Abstract Paleoproterozoic (Rhyacian) gold deposits of the Loulo district in western Mali contain >17 million ounces (Moz) Au and form part of the second most highly endowed region within West Africa. The deposits are located within siliciclastic, marble, and evaporitic rocks of the ca. 2110 Ma greenschist facies Kofi series, which were folded and inverted between ca. 2100 and 2070 Ma, prior to gold mineralization. Deposits at Yalea and Gounkoto are located along discontinuous, low-displacement, albite- and carbonate-altered shear zones, whereas Gara is confined to a tourmaline-altered quartz sandstone unit. Lodes typically plunge gently to moderately, reflecting the attitude of folds in the adjacent rocks and bends in the host shear zones, both of which influenced their location. Gold mineralization in the Loulo district was broadly synchronous with emplacement of the Falémé batholith and associated Fe skarn mineralization, which intrude and overprint the western margin of the Kofi series, respectively. However, hydrothermal fluids generated during metamorphic devolatilization of the Kofi series rocks appear responsible for gold mineralization, albeit within a district-wide thermal gradient associated with emplacement of the Falémé batholith. The regional-scale Senegal-Mali shear zone, commonly cited as an important control on the location of gold deposits in western Mali, is absent in the Loulo district.

Abstract The Kibali district in the Democratic Republic of Congo hosts the large Karagba-Chaffeur-Durba (KCD) deposit and smaller satellite deposits that together contained 20 million ounces (Moz) of gold when mining recommenced in 2013. An additional 3 Moz of gold was probably mined from the district before 2013. Gold deposits in the Kibali district are located along the KZ trend, a series of folds, contractional shear zones, and altered lithostratigraphic units that coincide with the margin of an earlier 2630 to 2625 Ma intraorogenic basin within the Neoarchean Moto belt. Fluids first responsible for barren carbonate-quartz-sericite alteration, and later for siderite and/or ankerite (±quartz, magnetite, pyrite, and/or chlorite) alteration with associated auriferous pyrite ± rare arsenopyrite veinlets, infiltrated and replaced the siliciclastic, banded iron formation (BIF), and chert host rocks via fold axes, shear zones, and reactive BIF horizons. The complex shape and gentle northeast plunge of the lodes across the Kibali district reflect the shape and plunge of coincident folds that formed during early barren alteration. Many other folded BIF horizons across the wider Moto belt remain barren or only weakly mineralized, suggesting deep extensional structures that may have developed in the vicinity of the KZ trend during basin opening and prior to gold mineralization, were important fluid pathways during later contractional deformation and mineralization.

Abstract The giant (>20 Moz) Telfer Au-Cu deposit is located in the Paterson Province of Western Australia and is hosted by complexly deformed marine Neoproterozoic metasedimentary siltstones and quartz arenites. The Telfer district also contains magnetite- and ilmenite-series granitoids dated between ca. 645 and 600 Ma and a world-class W skarn deposit associated with the reduced, ~604 Ma O’Callaghans granite. Based on monazite and xenotime U-Pb geochronology, Telfer is estimated to be older than O’Callaghans, forming between 645 and 620 Ma. Au-Cu mineralization at Telfer is hosted in multistage, bedding-parallel quartz-dolomite-pyrite-chalcopyrite reefs and related discordant veins and stockworks of similar composition that were emplaced into two NW-striking doubly plunging anticlines or domes. Mineralization is late orogenic in timing, with hot (≤460°C), saline (<50 wt % NaCl equiv) ore fluids channeled into preexisting domes along a series of shallow, ENE-verging thrust faults and associated fault-propagated fold corridors. A combination of fault-propagated fold corridors acting as fluid conduits below the apex of the Telfer domes and the rheology and chemical contrast between interbedded siltstone and quartz arenite units within the dome are considered key parameters in the formation of the Telfer deposit. Based on the presence of the reduced Au-Cu-W-Bi-Te-Sn-Co-As assemblage, saline and carbonic, high-temperature hydrothermal fluids in Telfer ore, and widespread ilmenite-series granites locally associated with W skarn mineralization, Telfer is considered to be a distal, intrusion-related gold deposit, the high copper content of which may be explained by the predominance of highly saline, magmatic fluids in gangue assemblages cogenetic with ore.

