Francisco Munizaga

Granitoids of the Coast Range of central Chile: Geochronology and geologic setting

The Southern Coastal Batholith of central Chile is mainly composed of calc-alkaline granitoids of Late Carboniferous to Permian age. There is no clear evidence of the existence of older plutonic rocks in the batholith. The granitoids are intruded into an accretionary prism which underwent contemporaneous metamorphism. Triassic high-level plutons of limited areal extent occur as post-tectonic bodies in the high P/T metamorphic belt of the subduction complex. Jurassic plutons, which increase in volume toward the northern limit of the area and beyond, are associated with a tectonomagmatic event which, in certain localities, remobilized and re-set the ages of the Paleozoic granitoids. Cretaceous plutons occur to the east of the Southern Coastal Batholith. Initial 87Sr/86Sr ratios of the Paleozoic granitoids (0.706 to 0.707) are higher than those of the Mesozoic plutons (∼0.704) which were derived from a source similar to that of the present-day volcanic rocks of this segment of the Andes. The northern limit of the Paleozoic rocks is presumed to be an east-west fault zone, apparently inactive since Jurassic times. The continuity of the Mesozoic granitoids is not affected by this boundary.

Relationship between copper tonnage of Chilean base-metal porphyry deposits and Os isotope ratios

Re-Os isotopes in pyrite and chalcopyrite from early high-temperature hypogene alteration assemblages in Chilean porphyry copper deposits identify the source of Os and, by inference, Cu in these ore systems. Typical concentrations for Os in both pyrite and chalcopyrite are between 7 and 30 ppt (10 ‐12 g/g), and for Re between 0.200 and 10 ppb (10 ‐9 g/g). Re-Os isochrons yield ages and initial 187 Os/ 188 Os ratios for the sulfides. The isotopic data reflect the relative contributions of copper from the mantle and crust in Chilean porphyry copper deposits. Seven ore deposits that reside in different tectonic terranes and represent distinct epochs of mineralization in Chile were studied. The initial osmium ratios of the first stage of mineralization at each of the deposits range from 0.15 to 5. These values are more radiogenic than the present chondritic mantle (~0.13), and indicate significant crustal contributions of Os to the magmatic and/or hydrothermal systems. There is a strong correspondence between the total copper content and initial Os isotopic ratios in base-metal porphyry deposits. The larger deposits have lower initial Os ratios than the smaller, less significant deposits. This relationship implies that larger deposits acquire a greater proportion of Os from the mantle. The initial Os ratio of samples in the central segment of porphyry copper deposits of northern Chile also decreases with decreasing age of the deposit. A plausible interpretation of the Re-Os data is that the later and larger deposits use regional tectonic and structural features that allow sampling of deeper more primitive magmatic sources.

Geochronology of the Lake Region of south-central Chile (39°–42°S): Preliminary results

Abstract Carboniferous, igneous, and metamorphic rocks, followed by Jurassic, Cretaceous, and particularly extensive Miocene granitoid plutons, crop out in the Andes of the Lake Region, as determined by new K-Ar and Rb-Sr whole-rock age determinations. Their spatial distribution appears to define the following: the westernmost and easternmost Paleozoic belts, an oblique belt of Jurassic age, a NNW belt of Cretaceous age developed mainly in Argentina but entering Chile at 39°30′S, and a N-S belt of Miocene batholiths and stocks. This distribution of plutons is unlike the west to east younging belts that have been described from the Andes between 28°S and 32°S. This difference could be related to the presence in the Lake Region of old lineaments oblique to the direction of the Andes and to the influence of the Liquine-Ofqui fault zone as a pathway for Miocene magmas. The narrowness of the zone of magmatism from late Paleozoic to Miocene times, compared to the wide outcrops of the Paleozoic accretionary wedge, could be explained by the lack of tectonic erosion during Mesozoic-Cenozoic subduction and the constant subduction geometry.

