Henriette Lapierre

Geodynamics of the Ogcheon Belt (South Korea)

Abstract The Ogcheon Belt is a stack of synmetamorphic southeastward verging nappes resulting from the deformation of a volcanosedimentary sequence deposited in an early Palaeozoic rift which was affected by polyphase tectonics. Its pre-Cretaceous structure results from three main orogenic events: the “Ogcheon” (late Silurian-Devonian (?)), Indosinian (late Permian-Triassic) and Yenshanian (middle Jurassic-early Cretaceous) tectonic phases. The external, autochthonous or para-autochthonous units represent platform zones with shallow-water deposits whereas rift facies characterize the internal domain. The Yeongweol sequence, forming an outer shelf on the southeastern margin of the rift, displays affinities with the Jiangnan basin of South China. It is in wrench fault contact with the easternmost Duwibong sequence correlated with the North China-North Korea platform. In the internal metamorphic units, post-Cambrian-early Ordovician pelitic and turbiditic rift metasediments overlie a thin Cambrian or Cambrian-Ordovician early platform sequence. Two subdomains are distinguished: the upper units (Chung Ju and Pibaryeong units) representing the deep “axial” part of the basin contain distal flyshoids and large olistoliths derived from the northwestern “narrow” margin; the lower units (Turung San, Poeun and Iwharyeong) are related to the “wide” margin located between the deep rift domain and the stable southeastern platform. The latter transitional domain is characterized by thick submarine debris flows, and volcanics of alkali or transitional tholeiitic affinity. The volcanic activity and mass gravity flow sedimentation are closely related to the crustal thinning and subsequent rifting and tilting. Lithostratigraphic evidence allows the correlation of the unfossiliferous rift metasediments with the Cambrian-Ordovician platform sequence in one single basin. The post-middle Silurian-pre-mid-Carboniferous “Ogcheon” ensialic orogeny resulted in the closure of the rift with relatively minor effects in the platform area. It gave rise to an intracontinental fold-thrust belt without ophiolites. This tectonic event may have been contemporaneous with early Palaeozoic tectonism in the Quinling Belt in Central China and in the Southeast China fold belt. The definitive constitution of the Sino-Korean craton took place before the mid-Carboniferous and “molasse” sedimentation was initiated on a single platform located on the western edge of the Yakuno oceanic basin of Southwest Japan. In late Permian and early Triassic times, the closure of the oceanic area and the subsequent westward collision of the Honshu Block are marked in Korea by the intracratonic “Indosinian” tectonism. Since late Triassic times, predominantly transcurrent movements have occurred along NNE-SSW faults associated with syntectonic intermontane troughs and important granitization.

Geochemical study of an early Paleozoic island-arc-back-arc basin system. Part 2: Eastern Klamath, early to middle Paleozoic island-arc volcanic rocks (northern California)

A systematic mineralogical, REE and Sr-Nd isotopic study of the eastern Klamath Mountains pre- to Early Devonian volcanism (Copley tholeiites, Lovers Leap volcanic rocks, Trinity ophiolitic basalts) show that all of the rocks belong to a low-K, island-arc, tholelitic suite, with the exception of the Late Ordovician Lovers Leap Butte calc-alkalic volcanic rocks. All of the latter have low ϵ Nd values (ϵ Nd(T) = 3.9), suggesting that they suffered crustal contamination. The Copley and Lovers Leap low-K tholeiites exhibit variable LREE enrichment or depletion and variable ϵ Nd values (ϵ Nd(T) range mostly between 5 and 8), suggesting that variable partial melting degree and/or contamination by sediments occur(s). The isotopic features of the Copley high-Mg andesites (boninitic affinities, ϵ Nd(T) range between 6.2 and 7.9) show that they were derived from the same source as the low-K tholeiites. The Trinity ophiolitic basalts, along with the LREE-depleted Copley and Lovers Leap tholeiites, show similar geochemical and isotopic features (ϵ Nd(T) = 7.3 and 7.4), suggesting that they belong to the same island-arc-back-arc basin system.