Abstract A heavy mineral, mineral chemical and detrital zircon study of Jurassic–Cretaceous (Bathonian–Valanginian) sandstones of the Andøya B borehole, Lofoten–Vesterålen, northern Norway, has revealed the existence of significant differences within the succession. These are related partly to changes in source and partly to variations in the extent of weathering during alluvial storage. Three mineralogical units have been identified. The main change takes place within the Bathonian, and is interpreted as marking a switch from eastern (West Troms) to western (Andøya–Lofoten High) sourcing, consistent with previously published sedimentological models. U–Pb age data indicate that most of the zircons were derived from Palaeoproterozoic rocks ( c. 1750–1860 Ma), with a subordinate Archaean group ( c. 2600–2800 Ma) and a small early Palaeozoic group (mostly in the 435–446 Ma range). These groups can all be tied back to lithological components of the Lofoten–Vesterålen and West Troms regions, including Palaeozoic rocks hosted in Caledonian allochthons. The provenance characteristics of the Andøya succession have no counterpart in Cretaceous and Paleocene sandstones of the Vøring Basin. This suggests that sediment fed into the basin from Lofoten–Vesterålen was of minor importance, and that prospective Cretaceous–Paleocene hydrocarbon reservoir sandstones in the Vøring Basin were mainly derived from either northern Nordland or northern East Greenland. Supplementary material: Zircon isotopic compositions and ages are available at http://www.geolsoc.org.uk/SUP18616 .

Abstract Combined U–Pb, Lu–Hf and O isotope data of detrital zircons from the Devonian sandstone of southern Libya provide important new boundary parameters for reconstruction of palaeosource areas and sediment transport and may lead to novel approaches to test current plate tectonic models, with important implications for our understanding of the evolution of northern Gondwana (in present-day coordinates) during the Palaeozoic. Detrital zircon U–Pb ages from Devonian sandstone of the eastern margin of the Murzuq Basin show four main age populations: 2.7–2.5 Ga (13%), 2.1–1.9 Ga (10%), 1.1–0.9 Ga (25%) and 0.7–0.5 Ga (46%). The ubiquitous occurrence of c. 1.0 Ga detrital zircons is characteristic of the Saharan Metacraton sedimentary cover sequence and provides new insights into palaeogeographic reconstructions of Gondwana-derived terranes in the Eastern Mediterranean and SW Europe. The Lu–Hf isotope data suggest that zircons crystallized within a narrow time interval from magmas with heterogeneous Hf isotope compositions. These magmas were derived by melting of pre-existing rocks, rather than being juvenile. The calculated Hf model ages range from 3.7 Ga to 1.3 Ga, with a major population between 2.8 Ga and 1.9 Ga, indicating prominent recycling of Archaean and Palaeoproterozoic crust.

Utilizing both sensitive high-resolution ion microprobe (SHRIMP) and conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) methods, crystallization and/or emplacement ages have been obtained for a suite of Cretaceous intermediate-composition plutonic samples collected along a roughly E-W–trending traverse through the northern Peninsular Ranges batholith. Previously noted petrologic, mineralogic, and textural differences delineated four major zonations from west to east and raised the need for detailed geochemical and isotopic work. U-Pb zircon geochronology establishes that these zonations are essentially temporally separate. Mean 206 Pb/ 238 U ages date the three older zones from west to east at 126–107 Ma, 107–98 Ma, and 98–91 Ma. Despite petrologic differences, a relatively smooth progression of magmatism is seen from west to east. A fourth zone is defined by magmatism at ca. 85 Ma, which represents emplacement of deeper-level plutons east of the Eastern Peninsular Ranges mylonite zone in an allochthonous thrust sheet in the northeastern Peninsular Ranges batholith. The age data presented here differ slightly from those presented in earlier work for similar rocks exposed across the middle and southern portions of the Peninsular Ranges batholith in that our data define a relatively smooth progression of magmatism from west to east, and that the transition from western-type to eastern-type plutonism is interpreted to have occurred at ca. 98 Ma and not at ca. 105 Ma. The progressive involvement of older crustal components in the enrichment of eastern Peninsular Ranges batholith–type magma sources is documented by the occurrence of Proterozoic zircon inheritance within samples of the eastern part of the batholith.

