Postmagmatic cooling and late Cenozoic denudation of the North Patagonian Batholith in the Los Lagos region of Chile, 41°−42°15′S

Zircon and apatite fission track (FT) thermochronology was applied to investigate the history of cooling and denudation of the Southern Andes between 41° and 42°15′S in relation to the late Cenozoic activity of the Liquine-Ofqui fault zone (LOFZ) and the northward migration of the Chile Triple Junction (CTJ). Fifty-six zircon and 51 apatite FT ages, plus 37 apatite confined track-length distributions were obtained mainly from plutonic rocks of the North Patagonian Batholith (NPB) in the main Andean Cordillera. Apatite FT ages and track lengths indicate a stage of rapid cooling at ∼5--3 Ma along both sides of the LOFZ, whereas older Miocene ages with monotonous cooling histories were obtained further away from the fault. Zircon FT ages range from Cretaceous to Pliocene, with marked differences observed along and across the LOFZ. Three different types of temperature-time histories characterise the post-magmatic cooling of the NPB in the region: deep intrusions with moderate and steady cooling rates, intrusions in the upper crust with very slow cooling rates following a stage of initial rapid cooling, and rapidly cooled and exhumed shallow intrusions, the latter with younger ages towards the fault zone. The most prominent denudation episode along the LOFZ is late Miocene to Pliocene, coeval with plate tectonic reconstructions for the arrival and subduction of the Chile Rise beneath the Taitao Peninsula.

The Hudson Volcano and surrounding monogenetic centres (Chilean Patagonia): An example of volcanism associated with ridge–trench collision environment

Hudson Volcano (45°54′S, 72°58′W) represents the southernmost central volcano of the South Andean Volcanic Zone (SVZ). It lies about 280 km east of Nazca–Antarctic–South American Plate Triple Junction, where the Chile spreading ridge enters the Chile Trench. Hudson is located on the border of a volcanic gap separating the Southern from the Austral Volcanic Zone (AVZ), which is commonly interpreted as linked to a slab window. The volcano is characterised by a volcano-tectonic depression about 10 km wide, set in a morphological high of the Patagonian Batholith. The scarce pre-caldera products are made up of mafic lavas interbedded with hyaloclastite. The majority of the volcanic products out from the caldera rims consist of thick deposits of lahars or hyaloclastitic breccias, subordinate lava flows and pyroclastic deposits linked to historical large explosive eruptions. Rough volume calculations on pre-caldera and post-caldera products suggest that the caldera formed in steps in the lifetime of the volcano and that it is mainly linked to a volcano-tectonic collapse during partial emptying of the magma chamber. The monogenetic cones surrounding Hudson (Río Ibanez and Murta) erupted primitive basalts that gave important clues on magma sources. The volcanic products of Hudson, with composition ranging from basalts to dacites, have a calc-alkaline signature. Nevertheless, they have distinctly higher K 2O, TiO 2, Na 2O, LILE, HFSE and REE than the calc-alkaline rocks of SVZ. The basaltic lavas from the monogenetic cones show La/Nb, Ba/Nb and Th/Ta significantly lower than the calc-alkaline lavas. Furthermore, they have some ratios as Th/Yb and La/Yb remarkably low, with values approaching those of MORB-like basalts. All the geochemical characteristics of Río Ibanez and Murta cones are very similar to those of the basalts dredged in the Chile Ridge. Geochemical modeling suggests that the Hudson basalts may derive from the basalts erupted at the Río Ibanez centre through an assimilation plus fractional crystallisation process. On the basis of these data, we propose a geodynamical model. In correspondence to the monogenetic centres, close to the border of the incipient opening of the slab window, the magma features are still those of the depleted asthenosphere under the subducting ridge. In this zone, remnant fragments of subducted oceanic crust undergo partial melting processes, while the mantle wedge is deeply modified by subduction-related components. In correspondence to Hudson and the nearby monogenetic centres, the MORB-type primary source component is modified by altered oceanic crust and variably mixed with andesitic magmas having a calc-alkaline signature. A contribution from partial melting of the continental lithosphere may be present in the complex source mixing process at the border of the slab window.

