Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to Cambrian sedimentary and volcanic rocks in Utah, Nevada, and SE Idaho record a dynamically evolving landscape along the ...North American Cordillera margin, which included: (1) initial development of intracratonic basins with deposition of siliciclastic strata of the Uinta Mountain Group from ~770 to 740Ma; (2) early rifting and volcanism along a N–S (present day geographic coordinates) basin system with deposition of diamictite-bearing strata of the Perry Canyon and related formations from ~720 to 660Ma; (3) early, broad subsidence with deposition of mature siliciclastic strata of the lower Brigham and McCoy Creek groups from ~660 to 580Ma; (4) final rifting, volcanism, and transition to drift with deposition of variably immature siliciclastic strata of the Prospect Mountain and correlative formations from ~570 to 520Ma; and (5) regional subsidence along a passive margin with deposition of Middle Cambrian to Devonian carbonate-rich strata. The Uinta Mountain Group comprises fluvial to marine, feldspathic to quartzose sandstone, conglomerate, and mudstone, with detrital zircon (DZ) patterns recording a mix of local basement sources to the N and distal Laurentian sources to the SE. The lower Perry Canyon and related formations contain variably feldspathic sandstone, quartz-pebble diamictite deposited during an older glacial episode, and mudstone, with DZ patterns recording a mix of distal sources, local basement sources, and sediment recycling during early rifting. The upper Perry Canyon and related formations contain mafic volcanic rocks, polymict diamictite deposited during a younger glacial episode, volcaniclastic wacke, and mudstone, with DZ patterns recording local basement sources along an evolving rift margin and felsic volcanism from ~700 to 670Ma. Mafic volcanic rocks and trachyte to rhyolite clasts in diamictite have geochemical signatures typical of continental rifting. The lower Brigham and McCoy Creek groups contain mostly mature quartz arenite deposited in shallow marine environments, with DZ patterns recording distal Laurentian sources. The base of the Prospect Mountain and correlative formations is marked by an influx of feldspathic, coarse-grained sediment derived from local basement sources and ~570–540Ma basalt volcanism, which was followed by deposition of subfeldspathic strata with dominant 1.7–1.8Ga DZ grains, recording sources from the SE rift margin and a marked decrease in distal sources during uplift of the Transcontinental Arch. Overlying carbonate-rich strata were deposited in shallow marine settings, with episodic influx of siliciclastic sediment derived from basement exposed during regressions. Stratigraphic thickness-age relations of Neoproterozoic to early Paleozoic strata are consistent with two episodes of rifting concentrated at ca. 700–670Ma and 570–540Ma along western Laurentia, leading to final development of a passive margin. Early rifting was incomplete with an estimated 25–40% extension of initially thick lithosphere that was weakened by igneous activity. Final rifting of previously thinned lithosphere involved an estimated 20–35% additional extension, renewed igneous activity, and thermal thinning of mantle lithosphere, with localized extension culminating in final separation along the continental margin. Stratigraphic, geochronologic, and available paleomagnetic data are consistent with linkage of western Laurentia to Australia–East Antarctica within Rodinia, followed by protracted rifting and drift during Cambrian time.
The extent and nature of the Archaean–Palaeoproterozoic craton of Río de la Plata of southern South America, a major but poorly understood crustal component in Neoproterozoic plate reconstructions, ...as well as the depositional, metamorphic and magmatic history of the surrounding orogenic belts, are reviewed and reassessed, in part through the analysis of material recovered from deep boreholes in western Argentina that penetrated Palaeozoic cover into basement. U–Pb SHRIMP zircon ages for these samples of 2162
±
6 Ma (diorite), 2189
±
14 Ma (amphibolitic schist) and 2088
±
6 Ma (granite) encompass the range of ages determined for the major Palaeoproterozoic orogenic events in the exposed parts of the craton close to the Atlantic coasts of Uruguay and Argentina. Taken together with the geochemical and Nd-isotope characteristics of these samples and an olivine gabbro from a further borehole that failed to yield zircon, these results strongly suggest that the Río de la Plata craton is extremely uniform in its dominant chrono-tectonic and lithological make-up, and that it extends westwards as far as the 535–520 Ma Pampean orogenic belt, against which it probably has a fault contact.