Paleozoic and Lower Mesozoic magmas from the eastern Klamath Mountains (North California) and the geodynamic evolution of northwestern America

Abstract The Paleozoic to Early Mesozoic geology of the eastern Klamath Mountains (N California) is characterized by three major magmatic events of Ordovician, Late Ordovician to Early Devonian, and Permo-Triassic ages. The Ordovician event is represented by a calc-alkalic island-arc sequence (Lovers Leap Butte sequence) developed in the vicinity of a continental margin. The Late Ordovician to Early Devonian event consists of the 430–480 Ma old Trinity ophiolite formed during the early development of a marginal basin, and a series of low-K tholeiitic volcanic suites (Lovers Leap Basalt—Keratophyre unit, Copley and Balaklala Formations) belonging to intraoceanic island-arcs. Finally, the Permo-Triassic event gave rise to three successives phases of volcanic activity (Nosoni, Dekkas and Bully Hill) represented by the highly differentiated basalt-to-rhyolite low-K tholeiitic series of mature island-arcs. The Permo-Triassic sediments are indicative of shallow to moderate depth in an open, warm sea. The geodynamic evolution of the eastern Klamath Mountains during Paleozoic to Early Mesozoic times is therefore constrained by the geological, petrological and geochemical features of its island-arcs and related marginal basin. A consistent plate-tectonic model is proposed for the area, consisting of six main stages: 1. (1) development during Ordovician times of a calc-alkalic island-arc in the vicinity of a continental margin; 2. (2) extrusion during Late Ordovician to Silurian times of a primitive basalt-andesite intraoceanic island-arc suite, which terminated with boninites, the latter suggest rifting in the fore-arc, followed by the breakup of the arc; 3. (3) opening and development of the Trinity back-arc basin around 430–480 Ma ago; 4. (4) eruption of the Balaklala Rhyolite either in the arc or in the fore-arc, ending in Early Devonian time with intrusion of the 400 Ma Mule Mountain stock; 5. (5) break in volcanic activity from the Early Devonian to the Early Permian; and 6. (6) development of a mature island-arc from the Early Permian to the Late Triassic. The eastern Klamath Mountains island-arc formations and ophiolitic suite are part of the “Cordilleran suspect terranes”, considered to be Gondwana margin fragments, that have undergone large northward translations before final collision with the North American craton during Late Mesozoic or Cenozoic times. These eastern Klamath Mountains island-arcs could be associated with the paleo-Pacific oceanic plate that led to accretion of these allochthonous terranes to the American margin.

Geodynamic implications of Devonian silicic arc magmatism in the Sierra Nevada and Klamath Mountains, California

Most proposed geodynamic models integrate the northern Sierra Nevada and eastern Klamath Mountains in a single island-arc system. New petrological and geochemical data show that the Upper Devonian Sierra Buttes Formation of the northern Sierra Nevada displays important differences from the Lower Devonian Balaklala Formation of the eastern Klamath Mountains. The cale-alkalic affinity of the Sierra Buttes rhyolites (light rare earth element [REE] enriched, low e Nd ) suggests that they were erupted in a continent-based island arc. In contrast, the Lower Devonian Balaklala low-K tholeiitic rhyolites (light REE depleted, high e Nd ) are considered to be part of an intraoceanic island-arc sequence. Moreover, the Paleozoic tectogenesis of these two island-arc sequences is different. The Lower Devonian Balaklala rhyolites were affected by the Late Devonian-Early Carboniferous orogeny, whereas the Sierra Buttes Formation is on basement already affected by a pre-Late Devonian deformation. Therefore, at least after Carboniferous time and before Late Triassic time, the eastern Klamath and northeastern Sierra Nevada island arcs cannot be considered lateral counterparts.

Diversity of magma types in a lower Paleozoic island arc-marginal basin system (Eastern Klamath Mountains, California, U.S.A.)

Abstract A trace-element investigation has been conducted on Ordovician to Devonian volcanics of the eastern Klamath Mountains spatially associated to the Siluro-Ordovician Trinity peridotite and gabbros. All the studied volcanics (low-K tholeiites and calc-alkaline volcanics) present strong island-arc geochemical features (low TiO 2 contents, negative Nb, Zr and Ti anomalies, mild LREE enrichment and depletion). Some volcanics from Lovers Leap and Copley exhibit strong immature island-arc geochemical affinities (very depleted LREE patterns and very low Nb contents), suggesting that they were formed by high degrees of partial melting (20%) of a residual lithosphere. Diminution of the partial melting degree and addition of sediments to the source will explain the occurrence of less depleted or slightly LREE-enriched volcanics. The Grey Rocks basaltic pillow-lavas thrust onto the Trinity peridotitic sheet probably represent the extrusive member of the ophiolitic sequence. Their strong island-arc geochemical affinities suggest that the ophiolite was formed during the first stages of a back-arc basin development. Differentiation took place in the arc by melting of the deeper basaltic layers, leading to the formation of abundant rhyolitic and trondhjemitic magmatism representing the first stages of continental accretion.

