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Hornblende geobarometry has been applied to granitic rocks of the Middle Jurassic Nelson Batholith, British Columbia, locally containing magmatic epidote. Geobarometry suggests equilibration pressures of less than 4.5 kbar (1 kbar = 0.1 GPa) in the northern part of the batholith, which lacks magmatic epidote. This part of the pluton shows clear magmatic intrusive relations, and the contact metamorphic rocks contain andalusite, which suggests that the equilibration and emplacement pressures are compatible.In the southern part of the batholith, granitic rocks containing magmatic epidote have equilibration pressures of 4.8–6.4 kbar. South and west of Nelson, there is a distinct contrast in pressure between the pluton and the country rock. Both the contact metamorphic rocks and the low-grade regional metamorphic rocks suggest pressures in the 2–3 kbar range.The pressure difference of about 2 kbar across the southwestern contact and the variation in pressure within the batholith can be explained by a model combining a late postequilibration upsurge (diapiric) of a deeper part of the pluton in the south, with a much later rotation and tilting of the batholith, associated with Eocene motion on the upper listric portion of the Slocan Lake Fault. The late diapiric (?) upsurge may account for the pressure contrast across the southwestern contact.

The Slide Mountain terrane (SMT) in southern British Columbia consists of rocks of continental and oceanic affinity that are juxtaposed with parautochthonous rocks of the North American continental margin. In southern British Columbia, SMT consists dominantly of fine‐grained quartzose clastic rocks, limestone and lesser amounts of conglomerate and volcanic rocks of the Carboniferous McHardy assemblage, and predominantly mafic volcanic rocks of the Permian Kaslo Group. U‐Pb ages of individual detrital zircons from the McHardy assemblage are 1.7 Ga to 3.1 Ga and are similar to published ages of zircons from sedimentary rocks of the adjacent Kootenay terrane and the North American continental margin. These data and the petrology of McHardy assemblage sandstones and conglomerate suggest Kootenay terrane and the North American miogeocline as sources for McHardy assemblage detritus. U‐Pb zircon ages of granitoid clasts within McHardy assemblage conglomerate indicate that Silurian granitic rocks also provided detritus to the SMT. Mafic volcanic, ultramafic, and sedimentary rocks of the Kaslo Group conformably overlie the McHardy assemblage. New geochemical data demonstrate that the Kaslo Group consists of light rare earth element depleted basalts. On the basis of geochemical and geologic data, we suggest that Kaslo Group volcanics were erupted within an ocean ridge proximal to the North American continental margin and probably represent the eastern (continental) margin of a Permian marginal basin. Lithologie, stratigraphie, and U‐Pb geochronologic data suggest that the SMT was deposited on autochthonous, distal miogeoclinal rocks of the adjacent western North American craton and in close proximity to an early Paleozoic arc terrane. We infer that correlative late Paleozoic basinal terranes in western North American were deposited in a similar tectonic setting.

Conventional K-Ar and 40 Ar/ 39 Ar analyses on whole-rock samples are reported for basaltic samples retrieved on the Central and Southern Kerguelen plateaus during Ocean Drilling Program Leg 120. Sites 747, 749, and 750 recovered basalts from the plateau basement, whereas Site 748 drilled a lava flow interbedded with sediments of probable Albian age.The freshest core basalts from the basement yielded dates falling in the 110-100 m.y. interval. Sample 120-749C-15R-3 (26-31 cm) gave conventional K-Ar, total fusion, and plateau ^Ar/^Ar ages that are closely concordant: 111.5 ± 3.2 m.y., 109.9 ± 1.2 m.y., and 109.6 ± 0.7 m.y., respectively. Sample 120-750B-15R-5 (54-60 cm), when taking into account the analytical uncertainties, yields conventional K-Ar and 40 Ar/ 39 Ar plateau ages that can be considered similar: 101.2 ± 7.5 and 118.2 ± 5 m.y., respectively. Inspection of the 39 Ar/ 40 Ar vs. 36 Ar/ 40 Ar diagram does not reveal the occurrence of an initial argon component of radiogenic composition in the two samples. Accordingly, our results suggest that the formation of the basement of the Central Kerguelen Plateau was closed at 110 m.y.. Furthermore, these results are in agreement with a K-Ar age of 114 ± 1 m.y. mentioned in the literature for a basalt dredged in the 77°E Graben.The still scant amount of data indicates that the outpourings of the Central Kerguelen Plateau correspond rather well with widespread continental magmatism in Gondwanaland that is believed to mark the incipient opening of the eastern Indian Ocean. This implies a huge head for the mantle plume at the source of these liquids. Nevertheless, on land and at sea the exact duration of magmatism remains unknown. Therefore, a catastrophic pattern similar to that currently invoked for the Deccan Traps at the end of the Cretaceous, though possible, is not yet required by present geochronologic data.