U–Pb SHRIMP zircon detrital age patterns are presented for representative metasedimentary samples from the craton cover in the Tandilia belt of eastern Argentina, and from the Pampean (Cambrian) and Famatinian (Ordovician) belts to the west of the craton. Whereas the oldest cover rocks in Tandilia clearly show material derived from the underlying craton, such detritus only appears in the younger (Ordovician) units to the west. Sedimentary protoliths in the Pampean belt were dominated by Neoproterozoic (broadly ∼
600 Ma) and late Mesoproterozoic (broadly ∼
1100 Ma) provenance, and derivation from the Río de la Plata craton is highly unlikely. Regional considerations, including previously published zircon data, palaeocurrent and structural data, suggest that these rocks must have had an origin within Gondwana-forming blocks, for which the closest identifiable sources are ‘Brazilian’ and ‘African’ (Namaqua–Natal). Consequently, the preferred model for the Pampean orogeny is that the Río de la Plata craton reached its present position by large-scale dextral strike-slip movement against fore-arc sedimentary sequences that had developed on the southern and western margins of the Kalahari craton during the Early Cambrian. In the final stage the displaced sedimentary sequences outboard of the RPC collided with the Mesoproterozoic Western Sierras Pampeanas terrane, which was at the time attached to the large Amazonia craton and other smaller continental blocks, such as Arequipa–Antofalla and Río Apa. Protracted relative displacement of the RPC after the Pampean Orogeny led to its final position.
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.
A review of the post-Cambrian igneous, structural and metamorphic history of Patagonia, largely revealed by a five-year programme of U–Pb zircon dating (32 samples), geochemical and isotope analysis, ...results in a new Late Palaeozoic collision model as the probable cause of the Gondwanide fold belts of South America and South Africa.
In the northeastern part of the North Patagonian Massif, Cambro-Ordovician metasediments with a Gondwana provenance are intruded by Mid Ordovician granites analogous to those of the Famatinian arc of NW Argentina; this area is interpreted as Gondwana continental crust at least from Devonian times, probably underlain by Neoproterozoic crystalline basement affected by both Pampean and Famatinian events, with a Cambrian rifting episode previously identified in the basement of the Sierra de la Ventana. In the Devonian (following collision of the Argentine Precordillera terrane to the north), the site of magmatism jumped to the western and southwestern margins of the North Patagonian Massif, although as yet the tectonics of this magmatic event are poorly constrained. This was followed by Early Carboniferous I-type granites representing a subduction-related magmatic are and Mid Carboniferous S-type granites representing crustal anatexis. The disposition of these rocks implies that the North Patagonian Massif was in the upper plate, with northeasterly subduction beneath Gondwana prior to the collision of a southern landmass represented by the Deseado Massif and its probable extension in southeastern Patagonia. This ‘Deseado terrane’ may have originally rifted off from a similar position during the Cambrian episode. Intense metamorphism and granite emplacement in the upper plate continued into the Early Permian. Known aspects of Late Palaeozoic sedimentation, metamorphism, and deformation in the Sierra de la Ventana and adjacent Cape Fold Belt of South Africa are encompassed within this model. It is also compatible with modern geophysical and palaeomagnetic data that do not support previous hypotheses of southward-directed subduction and collision along the northern limit of Patagonia. Subsequent Permian break-off of the subducted plate, perhaps with delamination of the lower part of the upper plate, allowed access of heat to the overlying Gondwana margin and resulted in voluminous and widespread silicic plutonism and volcanism throughout Permian and into Triassic times. Thus the new model addresses and attempts to explain three long-standing geological enigmas—the origin of the Gondwanide fold belts, the origin of Patagonia, and the cause of widespread Permian silicic magmatism (Choiyoi province) in southern South America. Differing significantly from previous models, it has new implications for the crustal structure, mineral resources, and plant and animal distribution in this part of Gondwana, since the southern landmass would have had an independent evolution throughout the Early Palaeozoic.
The positions of Laurentia and other landmasses in the Precambrian supercontinent of Rodinia are controversial. Although geological and isotopic data support an East Antarctic fit with western ...Laurentia, alternative reconstructions favor the juxtaposition of Australia, Siberia, or South China. New geologic, age, and isotopic data provide a positive test of the juxtaposition with East Antarctica: Neodymium isotopes of Neoproterozoic rift-margin strata are similar; hafnium isotopes of ~1.4-billion-year-old Antarctic-margin detrital zircons match those in Laurentian granites of similar age; and a glacial clast of A-type granite has a uraniun-lead zircon age of ~1440 million years, an epsilon-hafnium initial value of +7, and an epsilon-neodymium initial value of +4. These tracers indicate the presence of granites in East Antarctica having the same age, geochemical properties, and isotopic signatures as the distinctive granites in Laurentia.
