Source and tectonic implications of tonalite-trondhjemite magmatism in the Klamath Mountains

摘要: In the Klamath Mountains, voluminous tonalite-trondhjemite magmatism was characteristic of a short period time from about 144 to 136 Ma (Early Cretaceous). It occurred 5 10 m.y. after ∼165 159 Josephine ophiolite thrust beneath older parts province during Nevadan orogeny (thrusting ∼155 148 Ma). The also corresponds slow or no subduction. Most plutons crop out in south-central Mountains California, but one occurs Oregon at northern end province. Compositionally extended members suite consist precursor gabbroic dioritic rocks followed by later, more tonalitic and trondhjemitic intrusions. almost entirely tonalite trondhjemite. Poorly-defined concentric zoning is common. Tonalitic are typically low-Al type trondhjemites generally high-Al type, even those that occur same pluton as tonalite. characterized low abundances K2O, Rb, Zr, heavy rare earth elements. Sr contents moderate (∼450 ppm) comparison with Sr-rich arc lavas interpreted be slab melts (up 2000 ppm). Initial 87Sr/86Sr, δ18O, ɛNd typical mantle-derived magmas crustally-derived metabasic source. Compositional variation within can modeled variable degrees partial melting heterogeneous metabasaltic source (transitional mid-ocean ridge island basalt), not fractional crystallyzation basaltic parent. Melting models require residual assemblage clinopyroxene+garnet±plagioclase±amphibole; plagioclase suggests deep crustal origin rather than subducted slab. Such consistent part Because T thrusting, an external heat probably necessary achieve significant melting; extracted melts, which were parental mafic precursors suite. Thus, under appropriate tectonic thermal conditions, melt form both low- magmas; necessary.