From island arc to oceanic islands: Fiji, southwestern Pacific

Актуальность работы. Магматические породы на Большом Кавказе присутствуют в разрезах от ордовика до четвертичного периода. Выделены десятки разновозрастных магматических комплексов и свит содержащих вулканические породы приуроченных к узким структурно-формационным зонам. Однако геохимических исследований пород, на основе анализа распределения редкоземельных и редкометалльных элементов, все еще недостаточно и в первую очередь эта проблема касается раннепалеозойских образований. Целью исследования является получение (количественных) аналитических данных для расшифровки геодинамики раннего палеозоя. Объектом для исследований были выбраны тела догерцинских, огнейсованных габбро-диоритов аманкольского комплекса, распространенные в верховьях р. Кубань. Методы. Методы изучения, кроме геологического и петрографического описания, включали количественные RFA и ICP-MS анализы магматитов, рассмотрение геохимических характеристик, построение дискриминационных диаграмм, сравнение их с породами (эталонами) близких петрохимических типов, сформировавшихся в разнообразных геодинамических обстановках. Полученные результаты и их обсуждение. В результате комплексного изучения установлено, что габбро-диориты являются высокоглиноземистыми, умеренно титанистыми, умеренно магнезиальными породами известково-щелочной серии. Предполагается, что исходный расплав основного состава образовался при невысокой (13) степени плавления шпинелевых перидотитов. Низкие значения Mg 0,420,54, и содержания Ni позволяют считать, что, в процессе эволюции расплава происходило фракционирование оливина и пироксенов. Несовместимые элементы в габбро-диоритах и REE, нормированные по N-MORB и хондриту, образуют спектры наклонных линий, расположенных между профилями OIB и E-MORB, Laн/Ybн отношение в пределах 6,48,0. Порода характеризуется повышенными концентрациями LILe, LREE, MREE и элементов индикаторов флюидного субдукционного компонента Ba, U, Sr. На петрогенетических диаграммах фигуративные точки габбро-диоритов тяготеют к полям островодужных толеитов, андезитов океанических островов или активных континентальных окраин. Негативные аномалии Nb, Ta, P, Ti, Hf указывают на то, что породы формировались в надсубдукционных условиях. Выводы. Сравнение габбро-диоритов с породами близких петрохимических типов, показало, что они хорошо сопоставимы с породами зрелых вулканических островных дуг. Геохимическая специализация пород аманкольского комплекса литофильно-халькофильная. На основании полученных результатов сделано предположение о том, что в силурийское время (450400 млн лет назад), на севере Палеотетиса в Прото-Кавказском регионе существовала островодужная геодинамическая обстановка The relevance of the work. Igneous rocks in the Greater Caucasus are present in sections from the Ordovician to the Quaternary. Dozens of different ages of magmatic complexes, confined to narrow structural and formation zones, are identified here. However, geochemical studies of rocks, based on an analysis of the distribution patterns of rare-earth and rare-metal elements, are still insufficient and this problem primarily concerns the Early Paleozoic formations. Object. In connection with this object, the bodies of pre-Hercynian, fire-bound gabbro-diorites of the Amankol complex, locally distributed in the upper part of Kuban river, were selected for research. Methods. The research methods, in addition to geological and petrographic descriptions, included quantitative RFA and ICP-MS analyzes of magmatites, consideration of geochemical characteristics, construction of discriminatory diagrams, their comparison with rocks (samples) of similar petrochemical types formed in various geodynamic settings. Results. As a result of a comprehensive study, it was found that gabbro-diorites are high-alumina, moderately titanic, moderately magnesian rocks of the calc-alkaline series. It is assumed that the initial melt of the basic composition was formed at a low (1-3) degree of melting of spinel peridotites. The low values of Mg 0.420.54, and the Ni content suggest that, during the evolution of the melt, olivine and pyroxenes were fractionated. Incompatible elements in gabbro-diorite and REE, normalized to N-MORB and chondrite, form slanted line spectra located between the OIB and E-MORB profiles, Laн/Ybн ratio in the range of 6.48.0. The rock is characterized by increased concentrations of LILe, LREE, MREE and elements indicators of the fluid subduction component - Ba, U, Sr. On the petrogenetic diagrams, the figurative points of gabbro-diorites gravitate toward the fields of island-arc tholeiites, andesites of oceanic islands, or active continental margins. Negative anomalies of Nb, Ta, P, Ti, Hf indicate that rocks were formed under suprasubduction conditions. Comparison of gabbro-diorites with rocks of similar petrochemical types showed that they are well comparable with the rocks of mature volcanic island arcs. Geochemical specialization of rocks of the Amankol complex is lithophilic-chalcophilic. Based on the results obtained, it was assumed that in the Silurian time (450400 million years ago), an island-arc geodynamic situation existed in the north of the Paleotetis in the Proto-Caucasian region

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Geochemistry of the upper Snooks Arm Group basalts, Burlington Peninsula, Newfoundland: evidence against formation in an island arc

Canadian Journal of Earth Sciences ◽

10.1139/e80-087 ◽

1980 ◽

Vol 17 (7) ◽

pp. 888-900 ◽

Cited By ~ 12

Author(s):

G. A. Jenner ◽

B. J. Fryer

Keyword(s):

Subduction Zone ◽

Oceanic Crust ◽

Sedimentary Rocks ◽

Oceanic Islands ◽

Oceanic Island ◽

Island Arc ◽

Geochemical Data ◽

Basaltic Rocks ◽

Arc Setting ◽

Back Arc

The Snooks Arm Group of the Newfoundland Appalachians, which includes the Betts Cove ophiolite at its base, has been interpreted as oceanic crust overlain by island arc volcanic and sedimentary rocks. The limited geochemical data available on the upper Snooks Arm Group basalts have been used as evidence for and against their formation in an island arc environment.Reinvestigation of the chemistry of the basaltic rocks of the upper Snooks Arm Group establishes them as large ion lithophile enriched tholeiites. Similar basalts have been found in oceanic islands, on aseismic ridges, and possibly in back-arc basins. Chemically analogous rocks are notably lacking from island arc settings.The geochemistry and geology of the upper Snooks Arm Group suggest that these rocks may have formed in either an oceanic island setting or, as recently suggested by Upadhyay and Neale, as part of a marginal basin. It is not possible to distinguish between these alternate models, although the most similar basaltic rocks occur in the former environment. It is most unlikely that these rocks formed in an early island arc setting and indeed there may be no need for them to be associated with a major subduction zone.