Abstract
Intrusive rocks related to porphyry copper mineralization are part of the wide diversity of subduction-related, mantle-derived, igneous rocks generated in convergent margin settings. What ...differentiates them from barren igneous rocks results ultimately from the multi-component and multi-stage processes that condition magma composition in these settings. Unfortunately, the petrogenetic history is largely obscured by the pervasive alteration that affects rocks in these deposits. We address this issue through the study of zircon grains from El Teniente, one of the largest known porphyry Cu–Mo deposits in the world. El Teniente belongs to the Miocene–Pliocene Cu–Mo belt of the Central Chilean Andes, which formed in a short timespan during the Cenozoic constructive period of the orogen. Previously U–Pb dated zircon grains were selected for re-examination of their morphological characteristics and in situ analysis of chemical (rare earth element, Hf, Y and Ti contents) and isotopic (Hf, O) composition. They are from six intermediate to felsic syn- to late-mineralization, intrusive units covering a timespan of ∼1·6 Myr. The El Teniente zircons have compositional and morphological characteristics indicating crystallization from a series of cogenetic melts. However, a minor hydrothermal imprint is documented in the presence of crystals with mottled surfaces that correspond to thin high U–Th overgrowth rims (low-luminescent features in cathodoluminescence images). In terms of any other chemical and isotopic characteristic, these are indistinguishable from the main mineral populations. Zircons define morphological and chemical trends reflecting an evolution towards more differentiated magma compositions, lower crystallization temperatures and increased cooling rates with decreasing age of intrusion. Hf and O isotopic compositions are remarkably uniform at grain, sample and deposit scale. This, together with the general absence of older inherited zircon components, the lack of correlations between isotopic signature and whole-rock composition and high initial εHf values (total average 7·4 ± 1·2; 2σ), rules out involvement of any significant crustal contamination in the genesis of the El Teniente magmas. The Hf isotopic composition indicates a relatively juvenile source, but with some crustal residence time. The δ18OZrc weighted mean of 4·76 ± 0·12‰ (2σ; 61 analyses) is at the lower limit of the normal mantle zircon range of 5·3 ± 0·6‰ (2σ), and might reflect crystallization from low-18O magmas. Hf isotopic compositions have a restricted range in initial εHf values between +6 and +10, identical to preceding Cenozoic barren magmatic activity in Central Chile. These igneous rocks are the product of nearly 25 Myr of subduction-related magmatic activity, developed under contrasting tectonic regimes and margin configurations. This suggests a primary control of the isotopic signature by a stable long-lived MASH-type (melting, assimilation, storage and homogenization) reservoir in the deep lithosphere. In the context of the Cenozoic evolution of Central Chile we argue that dehydration melting in the enriched MASH reservoir occurred as a consequence of increasing crustal thickness, and was prompted by a high-temperature thermal regime resulting from long-lasting preceding magmatism. This process can also fractionate O to generate low-18O magmas. At the time of El Teniente formation, dehydration melting occurred coevally with arc migration, which probably influenced the fertility of the magmas by increasing the melt component derived from this process relative to the component derived from primary basalt differentiation. At a regional scale, such reactions are expected to occur as a consequence of progressive crustal thickening during the constructive period of the Andes, and can explain the simultaneous generation of porphyry deposits in the Miocene–Pliocene Cu–Mo belt of Central Chile.
This paper describes a strategy for effective discrimination and provenance evaluation of sandstones transported by different sediment dispersal systems using an integrated programme of heavy mineral ...analysis, mineral chemistry and zircon age dating. Sandstones derived from different source regions are initially discriminated using provenance–sensitive heavy mineral parameters (ratios of abundances of stable, hydraulically equivalent minerals). Differences between the heavy mineral populations are reinforced by single-grain mineral chemical analysis, which also provides further information on the nature of the source terrains. Zircon age dating of representative samples places further constraints on the location of the source areas, by identifying the main crust-forming events. In addition, the validity of the mineralogical discrimination can be tested by the zircon age data.
The value of this integrated approach is illustrated by a case study of deep marine Late Cretaceous–Paleocene sandstones in the Norwegian Sea. Potential sources of coarse clastic sediment lie both to the east, in mid-Norway, and the west, in East Greenland. Three main sand types (MN1, MN2 and MN3) have been identified, each of which has a unique combination of heavy mineral ratios (rutile/zircon, monazite/zircon, chrome spinel/zircon and apatite/tourmaline) and mineral chemistry (garnet and tourmaline). Sand type MN1 has a simple zircon age spectrum, with a large Early–Middle Proterozoic group and a small Early Paleozoic group, indicating derivation from northern mid-Norway. Sand type MN2 has a complex age structure including Archaean and early Proterozoic zircons. It can be divided into two subtypes, MN2a and MN2b, and can be ascribed to a source in East Greenland. Sand type MN3 has a simple age structure dominated by Middle Proterozoic zircons, and it was sourced from southern mid-Norway. Since reservoir presence is a major area of uncertainty in the deep water Norwegian Sea, the use of heavy mineral, mineral chemical and zircon age data to discriminate sandstones of eastern and western origin is crucial to hydrocarbon exploration of the area.
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