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Lytwyn, J.N., 1993

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Bibliographic Reference

Lytwyn, J.N., 1993, Geochemistry and petrogenesis of forearc volcanics and intrusives possibly associated with ridge subduction: Examples from the Pozanti-Karsanti and Hatay ophiolites, southern Turkey, and Chugach-Prince William terranes, southern Alaska: Houston, Texas, University of Houston, Ph.D. dissertation, 318 p., illust., maps.

Abstract

The geochemistry of forearc volcanics and related intrusives generated through subduction of an active ridge system reflect the compositions of melts originating from the subducting spreading center, overlying mantle wedge and/or associated accretionary complex. Geochemical and petrological investigation of forearc magmatism include the Pozanti-Karsanti and Hatay ophiolites, southern Turkey, Resurrection and Knight Island ophiolites, southern Alaska, and near-trench intrusives from the Kenai Peninsula, southern Alaska. Mafic dikes that cross-cut the Pozanti-Karsanti (P.K.) Ophiolite were possibly emplaced in a forearc environment by a subducting Neo-Tethyan spreading center during the Late Cretaceous. The dikes compositionally range from basalts to basaltic andesites with the geochemical characteristics of island-arc tholeiites and boninites. Pillow basalts and sheeted dikes from the Hatay Ophiolite are geochemically similar to the P.K. dikes and probably formed along a forearc spreading center above a Neo-Tethyan subduction zone, although ridge subduction has not been demonstrated. Parental liquids for both the P.K. dikes and Hatay volcanics and sheeted dikes were successfully modeled as products of variable mixing of compositionally diverse melt increments in a polybaric mantle melting column. Melts from shallower portions of the melting column were extracted from more refractory sources relative to deeper melts. Parental liquids to P.K. dikes originated beneath a subducting spreading center whereas Hatay melts formed in a mantle wedge receiving contributions of LREE-enriched liquids from deeper, more fertile sources. Both ophiolites are discussed in the context of Neo-Tethyan evolution and demise. Late Cretaceous-early Tertiary ophiolites and mafic to felsic forearc intrusives from the Chugach and Prince William Terranes, southern Alaska, are possible relics of Kula-Farallon ridge subduction. The MORB-like geochemistry of pillow basalts and sheeted dikes from the Resurrection Peninsula and Knight Island ophiolites suggest formation along an intra-oceanic spreading center. Forearc dikes and related intrusives from the Seldovia Quadrangle, southern Kenai Peninsula, compositionally range from I-type basalts and basaltic andesites to S-type dacites and rhyolites of calc-alkaline affinities. Refractory, island-arc type basalts, injected into the overriding accretionary complex by the subducting spreading center, may have assimilated flyschoid metasedimentary rock during fractional crystallization to produce the Seldovia igneous suite. MORB-like lavas erupted along an intra-oceanic spreading center may evolve to island-arc compositions during ridge subduction because of changes in the dynamics of mantle melting due to the insulating effect of overriding lithosphere.

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