Miocene volcanism in the central Chilean Andes (31°30′S–34°35′S)

Abstract The name Farellones Formation is currently used to designate continental Miocene volcanic and volcaniclastic deposits that form a 400 km long and 26–65 km wide, N/S-oriented belt of outcrops located along the central Chilean Andes. These deposits are mainly located east of an Oligocene-Miocene volcanic belt and west of the present volcanic arc. The lava flows and pyroclastics of the Farellones Formation vary in composition from andesitic to rhyolitic and, in lesser proportion, from dacitic to basaltic. Major element geochemistry and some trace elements confirm a typical calc-alkaline nature of the continental margin. The Miocene volcanic activity that formed the Farellones Formation occurred between 19.3 and 7.4 Ma, according to available K/Ar data, but this activity was not totally synchronous along the belt. It is estimated that 15.000 km 3 of effusive material was extruded during this volcanic episode. The main Miocene volcanic activity can be related to an increase in the normal convergence rate between the Nazca and South American plates, which occurred between 26 and 9.6 Ma. The Farellones volcanic belt is not related to the present segmentation of the Nazca plate and thus represents an older Andean segment with a continuous and compositionally homogeneous volcanism possibly related to segmentation of the paleo-subduction zone.

Age and Evolution of the Upper Cenozoic Andesitic Volcanism in Central-South Chile

The first ten potassium-argon determinations made on andesitic rocks of the Andes and Coastal ranges of central-south Chile, between latitudes 36° and 42° South, demonstrate the existence of at least two main cycles of volcanic activity; one in the Miocene and the other in the Pliocene to Holocene. The volcanic activity lines seem to have changed eastward from the present Coastal volcanic belt in Miocene times to about 100 km inland in Pliocene and Holocene times. During late Pleistocene to Holocene, a gradual inversion in the direction of migration of volcanism occurred. The geochronological asymmetry of the volcanic chains seems to be a highly important feature in this continental border and plate-contact zone.

Re-Os isotope systematics of sulfides from felsic igneous rocks: Application to base metal porphyry mineralization in Chile

Dating sulfides and determining the source of ore-forming metals have been limiting factors in our understanding of processes that produce hydrothermal ore deposits. Here we analyze sulfides from base metal porphyry deposits from Chile to demonstrate the potential of the Re-Os system to determine both the age and the source of metals for hydrothermal mineralization. Cogenetic chalcopyrite, bornite, pyrite, and sphalerite from El Teniente (ca. 5 Ma) have 187 Re/ 188 Os ratios from 0.3 to 21.8 and initial 187 Os/ 188 Os ratios from 0.17 to 0.22. A paragenetically late pyrite that has an initial 187 Os/ 188 Os of 0.88 indicates that the ore deposit is not isotopically homogeneous in Os throughout the formation of the deposit. Pyrite samples from Andacollo (ca. 100 Ma) have 187 Re/ 188 Os ratios from 15 to 3600, and different isochrons yield ages between 87 and 103 Ma, consistent with different sericite K-Ar ages. Initial 187 Os/ 188 Os ratios of 0.2 to 1.1 are similar to those of El Teniente. The relatively homogeneous 187 Os/ 188 Os ratios (~0.19) for cogenetic sulfides from El Teniente suggest that most of the Os in the ore was from the causal intrusive. The latestage pyrite probably incorporated Os leached from the country rock by meteoric fluids, which mixed with magmatic fluids at the periphery of the hydrothermal system. We demonstrate that the Re-Os isotope system can be a powerful geochronological tool in hydrothermal ore deposits. The large range in Re/Os ratios of sulfides permits the age of mineralization to be well constrained, despite isotopic heterogeneities of the hydrothermal fluids.