Abstract The Fosdick migmatite–granite complex of West Antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of Gondwana, one in the Devonian–Carboniferous and another in the Cretaceous. The Hf–O isotope composition of zircons from Devonian–Carboniferous granites is explained by mixing of material from two crustal sources represented by the high-grade metamorphosed equivalents of a Lower Palaeozoic turbidite sequence and a Devonian calc-alkaline plutonic suite, consistent with an interpretation that the Devonian–Carboniferous granites record crustal reworking without input from a more juvenile source. The Hf–O isotope composition of zircons from Cretaceous granites reflects those same two sources, together with a contribution from a more juvenile source that is most evident in the detachment-hosted, youngest granites. The relatively non-radiogenic ɛHf isotope characteristics of zircons from the Fosdick complex granites are similar those from the Permo-Triassic granites from the Antarctic Peninsula. However, the Fosdick complex granites contrast with coeval granites in other localities along and across the former active margin of Gondwana, including the Tasmanides of Australia and the Western Province of New Zealand, where the wider range of more radiogenic ɛHf values of zircon suggests that crustal growth through the addition of juvenile material plays a larger role in granite genesis. These new results highlight prominent arc-parallel and arc-normal variations in the mechanisms and timing of crustal reworking v. crustal growth along the former active margin of Gondwana. Supplementary material: Figs S1 and S2 are available at http://www.geolsoc.org.uk/SUP18625

Four different sand types (termed FSP1, FSP2, FSP3, and FSP4) have been recognized in the Paleocene succession of the Faroe-Shetland Basin, NE Atlantic, on the basis of conventional heavy mineral analysis, major element geochemistry of garnet, trace element geochemistry of rutile, U-Pb dating of detrital zircon, and palynofloral analysis. Sand types FSP1, FSP2, and FSP4 were all sourced from the eastern margin of the basin, whereas FSP3 was supplied from the west. No single technique discriminates all four sand types. Conventional heavy mineral analysis discriminates FSP3 from the other three sand types but does not discriminate FSP1, FSP2, and FSP4. Garnet geochemistry distinguishes FSP1, FSP2 and FSP4, but FSP3 garnet populations overlap those of FSP1 and FSP2. Rutile geochemistry distinguishes FSP2 from FSP1 and FSP4 but cannot be easily applied to FSP3 owing to the scarcity of rutile in this sand type. Zircon age spectra in FSP1, FSP2, and FSP4 are similar to one another, but FSP4 can be recognized on the basis of a higher proportion of Archean zircons. Some of the individual techniques have certain limitations: e.g., one of the key conventional heavy mineral parameters is the presence of clinopyroxene, but this is not always reliable owing to the instability of this mineral during burial diagenesis. Likewise, garnet geochemistry cannot be applied to the most deeply buried sandstones in the Faroe-Shetland Basin owing to complete garnet dissolution. Furthermore, care is required when interpreting garnet data from sandstones that have undergone partial garnet dissolution, as there may have been modification of the range of garnet compositions as a result of the greater instability of Ca-rich garnets compared with Ca-poor types. Finally, the “Greenland flora,” which occurs in association with sand type FSP3, has been found in some wells that lack FSP3 sandstones. This discrepancy is attributed to the difference in hydrodynamic behavior of palynomorphs compared with sand particles. This chapter illustrates the importance of adopting an integrated approach, as significant detail would have been lost if only one technique had been applied, and integration of a number of different techniques overcomes limitations associated with individual approaches. An integrated approach also builds a more comprehensive picture of source area characteristics.

New U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from zircon and monazite document a 350 m.y. geologic evolution for the New Jersey Highlands. Two pulses of calc-alkaline magmatism that include the Wanaque tonalitic gneiss (1366 ± 9 Ma and 1363 ± 17 Ma) and Losee Suite tonalitic gneiss (1282 ± 7 Ma), dacitic gneiss (1254 ± 5 Ma), and dioritic gneiss (1248 ± 12 Ma) represent the southern continuation of eastern Laurentian margin arc activity. Supracrustal paragneisses, marble, and cogenetic metavolcanic rocks were deposited in a backarc basin inboard of the Losee arc. Ages of 1299 ± 8 Ma to 1240 ± 17 Ma for rhyolitic gneisses provide lower and upper limits, respectively, for the age of the supracrustal succession. Inherited cores in zircon grains from supracrustal rhyolitic gneiss and from Losee Suite rocks yield overlapping ages of 1.39–1.30 Ga and indicate proximity to an older arc source temporally equivalent to the Wanaque tonalitic gneiss. Location of the backarc inboard of the Losee arc implies a northwest-dipping subduction zone at this time beneath the eastern Laurentian margin. A-type granite magmatism of the Byram and Lake Hopatcong intrusive suites at 1188 ± 6 Ma to 1182 ± 11 Ma followed termination of arc and backarc magmatism and documents a change to decompression melting of delaminated lithospheric mantle by upwelling asthenospheric mantle. Waning stages of A-type granite magmatism include clinopyroxene granite (1027 ± 6 Ma) and postorogenic Mount Eve Granite (1019 ± 4 Ma). Overgrowths on zircon and monazite give ages of 1045–1024 Ma, fixing the timing of granulite-facies metamorphism in the New Jersey Highlands; other overgrowth ages of 996–989 Ma reflect the thermal effects of postorogenic felsic magmatism and hydrothermal activity associated with regional U–Th–rare earth element (REE) mineralization.