Pacific subduction coeval with the Karoo mantle plume: the Early Jurasssic Subcordilleran belt of northwestern Patagonia

Abstract The Early Mesozoic magmatism of southwestern Gondwana is reviewed in the light of new U-Pb SHRIMP zircon ages (181 ± 2 Ma, 181 ± 3 Ma, 185 ± 2 Ma, and 182 ± 2 Ma) that establish an Early Jurassic age for the granites of the Subcordilleran plutonic belt in northwestern Argentine Patagonia. New geochemical and isotopic data confirm that this belt represents an early subduction-related magmatic arc along the proto-Pacific margin of Gondwana. Thus, subduction was synchronous with the initial phase of Chon Aike rhyolite volcanism ascribed to the thermal effects of the Karoo mantle plume and heralding rifting of this part of the supercontinent. Overall, there is clear evidence that successive episodes of calc-alkaline arc magmatism from Late Triassic times until establishment of the Andean Patagonian batholith in the Late Jurassic involved westerly migration and clockwise rotation of the arc. This indicates a changing geodynamic regime during Gondwana break-up and suggests differential rollback of the subducted slab, with accretion of new crustal material and/or asymmetrical ‘scissor-like’ opening of back-arc basins. This almost certainly entailed dextral displacement of continental domains in Patagonia.

The lagos La Plata and Fontana fold-and-thrust belt: long-lived orogenesis at the edge of western Patagonia

Extensive fieldwork carried out in the inner sectors of the Cordillera Patagónica has revealed evidence of alternate of periods of crustal extension and orogen formation since the Jurassic. The Patagonian lithosphere was extended during the Mesozoic and fractured and collapsed into numerous depocenters within the arc and retroarc. The last pulse of extension during this period occurred in the Early Cretaceous, related to a series of Pacific transgressions that formed the embayment of the Rı́o Mayo basin, north of the Austral basin. The Rı́o Mayo basin is part of a widespread Early Cretaceous synrift episode found throughout the Patagonian region between 155 and 140 Ma. The last part of this rifting episode, between 140 and 125 Ma, was spatially more limited, restricted to the arc and inner retroarc. The area of extension can be related to an early Paleozoic suture between the Somuncura Massif and the hanging wall of the Deseado Massif. The correct identification of this suture zone helps delimit distinctive volcanic provinces, as well as metallogenic ore districts. Thermal subsidence ceased as a consequence of a sudden tectonic inversion related to a major lithospheric reorganization at 120 Ma, during the breakup of southern Gondwana. The closure of the small Rı́o Mayo intra-arc-retroarc basin occurred as a result of a fold-and-thrust belt, mainly by tectonic inversion of previous normal faults, and was partially related to thin-skinned deformation. The emplacement of the Eastern Patagonian batholith between 100 and 80 Ma destroyed the inner sectors of the fold-and-thrust belt. During this time, the collapse of the inner sectors allowed for the development of a second, more limited intra-arc basin. The orogenic wedge grew through the inversion of progressively deeper decollements and out-of-sequence thrusts and lasted until the Late Paleogene, with a period of probable quiescence during the highly oblique subduction of the Aluk (Phoenix) Plate in the early Paleogene.

Provenance and depositional setting of pre‐Late Jurassic turbidite complexes in Patagonia, Chile

Metaturbidites of the Eastern Andes Metamorphic Complex, which crop out east of the Mesozoic‐Cenozoic Patagonian Batholith, show significant differences in modal and chemical composition with those of the Duque de York Complex which crop out west of the batholith. The modal quartz content of the sandstones of the former is larger and does not spread out of the Transitional Continental field in QFL diagrams as does that of the latter. In the geochemical discrimination diagrams, sandstones from both units plot in distinct fields with little or no overlap. These features strongly suggest different provenance areas and depositional settings. The Eastern Andes Metamorphic Complex was deposited in a basin located on a passive continental margin, and the source area for the clastic rocks had cratonic affinities. By contrast, the rocks of the Duque de York Complex indicate deposition in an active continental margin, which included sedimentary, plutonic, and volcanic rocks, associated with a subduction zone and/or a strike‐slip setting. These results suggest that the two units are not coeval, and that they represent deposition before and after the establishment of the active continental margin at the Pacific rim of Gondwanaland during the late Paleozoic or early Mesozoic.