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Using fossils to identify allochthonous oceanic islands in the ancient geologic record

The Paleontological Society Special Publications ◽

10.1017/s2475262200008352 ◽

1992 ◽

Vol 6 ◽

pp. 275-275

Author(s):

Constance M. Soja

Keyword(s):

Carbonate Platform ◽

Hot Spot ◽

Deep Ocean ◽

Taxonomic Diversity ◽

Oceanic Islands ◽

Oceanic Island ◽

Island Arc ◽

Plate Boundaries ◽

Diagnostic Features ◽

Geologic Record

Silurian organisms preserved in southeastern Alaska (Alexander terrane) inhabited marine environments within an island-arc complex during a phase of waning volcanism and are fossilized in a diversity of shallow-marine platform and deep-water deposits. These fossils exhibit a distinctive suite of characteristics and share fundamental similarities with biotas of Paleozoic-Mesozoic age that are preserved in other accreted island terranes of North America. These special attributes reflect the colonization, evolution, and diversification of marine organisms adjacent to subconical/conical volcanic edifices characterized by relatively high rates of subsidence, steep submarine slopes, tectonic instability, and biogeographic isolation. Recognition of these diagnostic features enables many ancient island faunas to be distinguished from those that lived on the craton and enhances differentiation of island biotas from pelagic assemblages that accumulated as oozes in deep ocean basins.Although island faunas exhibit a high degree of variability in taxonomic diversity, levels of endemism, and provincial affinities, many share a significant number of similarities. Several of these shared attributes reflect organismal evolution in biogeographic isolation at island sites separated from continental and other source regions by considerable geographic distances or at locations unfavorably situated with respect to oceanic currents transporting teleplanic larvae. Comparison of Silurian island-arc faunas from Alaska with coeval assemblages from different tectonic settings and with modern volcanic islands shows that oceanic island biotas commonly are characterized by: (1) initially impoverished, normal marine faunas of low diversity and abundance that are preserved in exceptionally thick platform sequences; (2) sequential development of organic structures from fringing to barrier reefs on the outer shelf during thermal subsidence and lateral expansion of the carbonate platform; (3) restricted faunas devoid of normal marine shelly benthos and tolerant of quiet-water conditions, muddy substrates, and fluctuations in salinity, temperature, and oxygen concentrations in back-reef lagoons; (4) extensive taphonomic redistribution of organisms along bathymetric gradients and downslope preservation in debris flows, slumps, and turbidites of mixed fossil assemblages derived from shelf and shelf-margin habitats; (5) rapid lateral and vertical changes in biofacies, reflecting complex depositional systems in fault-block basins; (6) insular biotas with relatively high levels of endemism; (7) complex paleobiogeographic affinities expressed in assemblages that comprise mixtures of taxa from different faunal regions; and (8) relict biotas that may represent the protracted survival of some organisms in island refugia.Because many accreted islands are poorly preserved and highly deformed, recognizing these distinctive features in oceanic island faunas enhances identification of allochthonous volcanic arcs, seamounts, atolls, and hot-spot islands in the ancient geologic record. Using fossils to identify islands in accreted terranes is especially important when island origins of strata are suspected but difficult to prove because calc-alkaline volcano-plutonic rocks or derivative volcanogenic and quartz-poor siliciclastic deposits are absent or not exposed. Hence, relying on fossils to recognize oceanic islands that survived destructive tectonic processes offers an expanded list of geologic criteria to aid in reconstructing plate boundaries marking ancient zones of convergence and to use in unraveling the tectonic history of ocean basins recorded in suspect terranes.

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Triassic sponges (Sphinctozoa) from Hells Canyon, Oregon

Journal of Paleontology ◽

10.1017/s0022336000018333 ◽

1988 ◽

Vol 62 (03) ◽

pp. 419-423 ◽

Cited By ~ 6

Author(s):

Baba Senowbari-Daryan ◽

George D. Stanley

Keyword(s):

Endemic Species ◽

Upper Triassic ◽

Island Arc ◽

Tropical Island ◽

The Family ◽

Wallowa Terrane ◽

Unique Condition ◽

Hells Canyon

Two Upper Triassic sphinctozoan sponges of the family Sebargasiidae were recovered from silicified residues collected in Hells Canyon, Oregon. These sponges areAmblysiphonellacf.A. steinmanni(Haas), known from the Tethys region, andColospongia whalenin. sp., an endemic species. The latter sponge was placed in the superfamily Porata by Seilacher (1962). The presence of well-preserved cribrate plates in this sponge, in addition to pores of the chamber walls, is a unique condition never before reported in any porate sphinctozoans. Aporate counterparts known primarily from the Triassic Alps have similar cribrate plates but lack the pores in the chamber walls. The sponges from Hells Canyon are associated with abundant bivalves and corals of marked Tethyan affinities and come from a displaced terrane known as the Wallowa Terrane. It was a tropical island arc, suspected to have paleogeographic relationships with Wrangellia; however, these sponges have not yet been found in any other Cordilleran terrane.

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