U-Pb zircon geochronology and field relations provide insights into metavolcanic and associated rocks in the Central Appalachian Piedmont of Maryland and northern Virginia. Ordovician ages were determined for volcanic-arc rocks of the James Run Formation (Churchville Gneiss Member, 458 ± 4 Ma; Carroll Gneiss Member, 462 ± 4 Ma), Relay Felsite (458 ± 4 Ma), Chopawamsic Formation (453 ± 4 Ma), and a Quantico Formation volcaniclastic layer (448 ± 4 Ma). A previously dated first phase of volcanism in the Chopawamsic Formation was followed by the second phase dated here. The latter suggests a possible source for contemporaneous volcanic-ash beds throughout eastern North America. Dates from the Chopawamsic and Quantico Formations constrain the transition from arc volcanism to successor-basin sedimentation. Ordovician metatonalites of the Franklinville (462 ± 5 Ma) and Perry Hall (461 ± 5 Ma) plutons are contemporaneous with the James Run Formation, whereas granitoids of the Bynum Run (434 ± 4 Ma) and Prince William Forest (434 ± 8 Ma) plutons indicate an Early Silurian plutonic event. The Popes Head Formation yielded Mesoproterozoic (1.0–1.25 Ga, 1.5–1.8 Ga) detrital zircons, and metamorphosed sedimentary mélange of the Sykesville Formation yielded Mesoproterozoic (1.0–1.8 Ga) detrital zircons plus a minor Archean (2.6 Ga) component. A few euhedral zircons (ca. 479 Ma) in the Sykesville Formation may be from granitic seams related to the Dalecarlia Intrusive Suite. A Potomac orogeny in the Central Appalachian Piedmont is not required, but the earliest Taconic orogenesis remains poorly constrained.

Geologic mapping of Mesoproterozoic lithologies and foliations, and sensitive high-resolution ion microprobe (SHRIMP) U-Pb crystallization ages of 43 samples of orthogneisses and metagranitoids from the northern Blue Ridge establish new subdivisions: group 1 (1183–1144 Ma), group 2 (1143–1111 Ma), and group 3 (1078–1028 Ma). Protoliths of group 1 were metamorphosed at amphibolite- to granulite-facies conditions and strongly deformed between ca. 1153 and ca. 1144 Ma. Metagranitoids of groups 2 and 3 were emplaced continually for another 115 m.y. and display only local effects of diminishing deformation events. Ages of zircon overgrowths overlap temporally with igneous crystallization ages of group 3, but continued until ca. 960 Ma.

New geologic mapping, petrology, and U-Pb geochronology indicate that Mesoproterozoic crust near Mount Rogers consists of felsic to mafic meta-igneous rocks emplaced over 260 m.y. The oldest rocks are compositionally diverse and migmatitic, whereas younger granitoids are porphyritic to porphyroclastic. Cathodoluminescence imaging indicates that zircon from four representative units preserves textural evidence of multiple episodes of growth, including domains of igneous, metamorphic, and inherited origin. Sensitive high-resolution ion microprobe (SHRIMP) trace-element analyses indicate that metamorphic zircon is characterized by lower Th/U, higher Yb/Gd, and lower overall rare earth element (REE) concentrations than igneous zircon. SHRIMP U-Pb isotopic analyses of zircon define three episodes of magmatism: 1327 ± 7 Ma, 1180–1155 Ma, and 1061 ± 5 Ma. Crustal recycling is recorded by inherited igneous cores of 1.33–1.29 Ga age in 1161 ± 7 Ma meta-monzogranite. Overlapping ages of igneous and metamorphic crystallization indicate that plutons of ca. 1170 and 1060 Ma age were emplaced during episodes of regional heating. Local development of hornblende + plagioclase + quartz ± clinopyroxene indicates that prograde metamorphism at 1170–1145 Ma and 1060–1020 Ma reached upper-amphibolite-facies conditions, with temperatures estimated using Ti-in-zircon geothermometry at ~740 ± 40 °C during both episodes. The chemical composition of 1327 ± 7 Ma orthogranofels from migmatite preserves the first evidence of arc-generated rocks in the Blue Ridge, indicating a subduction-related environment that may have been comparable to similar-age systems in inliers of the Northern Appalachians and the Composite Arc belt of Canada. Granitic magmatism at 1180–1155 Ma and ca. 1060 Ma near Mount Rogers was contemporaneous with anorthosite-mangerite-charnockite-granite (AMCG) plutonism in the Northern Appalachian inliers and Canadian Grenville Province. Metamorphism at ca. 1160 and 1060 Ma correlates temporally with the Shawinigan orogeny and Ottawan phase of the Grenvillian orogeny, respectively, suggesting that the Blue Ridge was part of Rodinia dating back to ca. 1180 Ma.