Late Cenozoic transpressional ductile deformation north of the Nazca–South America–Antarctica triple junction

The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the Cenozoic, which has been causally related to either oblique subduction or ridge collision. However, few structural and chronological studies of regional deformation are available to support one hypothesis or the other. We address along- and across-strike variations in the nature and timing of plate boundary deformation to better understand the Cenozoic tectonics of the southern Andes. Two east–west structural transects were mapped at Puyuhuapi and Aysén, immediately north of the Nazca–South America–Antarctica triple junction. At Puyuhuapi (44°S), north–south striking, high-angle contractional and strike-slip ductile shear zones developed from plutons coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Aysén (45–46°), top to the southwest, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral strike-slip shear zones develop within it. New 40Ar– 39Ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike-slip, oblique-slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4±0.3 Ma, laser on muscovite–biotite aggregates, Aysén transect, 45°S) and on mylonitic plutonic rocks with contractional deformation (3.8±0.2 to 4.2±0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain age is documented at the Canal Costa shear zone (45°S) and at the Queulat shear zone at 44°S. Published dates for the undeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip shear zones farther north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern Andes suggest different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation. The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern margins of the Miocene belt of the Patagonian batholith define a large-scale pop-up structure by which deeper levels of the crust have been differentially exhumed since the Pliocene at a rate in excess of 1.7 mm/year.

SHRIMP zircon U–Pb evidence for extended Mesozoic magmatism in the Patagonian Batholith and assimilation of Archean crustal components

The Patagonian Batholith forms the core of the southern Andes for about 1900km. We investigated the ages of magmatic evolution of this major granitic batholith with SHRIMP, or sensitive high mass-resolution ion microprobe. Zircons from four samples were dated near 45°S along a profile from the core of the batholith in Chile to pre-Andean Patagonia in Argentina, yielding ages of 191Ma±3,107Ma±2,96.3Ma±1.3, and 81.7Ma±1.3. An unexpected result is the presence of Precambrian zircon xenocrysts in the pre-Andean batholith apophyses, concentrated near 3.40–3.00, 2.75–2.50, and 2.15–1.95Ga. This demonstrates the presence of rocks derived from a Paleoproterozoic–Archean basement, which contaminated the gabbro and two granitoids.

Mesozoic‐Cenozoic denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes

Fission track (FT) analysis is applied to assess the Mesozoic and Cenozoic thermal and denudational history of the Patagonian Andes between 44° and 51°S and the geologic and geomorphic response of late Cenozoic subduction of the active Chile rise mid‐oceanic spreading center on the overriding plate. Seventy‐two FT ages from 43 samples are presented. Zircon FT ages indicate fast post intrusion cooling of Cretaceous parts of the Patagonian batholith and previously unreported Miocene magmatic rocks south of 48°S. Metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early Carboniferous and Late Permian. Apatite FT data reveal initiation of accelerated cooling and denudation at ca. 30 Ma at the western margin of southern continental South America followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca. 12–8 Ma at the position of the present‐day main topographic divide. This migration is proposed to be related to either coeval eastward migration of the retroarc deformation, the effects of subduction erosion in the overriding plate at the Peru‐Chile trench or less likely, shallowing of the angle of subduction. East of the divide, <3 km of denudation has occurred since the Late Cretaceous. Enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca. 28–26 Ma that triggered orographically enhanced precipitation on the west side of the Patagonian Andes allowing increased erosion by fluvial incision and mass transport processes. The actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation.