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In the Middle Jurassic Nelson Batholith, southern British Columbia, young 4 0~r / 3 9~r ages (i.e., 50-60 Ma) and distorted isobaric surfaces in the batholith suggest the possibility of Paleocene granitic plutonism. We present the results of a study undertaken to evaluate this possibility. Geochemical criteria successfully distinguish a suite of granitoids within the Nelson Batholith that differ from Nelson granites of similar SiO, content. The granitoid suite is composed of 71.6-75.7 wt. % SiO, leucocratic biotite granite and quartz monzonite with strong enrichments in alkaline, alkaline earth, and rare earth elements. Nd and Pb isotopic compositions suggest that biotite granite and quartz monzonite are not related. Biotite granite yields a U-Pb age of 158.9 + 0.6 Ma (concordant zircons). Quartz monzonite crystallized at 61 f 1 Ma, based on interpretation of titanite and zircon analyses. Zircons from this sample lie along a line from 61 to 160 Ma and demonstrate the presence of Middle Jurassic inheritance. Based on its petrographic and isotopic similarity to other Middle Jurassic plutons in the Nelson Batholith -Valhalla Complex area, we include the 159 Ma biotite granite with the Jurassic plutonic suite. This result demonstrates that magmatism in southern British Columbia was active at least until the early Late Jurassic (Oxfordian). The Paleocene (61 Ma) quartz monzonite that intrudes the southern Nelson Batholith is the structurally highest occurrence of "Ladybird" granite yet documented in southern British Columbia. Comparison of new and published geochemical and isotopic data for Paleocene granitoids throughout the southern Omineca Belt, British Columbia, suggests that these granitoids were not derived from a single, old crustal source. Les Lges 40Ar/39Ar jeunes (c.-h-d., 50-60 Ma) fournis par le Batholite de Nelson (Jurassique moyen), dans le sud de la Colombie-Britannique, et les surfaces isobares distordues qui caractkrisent le batholite suggkrent la possibilitC d'un plutonisme granitique palCogkne. Nous prksentons les rksultats d'une Ctude dont l'objectif Ctait d'kvaluer cette interprktation. Les critkres gtochimiques permettent de reconnaitre dans le Batholite de Nelson une suite de granitoides diffkrente des granites de Nelson ayant sensiblement la mCme teneur de SiO,. La suite granitoide est composCe d'un granite h biotite leucocratique contenant 71,6 i 75,7% de SiO, par poids et de monzonite quartzique, elle est fortement enrichie en ClCments -. . alcalins, alcalino-terreux et terres rares. Les compositions isotopiques d e~d et ~b suggkrent qu'il n'existe pas de relation entre le granite B biotite et la monzonite quartzique. Le granite h biotite a fourni un Lge U-Pb de 158,9 f 0,6 Ma (zircons concordants). L'Lge de cristallisation de la monzonite quartzique fond6 sur l'interprktation des analyses du zircon et de la titanite est Ctabli h 61 + 1 Ma. Les Lges des zircons extraits de cet Cchantillon sont CchelonnCs sur une ligne entre 61 et160 Ma, ce qui dCmontre la prCsence d'un hCritage du Jurassique ...

During Leg 120 basalts were recovered at four drill holes on the Kerguelen-Heard Plateau. This paper reports the trace element and Sr, Nd, Hf, and Pb isotopic characteristics of these basalts and compares these basalts with Indian Ocean basalts and Kerguelen and Heard island volcanics. Kerguelen-Heard Plateau basalts are extremely heterogeneous in character. Intersite variations are larger than intrasite variations. Part of the chemical variations of the plateau volcanics overlap with those characteristics of Kerguelen Island volcanics, which indicates tapping of the same mantle source during the two different periods of activity. The estimates of the degree of melting for the plateau basalts (smaller degree of melting than for mid-ocean ridge basalts) and the heterogeneous character of the plateau exclude an origin that requires large degrees of melting or more rigorous convection than at ocean ridges. However, all characteristics indicate an oceanic origin for the Kerguelen-Heard Plateau.