Abstract

Mafic meta-igneous supracrustal rocks in the east-central part of the western Triassic and Paleozoic belt are interlayered with and appear to predominantly overlie fine-grained clastic and cherty metasedimentary strata. This complex constitutes a lithostratigraphic terrane exposed in the vicinity of Sawyers Bar, California. Basaltic flows, dikes, and sills are informally referred to as the "Yellow Dog greenstones." Dark-colored, amygdaloidal flow breccias rich in titanium, iron, phosphorus, and light rare-earth elements (LREE's) constitute the lowest, mildly alkalic members of the mafic flow series, intimately interlayered with fine-grained terrigenous detritus. Element proportions suggest that the dark, Ti + Fe* + P-rich basaltic lavas may have been extruded in an oceanic intraplate setting. Interstratification with distal turbiditic strata indicates concurrent submarine volcanism and deep-sea sedimentary deposition. The main mass of the stratigraphically higher extrusive sequence is a light-colored, massive tholeiite series, poorer in Ti + Fe* + P + LREE's. Hypabyssal rocks belong exclusively to this second igneous suite. Compositions of the pale-colored, overlying basaltic/diabasic mass of the Yellow Dog greenstone section are consistent with eruption in an immature magmatic arc. Dikes presumably comagmatic with terminal-stage Yellow Dog volcanism cut the overlying, more eastern Stuart Fork terrane.

The Yellow Dog greenstones contain relict magmatic clinopyroxene and/or pargasitic hornblende. Both melt series include Mg-rich units (ave17: MgO = 14.4 ± 1.9 wt. %; Cr = 650 ± 200 ppm; Ni = 270 ± 100 ppm). Sea-floor alteration apparently affected these mafic rocks in the marine environment following eruption, as reflected by δ18O, MgO, and CaO/Al2O3 values and by minor alkali and silica metasomatism. Yellow Dog igneous activity was extinguished during mid-Jurassic suturing of the oceanward Sawyers Bar terrane against and beneath the pre-existing landward Stuart Fork blueschist complex of Late Triassic metamorphic age. Subsequently, the terrane amalgam was invaded by Middle/ Late Jurassic granitoids. Minor chemical alteration, including 18O enrichment, also probably accompanied later regional, and especially contact, metamorphism. Nevertheless, many major-, minor-, and rare-earth-element variations in the Yellow Dog greenstones are compatible with igneous fractional crystallization trends but exclude the assimilation of significant amounts of sialic crust.

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