Volcanisme triasique calco-alcalin
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摘要

Two types of island-arc occur in the North American Cordillera during the Permian-Triassic times. The first type is exposed in the eastern Klamath and Blue Mountains (fig. 1). Its stratigraphy is continuous from Permian to Triassic, and is composed of arc-tholeiites with minor calc-alkaline lavas. This suite shows high epsilon Nd (sub (T)) values similar to the range of intra-oceanic island-arc [Lapierre et al., 1987; Brouxel et al., 1987, 1988; Charvet et al., 1990; Lapierre et al., 1990, 1994]. In contrast, the second type, exposed in northern Sierra Nevada and central-western Nevada (Black Dyke) (fig. 1), is characterized by an early Permian calc-alkaline suite, with positive to negative epsilon Nd (sub (T)) values. Its basement is inferred to present continental affinities [Rouer et Lapierre, 1989; Rouer et al., 1989; Blein et al., 1996, 2000]. In western Nevada, volcanic rocks of early Triassic age are present in few localities: (1) the Triassic Koipato Group in central Nevada (fig. 1); (2) the Pablo Formation in the Shoshone mountains and the Paradise Range (figs. 1 and 2); and (3) the Garfield Flat formation in the Excelsior mountains (figs. 1 and 2). Silberling [1959] has subdivided the Pablo formation into three members: clastic, limestone, and greenstone (fig. 3). The clastic member consists of andesites, interbedded with volcaniclastic turbidites. The contact between the clastic and the limestone members is gradational and interlensing. The limestones are locally bioclastic with shell fragments, indicating a shallow-water deposition. They yielded a reworked late Permian fauna which suggests a late Permian or younger age. The clastic and limestone members could represent the recurrent rapid deposition in a shallow marine basin of volcanic flows, reworked material from a nearby terrane of volcanic, granitic, and sedimentary rocks. The greenstone member is composed of andesites, volcanic breccias and tuffs. The middle Triassic Granstville formation rests conformably on the Pablo formation. Both formations are affected by Mesozoic polyphase deformations [Oldow, 1985]. The Permian and/or Triassic Garfield Flat formation is composed of ignimbrites and pyroclastic breccia interlayered with conglomerates, sandstones, calcareous and red pelites (fig. 4). The Jurassic-Triassic Gabbs-Sunrise formation rests unconformably on the Garfield Flat formation. Both formations are affected by Mesozoic polyphase deformations [Oldow, 1985]. In the Pablo formation, lavas are shoshonitic basalts and calc-alkaline andesites, while calc-alkaline andesites and rhyolites predominate in the Garfield Flat formation. Basalts and andesites exhibit enriched LREE patterns (fig. 6) with slight negative anomalies in TiO 2 , Nb and Ta typical of subducted-related magmas in the primitive mantle-normalized spidergrams (fig. 7). The lavas show epsilon Sr (sub (T)) and epsilon Nd (sub (T)) values which range between -0.4 to +19.6, and -1.4 to +0.8 respectively (fig. 8). Most of the samples are displaced from the mantle array toward higher epsilon Sr (sub (T)) values, due to the alteration. The epsilon Nd (sub (T)) values, close to the Bulk Earth composition, record an interaction between material from a juvenile pole (mantle or young crust) and from an old crust. The Pablo and Garfield Flat formations differ from the Permian Black Dyke formation. This latter is characterized by calc-alkaline basalts and mafic andesites enriched in LREE, and a mantle source contaminated by subducted sediments or arc-basement [Blein et al., 2000]. The Pablo and Garfield Flat formations show many similarities with the Koipato Group. In central Nevada, the Koipato Group is a sequence of andesites, dacites and rhyolites interbedded with tuffs and volcaniclastic sediments. It rests with a marked angular unconformity on folded Upper Paleozoic oceanic rocks [Silberling and Roberts, 1962]. Fission-track dating on zircon [McKee and Burke, 1972] indicate an age of 225+ or -30 Ma for the Koipato Group. Ammonites, near the top, are considered to be upper early Triassic [Silberling, 1973]. The Pablo and Garfield Flat lavas share in common with the Koipato Group: (1) late Permian to middle Triassic ages; (2) abundant andesites and rhyolites with minor basalts, associated with felsic pyroclastic breccias; (3) LILE and LREE enrichement; (4) low epsilon Nd (sub (T)) values suggesting a juvenile source with slight contamination by a crustal component; (5) La/Nb ratios close to the lower limit of orogenic andesites [Gill, 1981]; and (6) high Nb/Zr ratios suggesting a generation far from a subduction zone [Thieblemont and Tegyey, 1994]. This Triassic high-K calc-alkaline to shoshonitic magmatism is enriched in K, Rb, Th, Nb and Ta relative to the calc-alkaline Black Dyke lavas, and is mainly juvenile judging from Nd isotopic ratios. The source may correspond either to a juvenile crust composed of high-K andesites [Roberts and Clemens, 1993], which could be the Black Dyke lavas, or to phlogopite-K-richterite enriched lithospheric mantle. In both cases, the generation of the high-K calc-alkaline magmatism needs the former existence of an important subduction phase to generate its source. The lavas of the Pablo and Garfield Flat formations are similar to calc-alkaline and shoshonitic lavas emitted in post-collisional setting. Post-collisional arc/continent magmatism is varied from intermediate to felsic, calc-alkaline to shoshonitic, low to high-K and meta-aluminous to hyper-aluminous. The studied lavas may be compared to the arc/passive margin collision of Papua-New Guinea, where a post-collisional magmatism characterized by high-K basalts, andesites and shoshonites [McKenzie, 1976]. In Nevada, this post-collisional event develops after the accretion of the Permian Black Dyke island-arc (Type 2), and before the accretion of the intra-oceanic Permo-Triassic arc (Type 1).

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