Paleomagnetic evidence for a very large counterclockwise rotation of the Madre de Dios Archipelago, southern Chile

A paleomagnetic study of Late Paleozoic exotic units exposed in the Madre de Dios Archipelago and Diego de Almagro island, southern Chile, is reported. These units correspond to the Tarlton Limestone and the metabasaltic pillow lavas of the Denaro Complex. Nineteen sites were drilled (14 in limestone and five in lavas). Standard demagnetization procedures show that the primary remanence is not present in the investigated rocks, preventing the determination of the original latitudinal position of these rocks. However, a post-tectonic secondary magnetic component was isolated in eight sites at the Madre de Dios Archipelago, in both limestone and basaltic pillow lavas. Rock magnetic experiments and microscopic observations indicated that the carrier of this remanence is secondary titanomagnetite in the limestone and titanomagnetite–titanomaghemite in the lavas. The most likely event to produce this secondary magnetization is the nearby intrusion of the south Patagonian batholith in the Early Cretaceous. Mean direction from eight sites shows a very large (117.7±29.9°) counterclockwise rotation of the study area with negligible paleolatitude anomaly. This rotation is only constrained as post-Early Cretaceous. Restoration of the structures to their pre-rotation position indicates that accretion of the exotic block took place from the northwest and not from the south as previously proposed. This is more consistent with a low latitude origin for the limestone and the general paleogeographic framework. Different tectonic mechanisms and ages for this large rotation are possible.

Jurassic to Miocene K–Ar dates from eastern central Patagonian Cordillera plutons, Chile (45°–48° S)

Thirty-nine K–Ar and one Ar–Ar radiometric dates from the eastern central Meso-Cenozoic Patagonian Batholith and eastern satellite plutons of the Aysén Region, of southern Chile between latitudes 45° and 48° S, combined with previous dating of seven plutons, have yielded six age groups: (1) Middle to Late Jurassic, (2) Early Cretaceous, (3) mid-Cretaceous, (4) Late Cretaceous, (5) Oligocene and (6) Miocene. In general, the Cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith, but for the satellite plutons to the east show a wider age spectrum than the previously accepted Late Miocene dates. These results indicate a relatively continuous Late Jurassic to mid-Cretaceous plutonism, known to have been coeval with volcanic activity, followed by intermittent magmatism. Biotite K–Ar dates of c. 143–151 and 106–109 Ma, from cataclastic granitoids, may be marking the time of deformation. A review of all radiometric data on magmatic rocks from the region between 45° and 48° S in Chile shows a gap in Palaeocene ages that may correlate with a period of low-angle (flat slab) subduction between 65–50 Ma.

Paleomagnetism of the North Patagonian batholith, southern Chile. An exercise in shape analysis

Over 100 sites in granitic country rock, mafic inclusions within granitic rock, and mafic dikes were sampled in the northernmost 550 km of the Patagonian batholith. After magnetic cleaning, some sites were rejected as magnetically unstable, and others were combined because they were contiguous and carried similar directions, hence probably representing duplicate samplings of the same direction of the geomagnetic field. A total of 42 well-determined directions — 29 from granitic country rock and inclusions and 13 from dikes — survived to be included in the interpretation. Most of these had a normal magnetic polarity, probably indicating that a disproportionate fraction of the North Patagonan batholith formed during the mid-Cretaceous long normal interval. Tectonic interpretation of paleomagnetic directions from intrusive rocks often is difficult because paleohorizontal indicators in such rocks are usually lacking. Rather than struggle against this limitation, we yielded to it and elected to concentrate on the shapes of our various data sets. The method used is simple and straightforward, involving Bingham statistics and small-circle fitting. From this analysis, it appears that the main structural feature within the North Patagonian batholith (NPB) — the Liquiñe-Ofqui fault zone — is a site of crustal detachment; a sliver of South America west of the fault is detached from the rest of the continent and has moved relatively northward. Because the main strands of the Liquiñe-Ofqui fault are curved and concave to the west, northward movement was accompanied by counterclockwise rotation. East of the fault, the NPB appears to have been the locus of distributed shear, mainly dextral. The principal period of deformation of the batholith appears to have been mid- or late Cenozoic, and may have been initiated by the arrival of the South America–Nazca (Farallon)–Phoenix (Aluk) triple junction.

