Origin and Petrogenesis of the Trinity Ophiolite Complex (California): New Constraints from REE and Nd Isotope Data

Abstract

Sm-Nd isotopic results, as well as concentrations of major elements and REE, are reported for nine whole-rock samples of plagioclase lherzolite, lherzolite, harzburgite, clinopyroxene-rich dykes, layered and isotropic gabbros, and a plagiogranite dyke from the central part of the Silurian Trinity Ophiolite Complex (Toad Lake area) of northern California. The data provide new constraints on the origin and petrogenesis of this ophiolite complex and have potentially important implications with regard to the origin of mid-ocean ridge basalts (MORBs) and differentiation of the oceanic lithosphere. Although Sm-Nd isotopic data published earlier on samples from the northern (China Mountain) and southern (Gray Rock) parts of the complex yielded discordant ages and initial εNd values for peridotites [T=472 ± 32 Myr; εNd(T)=+ 10·4 ± 0·4], and crustal rocks from the overlying magma chambers or intrusive dykes [T∼440 Myr; εNd(T)∼ +7·0], all the samples investigated in this study define a single array in the Sm-Nd isochron diagram, allowing an age of 423 ± 48 Myr and an εNd(T) value of +6·4 ± 0·7 to be calculated. Furthermore, although the results published so far on peridotites and gabbroic dykes from Trinity indicated variable LREE depletion, all the basicultrabasic samples from Toad Lake are found to exhibit similar, strongly LREE-depleted patterns [0±17<(Ce/Sm)N<0·34]. As elsewhere in the Trinity Ophiolite Complex, peridotites in the Toad Lake area preserved evidence of partial melting (e.g., plagioclase lenses tracking the high-temperature flow plane) and melt extraction (e.g., depleted borders along cross-cutting dykes). Comparison of the new Sm-Nd results with U-Pb zircon ages for plagiogranite samples suggests that 423 ± 48 Myr is a geologically meaningful age, corresponding to the crystallization of plagioclase lenses in plagioclase lherzolites, as well as to the crystallization of dykes and rocks of the Toad Lake magma chamber. Because of the discrepancy in Sm-Nd ages, initial εNd(T) values, and REE patterns, peridotites and crustal rocks of the Trinity Ophiolite Complex have so far been regarded as genetically unrelated. The results from Toad Lake argue for partial reassessment of this view. As far as Sm-Nd isotopic data and REE patterns are concerned, peridotites, dykes, and gabbros from this area have features consistent with a cogenetic rock suite. Clinopyroxene-rich dykes near Toad Lake have been regarded as the remnants of the conduits which fed the magma chamber, a view which is fully confirmed by the present geochemical and isotopic results. Careful evaluation shows that these dykes have petrographic and geochemical features more representative of crystallized melts, rather than mineral segregates, hence allowing us to propose that their actual picritic composition (MgO∼ 16%) might reflect the composition of the magma which entered the Toad Lake magma chamber. Comparison of the present results with those published earlier shows the Trinity peridotite to be an isotopically heterogeneous body. Moreover, a difference in age may exist between peridotites from the northern and southern parts of the complex. It is possible that the Trinity Ophiolite Complex represents the remnant of the opening of an ocean, perhaps in an area similar to a modern back-arc or inter-arc basin.