THE POLYGENETIC TRINITY OPHIOLITIC COMPLEX, AN OCEANIC COLLAGE, EASTERN KLAMATH MOUNTAINS, CALIFORNIA

The Trinity Complex (TC) includes four geologic components formed over a 200 m.y. period from late Neoproterozoic to Middle Devonian. A 5^th component sometimes correlated with TC is a separate terrane. Each has a different age, composition, and tectonic history. 1) SRB: “Scott River block” – intensely deformed, synformal late Neoproterozoic ophiolite consisting of serpentinized harzburgite, amphibole metagabbro (579-556 Ma), plagiogranite (571-565 Ma) cut by dikes and sills, and pillow basalt along Scott River’s East Fork. A ductile shear zone sutures this block to: 2) TRB: “Trinity River block” – Cambrian peridotite consisting of harzburgite and lherzolite with subordinate dunite and feldspathic lherzolite exposed along the Trinity River and east to I-5. Feldspathic partial melt dikes dated 472 ± 32 Ma (Sm-Nd) crosscut mantle tectonite structures and are boudinaged, indicating late syntectonic deformation during Early Ordovician uplift from mantle depths. Undeformed layered gabbro and microgabbro that intrude both SRB and TRB range from 480 to 470 Ma and suggest ductile deformation and juxtaposition of SRB and TRB ended by Early Ordovician. Thus mantle tectonite structures in the TRB formed in the Cambrian. 3) SSZ: Siluro-Devonian supra-subduction zone ophiolite that intruded/erupted over the composite SRB/TRB. Undeformed SSZ components (435-412 Ma) consist of pegmatitic gabbro and associated pyroxenite, hornblende gabbro, diorite, and plagiogranite that extensively intrude the two older entities, locally overlain by sheeted dikes and pillow basalts. 4) TAB: “Trinity Alps block” – Middle Devonian non-feldspathic harzburgite with dunite and minor orthopyroxenite, metamorphosed and interfolded with the Central Metamorphic terrane (CMT). The CMT underthrust TAB ~380 Ma along Irwin’s (1981) Trinity Fault, which is not traceable north into the Yreka and Fort Jones terranes. TAB is faulted against TRB along a tremolite schist shear zone. Because SSZ intrusions occur only east of the shear zone, this block joined the collage between 412 ± 10 and 380 Ma. 5) FMT: Forest Mountain ophiolite terrane – a thin belt of harzburgite, dunite, and minor lherzolite extends from Callahan to Yreka east of Scott Valley. It steeply underthrust the composite Trinity-Yreka terrane ~407-391 Ma, Early Devonian.