Solonker ophiolite in Inner Mongolia, China: A late Permian continental margin-type ophiolite (original) (raw)
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Geochemical constraints on the origin of the Hegenshan Ophiolite, Inner Mongolia, China
Journal of Asian Earth Sciences, 1999
The Hegenshan ophiolite in Inner Mongolia is a remnant of oceanic lithosphere of probable Devonian age. The ophiolite consists of several blocks composed chie¯y of serpentinized ultrama®c rocks with lesser amounts of troctolite and gabbro, and sparse lavas and dikes. The ultrama®c rocks consist chie¯y of depleted harzburgite and minor dunite and are interpreted as mantle tectonites. In the Hegenshan block dunite is relatively abundant and is typically associated with podiform chromitite. Both the chromite ore and the residual chromites in this body are relatively aluminous with average Cr numbers of 44±54. A few small chromite bodies and some of the residual chromites have much higher Cr numbers (72±76). Several blocks have welllayered cumulate sequences of gabbro and troctolite. Sheeted dikes are absent but small ma®c dikes are common in some of the ultrama®c sections. Most of the ma®c dikes have¯at chondrite-normalized REE patterns and are strongly depleted in incompatible elements, similar to depleted tholeiites from immature island arcs. The basaltic lavas of the Hegenshan ophiolite have two distinctly dierent chemical signaturesÐone similar to the ma®c dikes and one similar to ocean island basalts. The entire complex was probably formed within an island arc±marginal basin system that was later accreted to the southern margin of the Siberian Altaids. #
Structural and lithological characteristics of the Bayankhongor Ophiolite Zone, Central Mongolia
Journal of the Geological Society, 2001
The mechanism of continental growth of Central Asia is currently debated between models invoking continuous subduction-accretion, or punctuated accretion due to closure of multiple ocean basins. Ophiolites in Central Asia may represent offscraped fragments in an accretionary complex or true collisional sutures. The Bayankhongor ophiolite, a NW-SE-striking sublinear belt 300 km long and 20 km wide, is the largest ophiolite in Mongolia and possibly Central Asia. We present results of the first detailed structural and lithological study of the ophiolite. The study area is divided into four zones: Baidrag complex, Burd Gol, Bayankhongor, and Dzag zones. The Archaean Baidrag complex comprises tonalitic granulites and metasediments. The Burd Gol zone is a metamorphosed sedimentary and igneous mélange. The Bayankhongor zone contains the dismembered ophiolite forming a serpentinite mélange. The Dzag zone consists of asymmetrically folded chlorite-mica schists resembling meta-turbidites. The structure is dominated by steeply dipping, NE directed thrusts and NE-vergent folds. We suggest the Bayankhongor ophiolite marks the closure of an ocean separating two microcontinents: the Baidrag complex with the Burd Gol accretionary complex to the south, and a northern continent that forms the basement for the Hangai region. Subduction was towards the SW with NE-directed ophiolite obduction onto a passive margin represented by the Dzag zone.
The Central Ailaoshan ophiolite and modern analogs
Gondwana Research, 2013
a b s t r a c t The Central Ailaoshan (CAL) ophiolite represents an important tectonic component of the Jinshajiang-Ailaoshan-Song Ma suture zone separating the South China and Indochina blocks in the mainland SE Asia. The CAL ophiolite occurs as a complex tectonic mélange, and preserves the history of the opening and closure of the once vast Jinshajiang-Ailaoshan-Song Ma branch of the Paleotethys. New and existing geological data indicate that the CAL ophiolite contains magmatic rocks generated by: (1) L. Devonian-E. Carboniferous (ca. 380-330 Ma) volcanic passive margin-breakup development in the NW Gondwana margin; (2) L. Permian (ca. 258 Ma) Emeishan large igneous province-related continental rift magmatism, together with (and intruded by) (3) earliest M. Triassic (ca. 244 Ma) continent-continent syn-collisional S-type granitoids. The Devono-Carboniferous suites of the CAL ophiolite are highly comparable with many continental margin-type Alpine Tethyan ophiolites. In addition, the various CAL magmatic suites have strong South China block-affinities with coeval magmatism particularly in the western South China block, Jinshajiang-, Song Ma-, and Song Da terranes.