Paleomagnetism of the Patagonian Plateau Basalts, Southern Chile and Argentina

A total of 505 paleomagnetic samples were collected from 65 sites (volcanic flows) of the Patagonian plateau basalts at four locations in southern Chile and Argentina. K/Ar analyses indicate that 38 flows from two locations form a Late Cretaceous group (64–79 Ma), while the remaining 27 flows are Eocene in age (42–56 Ma). Progressive demagnetization indicates that a characteristic remanent magnetization (ChRM) has been successfully isolated from 59 flows. Rock-magnetic properties and analysis of ChRM directions within and between sites allow the secure inference that the ChRM is a thermoremanent magnetization acquired during original cooling. The Eocene flows yield 15 independent samplings of the paleomagnetic field. However, these data fail the reversals test and probably do not adequately average geomagnetic secular variation. The Late Cretaceous flows yield 18 independent samplings of the paleomagnetic field, six of normal polarity and 12 of reversed polarity. These data pass the reversals test and have angular dispersion of virtual geomagnetic poles (VGPs) consistent with adequate sampling of geomagnetic secular variation. The mean of these 18 VGPs yields a Late Cretaceous paleomagnetic pole for South America: latitude = 78.7°S; longitude = 358.4°E; A95 = 6.3° (K = 31.6; S = 14.5°). This pole is consistent with similar age poles from North America and Africa when the Atlantic is reconstructed to 70 Ma. Contrary to previous indications, the South American apparent polar wander path does not reach the present rotation axis until after Late Cretaceous time.

Late Paleozoic K/Ar ages of blueschists from Pichilemu, Central Chile

Two crossite concentrates and one blueschist whole rock were analyzed by the K/Ar method. These samples belong to the high/intermediate pressure Western Series of the Chilean metamorphic basement and, in this area, are intruded by a small monzonite body. Ages obtained were 211 m.y. and 329 m.y. for the mineral concentrates and 211 m.y. for the whole rock. Discussion based on crystal size as a factor for retention of40Ar during localized re-heating of the metamorphic rocks due to the monzonitic intrusion leads to the acceptance of 329 m.y. as the minimum age of crossite crystallization. This age agrees with the whole rock Rb/Sr limiting reference isochrons (273–342 m.y.) previously obtained for the metamorphic basement of Central Chile which did not include samples of the present area. This age provides the first evidence of a Paleozoic blueschist assemblage in the eastern Pacific border and would suggest the existence of a Late Paleozoic subduction zone along the western margin of South America.

Fission track thermochronology of Neogene plutons in the Principal Andean Cordillera of central Chile (33-35°S): Implications for tectonic evolution and porphyry Cu-Mo mineralization

Apatite fission track data for Miocene plutons of the western slope of the Principal Andean Cordillera in central Chile (33-35°S) define a distinct episode of enhanced crustal cooling through the temperature range of the apatite partial annealing zone (~125-60°C) from about 6 to 3 Ma. This cooling episode is compatible with accelerated exhumation of the plutons at the time of Pliocene compressive tectonism, and mass wasting on the western slope of the Principal Andean Cordillera in central Chile. The timing coincides with the southward migration of the subducting Juan Fernandez Ridge and the development of progressive subduction flattening northward of 33°S. It also corresponds to the time of active magmatic-hydrothermal processes and rapid unroofing of the world class Rio Blanco-Los Bronces and El Teniente porphyry Cu-Mo deposits. Zircon fission track ages coincide with previous 40 Ar/ 39 Ar dates of the intrusions, and with some of the apatite fission track ages, being coherent with igneous-linked, rapid cooling following magmatic intrusion. The thermochronologic data are consistent with a maximum of about 8 km for Neogene exhumation of the plutons.

Mesozoic and cenozoic plutonic development in the Andes of central Chile (30°30′–32°30′S)

Abstract Mesozoic and Cenozoic granitoids of this segment of the Andes occur in three N/S-trending belts: western (WB), central (CB), and eastern (EB). The WB is formed by the Mincha and Illapel superunits; the CB includes the Cogoti superunit and the San Lorenzo unit; and the EB comprises the Rio Grande and Rio Chicharra superunits. KAr and RbSr ages show discrete ranges for each of the belts, with a pronounced eastward migration of magmatism with time: WB, Early Jurassic to Late Cretaceous; CB, early Tertiary; and EB, late Tertiary. The jumps in the sites of the magmatic belts correspond to essentially non-magmatic intervals (86–70 Ma and 39–26 Ma) and may relate to periods of subduction-erosion or changes in the dip angle of the subducted lithosphere. Periods of rapid migration correspond to specific changes in the Pacific Ocean spreading history. The superunits show relatively uniform major element oxide variation. The exceptions are the Limahuida granitoids which have characteristics of a confirm derivation of the parent magmas from the upper mantle with virtually no continental crustal involvment. This distinguishes the Mesozoic-Cenozoic granitoids from those of the Paleozoic belts of Chile, which have values systematically higher than 0.705.