Abstract Field-based investigation of ‘Infracambrian’ rocks cropping out on the eastern flank of Al Kufrah Basin (area 500 000 km 2 ) reveals a an approximately 500 m-thick clastic succession of massive and cross-bedded sandstones, separated by 60 m-thick mudrock intervals. New zircon age data indicate a maximum age of deposition of approximately 950 Ma; furthermore, the absence of zircons of Pan-African age suggests a minimum depositional age older than the Pan-African Orogeny. Previously unreported folding and spaced cleavage affects these deposits to produce a pronounced NE–SW-striking tectonic grain that is interpreted to result from NW–SE-directed orthogonal compression during the Pan-African Orogeny. These Infracambrian rocks are therefore unlikely to be suitable analogues for weakly deformed strata shown to exist beneath the Cambro-Ordovician strata of the Al Kufrah Basin. Earlier work mapped a series of Infracambrian marble outcrops along strike of the clastic deposits; thin section petrography reveals that some of these are basic igneous rocks metamorphosed to greenschist facies. Interpretation of gravity data over the Al Kufrah Basin shows NE–SW-striking faults, parallel to outcrop structures, and secondary NW–SE faults. The data do not support earlier interpretations of a rhomboidal geometry in the deep subsurface of the basin, which has previously been attributed to strike-slip (pull-apart) processes. This research impacts on earlier suggestions that the Al Kufrah Basin opened as one of a series of en echelon pull-apart basins situated along a 6000 km-long shear zone known as the Transafrican Lineament, stretching from the Nile to the Niger Delta.

Abstract In eastern Dronning Maud Land (DML), East Antarctica, there are several discrete, isolated magmatic and high-grade metamorphic regions. These are, from west ( c. 20°E) to east ( c. 50°E), the Sør Rondane Mountains (SRM), Yamato–Belgica Complex (YBC), Lützow-Holm Complex (LHC), Rayner Complex (RC) and Napier Complex (NC). To understand this region in a Gondwanan context, one must distinguish between Pan-African and Grenvillian aged magmatic and metamorphic events. Sensitive high-resolution ion microprobe U–Pb zircon ages and Nd model ages for metamorphic and plutonic rocks are examined in conjunction with published geological and petrological studies of the various terranes. In particular, the evolution of the SRM is examined in detail. Compilation of Nd model ages for new and published data suggests that the main part of eastern Dronning Maud Land, including the SRM, represents juvenile late Mesoproterozoic ( c. 1000–1200 Ma) crust associated with minor fragments of an older continental component. Evidence for an Archaean component in the basement of the SRM is lacking. As for central DML, 1100–1200 Ma extensive felsic magmatism is recognized in the SRM. Deposition of sediments during or after magmatism and possible metamorphism at 800–700 Ma is recognized from populations of detrital zircon in metasedimentary rocks. The NE Terrane of the SRM, along with the YBC, was metamorphosed under granulite-facies conditions at c. 600–650 Ma. The SW and NE Terranes of the SRM were brought together during amphibolite-facies metamorphism at c. 570 Ma, and share a common metamorphic and magmatic history from that time. High-grade metamorphism was followed by extensive A-type granitoid activity and contact metamorphism between 560 and 500 Ma. In contrast, T DM and inherited zircon core ages suggest that the LHC is a collage of protoliths with a variety of Proterozoic and Archaean sources. Later peak metamorphism of the LHC at 520–550 Ma thus represents the final stage of Gondwanan amalgamation in this section of East Antarctica.