Age, geochemistry and tectonic setting of Buqingshan ophiolites, North Qinghai-Tibet Plateau, China
Journal of Asian Earth Sciences, 2004
The Buqingshan ophiolite complex is a sector of the A'nyemaqen ophiolite belt in the East Kunlun southern marginal suture zone in the northern part of the Qinghai-Tibet Plateau. The WNW-trending ophiolite complex consists of metaperidotite, gabbro, diabase, pillow basalt, massive basalt and pelagic sedimentary rocks including radiolarian chert. We have determined that Early Paleozoic and Early Carboniferous -Early Permian ophiolites are present in this ophiolite complex that were previously thought of Permian -Middle Triassic age. A zircon U -Pb age of 467.2^0.9 Ma for gabbro, and a Rb-Sr isochron age of 481^130 Ma for diabase and gabbro have been obtained from the Early Paleozoic ophiolite slice, and Middle-Late Ordovician acritarchs have been found in the mélange matrix that envelopes the sliver. We interpret the first two ages to represent the formation age of the ophiolite. Granodiorite -tonalite plutons with a zircon U-Pb age of 402^24 Ma intrude the slice. Early Carboniferous -Early Permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice. Geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle. Most mafic rocks in the Early Paleozoic ophiolite are of N-MORB type with small amount of T-MORB, and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination. The basalts in the Early Carboniferous -Early Permian ophiolite have similar characteristics to the mafic rocks and also show DUPAL anomaly and a mixed source of DMM and EM II. The original formation environments of the two-stage ophiolites are both mid-ocean ridges. These lines of evidence suggest that a mature Early Paleozoic Kunlun-Qilian -Qinling ocean basin and a mature Paleotethyan ocean basin once existed in the study area. q
Science, 2001
We report a thick, laterally extensive 2505 6 2.2Ðmillion-year-old (uraniumlead ratio in zircon) Archean ophiolite complex in the North China craton. Basal harzburgite tectonite is overlain by cumulate ultramaÞc rocks, a maÞc-ultrama Þc transition zone of interlayered gabbro and ultramaÞc cumulates, compositionally layered olivine-gabbro and pyroxenite, and isotropic gabbro. A sheeted dike complex is rooted in the gabbro and overlain by a mixed dikeÐ pillow lava section, chert, and banded iron formation. The documentation of a complete Archean ophiolite implies that mechanisms of oceanic crustal accretion similar to those of today were in operation by 2.5 billion years ago at divergent plate margins and that the temperature of the early mantle was not extremely elevated, as compared to the present-day temperature. Plate tectonic processes similar to those of the present must also have emplaced the ophiolite in a convergent margin setting.
International Geology Review, 2003
The Aoyougou mafic-ultramafic complex lies in the Paleozoic orogenic belt of the western part of the North Qilian Mountains near Aoyougou valley in Gansu Province, northwestern China. It consists of serpentinite, a cumulate sequence of gabbro and diorite, pillowed and massive lavas, diabase dikes, and chert, an assemblage tentatively interpreted as an ophiolite. SHRIMP dates on zircons from the diabase dikes indicate a crystallization age of 1777 ± 28 Ma. The basalts show light rareearth element enrichment and have relatively high TiO 2 and low Al 2 O 3 contents, characteristic of present-day E-MORB or ocean-island lavas. All of the lavas have relatively low MgO contents and Mg numbers [100Mg/(Mg+Fe)], indicating a somewhat evolved character. The diabase dikes have flat chondrite-normalized REE patterns and are significantly more primitive than the lavas. Based on the lava geochemistry, the nature of the serpentinite, and the regional geology, the Aoyougou complex is interpreted as a fragment of oceanic lithosphere that may have formed at an oceanic spreading axis. Later subduction of oceanic lithosphere in the Middle Proterozoic produced a trench-arc-basin system, which is preserved in the North Qilian Mountains. The tectonically dismembered Aoyougou complex is similar in many respects to Phanerozoic ophiolites and suggests that modern-style plate tectonic processes in China may have begun as early as the Middle Proterozoic.