IS THE TRINITY OPHIOLITE A PIECE OF HYPEREXTENDED CONTINENTAL MARGIN? (Invited Presentation)

The Trinity Ophiolite of the Eastern Klamath Terrane is considered to be a composite Neoproterozoic to Devonian ophiolite with a complicated tectonic history. The pre-Silurian history of the unit is exposed at Lover’s Leap, near the town of Callahan, where a section of peridotite, amphibolite-facies gabbro, plagiogranite (tonalite), and basalt is exposed.

Although this unit is thought to have formed at a suprasubduction zone (forearc or island arc) or mid-ocean ridge setting, there is some reason to reconsider these interpretations:

1) The unit interpreted as oceanic plagiogranite is older than the rest of the ophiolite. Dates from the literature indicate a ~565 Ma age for the plagiogranites, over 90 Myr older than the 472 Ma peridotite and 476 Ma amphibolitic gabbro. 2) The plagiogranite has K₂0 of up to 2.32% with a REE pattern of HREE < LREE that is indicative of generation from a garnet-bearing source. These observations are inconsistent with oceanic plagiogranites. 3.) The basaltic section also has a HREE < LREE pattern; they plot in the G-MORB (continental margin basalt) field on a (Dy/Nb)_N and (Ce/Nb)_N discrimination diagram. 4) Hornblendes from the mylonitic gabbro indicate pressures of >8 kb which is too deep to have formed in oceanic crust. 5) A detachment fault separates the basalt from the mylonitic gabbros.

These observations suggest that the unit described as oceanic plagiogranite is instead a tonalite of continental origin. Furthermore, trace element geochemistry of the basalt indicates a garnet in peridotite source consistent with the generation of melt in a deep subcontinental setting rather than in an oceanic setting.

In this case, the tonalite would be an extensional allochthon of continental crust, and the Trinity Ophiolite may be a fragment of hyperextended continental margin, similar to what is exposed in accreted elements of the Alpine Tethys. In these settings, rifting occurs slowly with extension accommodated by normal faulting instead of generation of voluminous magmas.