Abstract Mesoproterozoic strata from east-central Idaho and the Belt Supergroup of southwest Montana (eight new samples) contain several age groupings of detrital zircon grains: from old to young: a) Laurentian grains older than 1.85 Ga, b) a flood of 1655 to 1790 Ma Paleoproterozoic grains; b) non-North American zircon populations (ca. 1510 to 1625 Ma) with no known source on Laurentia, and c) syn- Belt grains, with groupings at 1480 Ma (syn-lower Prichard) and 1450 Ma (upper Piegan Group). The 1450 Ma = grain age population overlaps a 1454 ± 9 Ma fallout tuff in Glacier National Park. Strata of the Yellowjacket, Hoodoo, Apple Creek, and western Gunsight formations of Idaho all contain the 1450 Ma population, sparse non-North American grains, and dominant Paleoproterozoic populations at 1670 to 1790 Ma. This detrital-zircon signature is comparable to that of the Wallace Formation of the Belt Supergroup. The Swauger Quartzite in Idaho and the eastern Gunsight Formation at the ca. 900 Ma Beaverhead impact site in southwest Montana contain 1710 and 1780 Ma zircon populations identical to those of the Missoula Group of the Belt Supergroup. The E member of the lower Belt Prichard Formation from Plains, Montana, contains a population of syndepositional zircons at 1479 ± 19 Ma. Like the Revett Formation of the Ravalli Group, the E member contains Mesoproterozoic non-North American detrital-zircon populations as well as Paleoproterozoic grains at 1750 to 1790 Ma. The thick east-central Idaho Mesoproterozoic section was deposited after 1450 Ma, with deposition of the 10 km thickness from the lower Yellowjacket Formation through the Gunsight Formation spanning only 10 to 20 My. Given this very high rate of deposition, previous correlations of the Apple Creek Formation with the Piegan Group are permissible if problematic. However, previous correlations of the eastern Gunsight and Swauger formations with the Missoula Group are supported. Their detrital-zircon grain populations are identical.

The Priest River, Clearwater, Bitterroot, and Anaconda metamorphic core complexes of the northern Rocky Mountains were exhumed in Eocene time by crustal extension, which was linked via dextral displacement on the Lewis and Clark fault zone. Detailed geochronology and thermochronology (U-Pb, 40 Ar/ 39 Ar, and fission-track) from the Bitterroot complex indicates that extension started at 53 ± 1 Ma and continued until after 40 Ma. New U-Pb zircon and 40 Ar/ 39 Ar data from the Anaconda complex and published geochronology from the Priest River complex indicate a similar timing for the onset of major extension and exhumation. 40 Ar/ 39 Ar data from the Clearwater complex, which formed within a relay between strike-slip splays of the Lewis and Clark fault zone, are consistent with exhumation during the same time span. The Lewis and Clark fault zone separates ENE-directed extension in the Priest River complex from ESE-directed extension in the Bitterroot and Anaconda complexes. Large-scale extension was transferred eastward on the south side of this fault zone, where stretching lineations in core complex mylonites are oriented ∼104°–110° and coincide with the general trend of the transcurrent faults. Extension and exhumation of middle crustal rocks along the Lewis and Clark fault zone was concentrated in areas that also experienced voluminous Eocene midcrustal magmatism. Extension was probably initiated by a change in plate boundary conditions combined with the rapid influx of heat from the asthenosphere as a slab window opened beneath the western Cordillera, which led to collapse of the Cordilleran orogenic wedge and widespread early Eocene magmatism.