The origin of geochemical variations in a late permian volcanic arc, eastern klamath mountains, California

Abstract Volcanic are remnants present in the eastern Klamath terranes record an intermittent volcanism from Ordovician to Early Jurassic times. The Upper Permian volcanics (Nosoni, Dekkas and Bully Hill formations) consist mainly of andesites with minor basalts and rhyolites, metamorphosed in the prehnite-pumpellyite facies. Their petrographic and geochemical characters indicate a mature island-arc setting, but their tholeiitic affinity suggests an intra-oceanic environment. Nd isotopic compositions of the Nosoni-Dekkas lavas ( e Nd T = +4 to +8), as well as a preferential enrichment of light rare-earth over high-field-strength elements and systematic negative Ce anomalies, point to MORB-type mantle sources contaminated by a few percent of slab-derived sediments. Large variations in the chondrite-normalized La/Yb ratio (0.95 to 3.7) are interpreted in terms of variable depth and degree of partial melting. Two distinctive basalt-types are found at the top of the Nosoni Formation (Dekkas Saddle) and the Bully Hill Formation. The former are high-Mg basalts showing a strong enrichment in incompatible elements (La/Yb ∼ 50, 8.7 ppm Th, ∼ 50 ppm Nb) but low TiO 2 , Al 2 O 3 , Yb, Zr and Sr contents, with a pronounced positive Eu anomaly. They may result from small degrees of partial melting of a refractory peridotitic source enriched by felsic melts derived from subducted MORB, with garnet in the residue. These lavas probably record a transient anomalous thermal regime in the deepest part of the subduction zone, and can be compared to some extent to the arc alkali series. On the other hand, the Bully Hill basalt shows boninitic affinities and probably originated from a shallower lithospheric mantle source. The large range of compositions recorded by the Permian arc lavas gives evidence of efficient extraction of melts segregated at different depths.

Comparison between two Palaeozoic island-arc terranes in northern California (eastern Klamath and northern Sierra Nevada): geodynamic constraints

Abstract The Palaeozoic geodynamic environment of northern California is considered to be one of offshore island arcs bounded by marginal seas. The petrological and geochemical features of the eastern Klamath and northern Sierra Nevada island-arc sequences are important indicators of the geodynamic evolution of the Cordillera. The eastern Klamath pre-Early Devonian island-arc sequence consists of pillowed basalts andesites and boninites, overlain by low-K rhyolites, both suites erupted in a submarine environment. These volcanics, characterized by almost flat REE patterns and high ϵ Nd represent a primitive intraoceanic island arc. The mid-Palaeozoic volcanic succession of northern Sierra Nevada is formed of calc-alkaline rhyolites conformably overlain by either calc-alkaline (in the north) or low-K tholeiite (in the south) basalts and andesites. The volcanic episode ended in the Early Carboniferous with a differentiated shoshonitic suite. All the massive or pillowed flows are interbedded with volcaniclastic sediments, suggesting that they were erupted partly in a shallow-water environment. The geochemical features of the volcanics, moderate to high LREE enrichment and low ϵ Nd values, are evidence for the involvement of a crustal component in the magma genesis. The mid-Palaeozoic volcanic sequence therefore represents a continent-based island arc. This microcontinent could be the Sonomia microplate (excluding the eastern Klamath plate). Thus, these two northern Californian Palaeozoic sequences cannot be considered as lateral counterparts but could represent the remnants of oceanic and continental island arcs as observed in the present-day East Pacific areas along the Eurasiatic margin.

Upper Jurassic mafic magmatic rocks of the eastern Klamath Mountains, northern California: Remnant of a volcanic arc built on young continental crust

Diabasic and gabbroic dikes intruding the lower Paleozoic Trinity Ophiolite in the Lovers Leap section, Klamath Mountains, California, display strong calc-alkalic petrological and geochemical features (occurrence of primary amphiboles, zoned plagioclase phenocrysts and biotite, low TiO 2 , high incompatible trace-element contents, and light rare earth element enrichment). These dikes, of Late Jurassic age (149 ±6 Ma by K-Ar), are petrographically and geochemically similar to the contemporaneous calc-alkalic ultramafic-mafic magmatism well developed through the Klamath Mountains. They present negative Nb, Zr, and Ti anomalies typical of subduction-related magmatism and probably belong to a volcanic arc on an active continental margin. Their ϵ Sr (between -9.7 and -12.5) and ϵ Nd (between 5.6 and 6.3) values compare with some western U.S. Mesozoic granites. The Nd isotopic values, lower than those of mid-oceanic ridge basalts and intra-oceanic island arcs, suggest that these dikes, deriving from a depleted mantle source, have been slightly contaminated by continental material, probably subducted sediments. Values of ϵ Nd suggest, moreover, that no old continental crust underlies the Klamath Mountains.