Contrasting nature of deformation along an intra-arc shear zone, the Liquiñe–Ofqui fault zone, southern Chilean Andes

The Liquiñe–Ofquifault zone (LOFZ) cuts the Patagonian batholith and the modern volcanic arc of southern Chile for ca 1000 km. The rock fabric along three transects of the LOFZ combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike. In the Liquiñe transect (39°S), a 1 km wide, northeast-striking subvertical mylonitic zone shows north-plunging stretching lineations. This mylonitic zone has been juxtaposed by high-angle reverse faulting against a nearly undeformed Miocene granitoid. Metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement. Deformation pre-dates a 100±2 Ma undeformed porphyritic dike (hornblende 40Ar– 39Ar mean age). In the Reloncavı́ transect (41–42°S), deformation in Cretaceous and Miocene plutons is predominantly brittle. Kinematic analysis of two fault populations yields compressional and dextral strike-slip stress regimes, interpreted as late Miocene in age. In the Hornopirén transect (42–43°S), a 4 km wide mylonitic zone, developed in plutons and wallrocks, shows subhorizontal stretching lineations and dextral displacement. A single fault population overprinting the mylonites supports a dextral strike-slip stress regime. Available U–Pb, K–Ar and 40Ar– 39Ar dates on deformed Cenozoic plutons and wallrock range from 9 to 13 Ma on hornblende and from 6 to 3 Ma on biotite. Microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies, similar to the biotite Ar closure temperature. Subparallel magmatic and solid-state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons. Dextral-oblique subduction of the Nazca plate beneath western South America has driven long-term intra-arc deformation at the southern Andes plate boundary zone; ridge collision, in turn, has favored dextral displacement at the leading edge of the continent since the Pliocene

The Miocene plutonic event of the Patagonian Batholith at 44° 30′ S: thermochronological and geobarometric evidence for melting of a rapidly exhumed lower crust

The Patagonian Batholith was formed by numerous plutonic events that took place between the Jurassic and the Miocene. North of 47° S, the youngest plutons occupy the axial zone adjacent to the Liquiñe-Ofqui Fault Zone, which is a major intra-arc strike-slip fault system active since the Miocene. The Queulat Complex, located at 44° 30′ S, includes two Miocene plutonic units: the Early Miocene Queulat diorite (QD) and the Late Miocene Puerto Cisnes granite (PCG). The QD includes hornblende + clinopyroxene diorites and tonalites, whereas the PCG includes slightly peraluminous garnet ± sillimanite granites and granodiorites.Eleven mineral Ar–Ar ages and three apatite fission track ages were obtained from the Queulat Complex and surrounding host rocks. Hornblende and biotite Ar–Ar ages ofc.16-18 Ma and 9-10 Ma, respectively, were obtained for the QD. The youngest ages of the QD are similar to the age of emplacement of the PCG as previously determined. Ar–Ar ages for muscovites and biotites of 6·6 ± 0·3 Ma and 5·6 ± 0·1 Ma, respectively, were obtained for the PCG. Biotites and muscovites from mylonites and pelitic hornfelses adjacent to the PCG yielded Ar—Ar ages between 5·1 Ma and 5·5 Ma. The apatite fission track ages of the QD and PCG overlap within the error margin (2•2 ± 1·1-3·3 ± 1·4 Ma).The Al-in-hornblende geobarometer yielded pressures for the QD emplacement equivalent to depths in the 19-24 km range, which is substantially higher than the 10 km depth estimated previously for the PCG emplacement. Exhumation rates (v) up to 2·0mm/yr were calculated for the time elapsed between the QD and PCG emplacements. Avvalue of 1·0mm/yr was calculated for the PCG subsequent to its emplacement. Using the silica—Ca-tschermak-anorthite geobarometer, we estimate the QD magma generation to be atc.33 km, which is similar to the current crustal thickness. Melting of mafic and metapelitic lower crust was possible at > 30km depth during a period whenvwas between 1·0mm/yr and 2·0mm/yr.