Abstract Combined isotopic dating indicates five episodes of felsic intrusion within the El Teniente orebody: (1) Sewell stock and other quartz diorite-tonalite intrusions of the eastern part crystallized from 6.46 ± 0.11 to 6.11 ± 0.13 Ma (zircon U-Pb); (2) quartz diorite-tonalite, immediately southeast of the orebody, with biotite 40 Ar/ 39 Ar plateau ages of 5.63 ± 0.12 and 5.47 ± 0.12 Ma—these ages agree with a hydrothermal overprint on zircons from the intrusions of the previous episode at 5.67 ± 0.19 to 5.48 ± 0.19 Ma (U-Pb); (3) Teniente dacite porphyry crystallized at 5.28 ± 0.10 Ma (zircon U-Pb); (4) a dacite ring dike encircling the Braden pipe crystallized at 4.82 ± 0.09 Ma (zircon U-Pb); and (5) minor dacite intrusions and dikes yielded a biotite 40 Ar/ 39 Ar plateau age of 4.58 ± 0.10 Ma, and sericite 40 Ar/ 39 Ar plateau ages of 4.56 ± 0.12 to 4.46 ± 0.10 Ma. All these felsic intrusions were emplaced within country rocks of late Miocene according to an apatite fission-track age of 8.9 ± 2.8 Ma for a mafic sill, in accord with previous K-Ar ages of 12.0 ± 0.7 to 6.6 ± 0.4 Ma for volcanic rocks from the district. Molybdenite Re-Os dating at El Teniente revealed ore deposition at 6.30 ± 0.03, 5.60 ± 0.02, 5.01 to 4.96, 4.89 ± 0.08 to 4.78 ± 0.03, and 4.42 ± 0.02 Ma, concurrent with the five intrusive episodes. The Re-Os system for molybdenite was unaffected by the various hydrothermal episodes. In contrast, the 40 Ar/ 39 Ar system of micas was reset by high-temperature (>350°C) fluid circulation and provides only a partial record of the latest history of development of this supergiant ore-forming system; biotite, sericite, and altered whole-rock samples collected throughout the orebody yielded 40 40 Ar/ 39 Ar plateau ages ranging from 5.06 ± 0.12 to 4.37 ± 0.10 Ma. These ages reveal a period of hydrothermal activity, which extended either continuously or episodically, for at least 0.69 ± 0.22 m.y. (±2σ) and that comprises a succession of three episodes of ore deposition. Separate hydrothermal episodes are thus interpreted to have lasted <0.69 ± 0.22 m.y. The Braden breccia pipe in the center of the deposit was formed as a single synmineralization event, probably related in time to the injection of the dacite ring dikes at 4.82 ± 0.09 Ma (zircon U-Pb). It was followed by quartzsericite alteration within and peripheral to, the pipe from 4.81 ± 0.12 to 4.37 ± 0.10 Ma (sericite 40 Ar/ 39 Ar). The successive intrusions of felsic bodies and their respective crystallization processes were immediately followed by genetically related, short-lived episodes of ore deposition, each associated with hydrothermal alteration. This multistage evolution, inferred from systematic dating, was not apparent from previous geochronologic data and is inferred to have contributed to the enormous volume and richness of the El Teniente. Thermal modeling of apatite fission-track data suggests that the porphyry system cooled very rapidly to temperatures below 105° ± 20°C, most likely before the intrusion of a postore hornblende-rich andesitic dike at 3.85 ± 0.18 Ma (hornblende 40 Ar/ 39 Ar). This dike cuts the southern part of the El Teniente deposit and marks the end of igneous activity in the orebody.

Siliciclastic rocks of the upper Byrd Group in the Transantarctic Mountains record rapid denudation and molasse deposition during Ross orogenesis along the early Paleozoic convergent margin of Gondwana. These rocks, which stratigraphically overlie Lower Cambrian Byrd carbonate deposits, are dominated by fresh detritus from proximal igneous and metamorphic sources within the Ross Orogen. Biostratigraphic evidence indicates that deposition of the siliciclastic succession is late Botomian or younger (<515 Ma). The largest modes of U-Pb and 40 Ar/ 39 Ar ages from detrital zircons and muscovites respectively in the siliciclastic molasse are Early to Middle Cambrian, but based on ages from crosscutting igneous bodies and neoblastic metamorphic phases, deposition of individual molasse units continued until ∼490–485 Ma (earliest Ordovician). The entire episode of interrelated tectonic, denudational, sedimentary, deformational, and magmatic events is restricted to a time interval of 7–25 m.y. in the late Early Cambrian to earliest Ordovician, within the resolution of these stratigraphic and geochronologic data. Stratigraphic relationships suggest that the detrital zircon and muscovite in the sediments came from the same source terrain, consistent with large volumes of molasse having been shed into forearc and/or marginal basins at this time, primarily due to erosion of igneous rocks and metamorphic basement of the early Ross magmatic arc. Rapid erosion and unroofing in the axial Ross Orogen is consistent with a sharp carbonate-to-clastic stratigraphic transition observed in the upper Byrd Group, reflecting an outpouring of alluvial fan and fluvial-marine clastic detritus. The short time lag between tectonism and sedimentary response is similar to that determined for the corresponding section of the Ross-Delamerian orogen in South Australia and other continental-margin arc systems, such as in the Mesozoic Peninsular Ranges of California. Mineral cooling ages from metamorphic basement adjacent to the orogen yield a syn- to late-orogenic cooling rate of ∼10 °C/m.y., which, combined with a known metamorphic geotherm, indicates a denudation rate of ∼0.5 mm/yr. Such denudation rates are comparable to those in recent convergent or collision orogens and suggest that crustal thickening associated with both magmatic intrusion and structural shortening was balanced by near-synchronous erosional exhumation.

