Abstract

New age determinations on radiolarian cherts, foraminifers, and volcanic rocks document the presence of allochthonous fragments of Cretaceous oceanic plate, suggesting accreted terrane, in the outer Mariana fore arc, more than 50 km from the trench. Three dredges, from 3km2area along a steep scarp, recovered a diverse assemblage of rocks representing an ophiolite suite (chert, mafic and intermediate lavas and intrusive rocks). Trace element patterns of the lavas suggest at least three tectonic associations (island arc, ocean island, and oceanic plate). The cherts contain two deep-water assemblages of radiolaria of middle to late Valan-ginian (131-138 Ma) and Albian (97-112 Ma) age. Foraminifers recovered with the chert are Aptian to Albian in age. The lavas record a wide range of K-Ar ages, 85 Ma for a metabasalt with trace-element signatures of mid-ocean ridge basalt, 71 Ma for a highly metamorphosed alkalic basalt, and 39 Ma for a fresh glassy boninite. These ages imply multiple volcanic events and at least two tectonic settings for magma genesis. The cherts and metabasalts are too old to have formed in situ or to be part of trapped West Philippine Basin crust. The mix of old oceanic plate with younger are rocks requires complex tectonic relations. We suggest that one or more fragments of Cretaceous oceanic plate (chert, mid-ocean ridge basalt, and alkalic lavas) were accreted to the Mariana fore arc and have been extensively faulted and probably intruded by arc lava (island-arc tholeiite and boninite).

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