Nd-Sr-Pb systematics and age of the Kings River ophiolite, California: implications for depleted mantle evolution

Abstract

Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with ε_Nd (483)= +10.7±0.5 and ε_Nd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma isochron. The 483 Ma data provide the first evidence of lower Paleozoic oceanic crust in the Sierran ophiolite belt. New U-Pb analyses of zircons from a plagiogranite lying on the 483 Ma Sm-Nd isochron yield upper and lower intercepts with the concordia of 430^(+200)_(−60) and 183±15 Ma. Published zircon ages have underestimated the primary age of the ophiolite by 200–300 m.y. due to the effects of polymetamorphism. The 483 Ma samples have initial ^(87)Sr/^(86)Sr=0.7023–0.7030, ^(206)Pb/^(204)Pb=17.14–17.82, ^(207)Pb/^(204)Pb=15.37–15.52, ^(208)Pb/^(204)Pb=36.80–37.38. The 285 Ma samples have similar initial 87Sr/86Sr, but more radiogenic Pb. The range in Sr and Pb compositions is probably due to introduction of radiogenic Sr and Pb during multiple post-emplacement metamorphic events. The high ε Nd, low ^(87)Sr/^(86)Sr, ^(206)Pb/^(204)Pb, ^(207)Pb/^(204)Pb, ^(208)Pb/^(204)Pb of the least disturbed samples are clearly diagnostic of a midocean ridge origin for the 483 Ma portion of the ophiolite. Igneous activity at 285 Ma is thought to have occurred in an arc or back-arc setting, or perhaps along a leaky transform. The initial ε_Nd (483)=+10.7 is indistinguishable from that of the similar age Trinity Peridotite (Jacobsen et al. 1984). This value is the highest yet reported for the Mesozoic or Paleozoic depleted mantle and requires either a mantle source that was depleted ∼ 850 m.y. earlier than average or a source more highly depleted than average. Alternatively, if such values were more typical of the early Paleozoic mantle than is currently thought, then there has been little evolution of the depleted mantle over the last ∼ 500 m.y. This requires that the modern mantle has been refluxed by material with low ε_Nd, such as continental crust.

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