The Fosdick Mountains, West Antarctica, form an 80 × 15 km migmatite dome comprising massive paragneisses that exhibit polyphase fabrics, nappe-scale folds that involve granodiorite to leucogranite intrusions, and diatexite. High strain zones developed on the NE flank of the dome. Multiple generations of leucogranite sheets, dikes and diatexite intrude the dome, and evidence for partial melt in structural sites is widespread. Macroscopic folds and the maximum anisotropy of magnetic susceptibility (AMS) direction are oriented NE-SW, generally parallel with the N65W regional finite strain axis determined from brittle faults and a mafic dike array outside the dome. The direction is oblique to the inherited fault that bounds the dome, to regional trends in the surrounding Ford Ranges, and to the nearby continental margin. Paragneiss assemblages yield thermobarometry results that indicate ≥18 km depth for growth of texturally early garnet and ∼10 km depth for growth of texturally late cordierite at the expense of biotite. Nodular and dendritic forms of cordierite that develop at shallow crustal depths completely overprint dynamic fabrics. The cordierite-K feldspar-sillimanite-garnet-biotite gneisses are determined by U-Pb SHRIMP (sensitive high-resolution ion microprobe) zircon analysis to contain inherited zircon populations of 1100–1000 Ma and 500 Ma age. The U-Pb distribution is characteristic of sediments shed from the Ross Orogen of the Paleozoic Gondwana margin, represented by Swanson Formation in the Ford Ranges. A granodiorite gneiss yields 375 Ma prismatic zircon grains characteristic of Ford Granodiorite in the region. Zircon rim ages in both rock types suggest a protracted growth history during polyphase high-temperature metamorphism. The peak of metamorphism was attained at 106–99 Ma, based on prior U-Pb monazite ages and regional relationships, followed by rapid cooling through the range of 40 Ar/ 39 Ar mineral systems between 101 and 94 Ma. The timing coincides with a change from convergent to divergent tectonics along the West Gondwana margin prior to breakup. Considered together, the partial melt evidence, decompression record and rapid thermal evolution of the partially molten rocks suggests diapiric processes in effect during emplacement the Fosdick Mountains dome along the Balchen Glacier fault. The consistent NE-SW orientation of folds, AMS strain axes, stretching direction from mafic and felsic dikes, kinematic axes from minor faults, and sparse mineral lineation attest to structural controls on dome emplacement. These are interpreted as evidence of dextral transcurrent strain across the region at ca. 100 Ma.

The Baltimore Gneiss, exposed in antiforms in the eastern Maryland Piedmont, consists of a suite of felsic and mafic gneisses of Mesoproterozoic age. Zircons from the felsic gneisses are complexly zoned, as shown in cathodoluminescence imaging; most zircon grains have multiple overgrowth zones, some of which are adjacent and parallel to elongate cores. Sensitive high-resolution ion microprobe (SHRIMP) analyses of oscillatory-zoned cores indicate that the volcanic protoliths of the felsic gneisses crystallized at ca. 1.25 Ga. These rocks were subsequently affected by at least three Mesoproterozoic growth events, at ca. 1.22, 1.16, and 1.02 Ga. Foliated biotite granite intruded the Baltimore Gneiss metavolcanic sequence at ca. 1075 Ma. The Slaughterhouse Granite (renamed herein) also is Mesoproterozoic, but extremely discordant U-Pb data from high-U, metamict zircons preclude calculating a precise age. The 1.25 Ga rocks of the Baltimore Gneiss are coeval with rocks emplaced in the Grenville Province during the Elzevirian orogeny, and the 1.22 Ga zircon overgrowths are coincident with a later stage of this event. Younger zircon overgrowths formed during the Ottawan phase of the Grenville orogeny. Backscattered electron imaging of titanites from felsic gneisses and foliated biotite granite reveals that many of the grains contain cores, intermediate mantles, and rims. Electron microprobe traverses across zoned grains show regular variations in composition. SHRIMP ages for titanite from the foliated biotite granite are 374 ± 8, 336 ± 8, and 301 ± 12 Ma. The ca. 374 Ma age suggests growth of titanite during a thermal event following the Acadian orogeny, whereas the late Paleozoic titanite growth ages may be due to greenschist-facies replacement reactions associated with Alleghanian metamorphism and deformation.