Amphibolite-tonalite relationships: Part I. Experimental investigation of the phase relationships and textural development of amphibolite dehydration-melting. Part II. The geology, petrology and geochronology of a tonalitic and mafic dike swarm (southwestern Foothills terrane, California).

摘要

Part I. Phase relationships have been determined for the dehydration-melting of a (powdered and solid) calcic, low-K, olivine tholeiitic amphibolite (hornblende 70%, plagioclase 30%), in runs at 10 kbar, 750 to 1000{dollar}spcirc{dollar}C, {dollar}fsb{lcub}rm Osb2{rcub}cong{dollar} Ni-NiO, and for 1 to 21 days. Hornblende is involved in a sliding reaction: hornblende + anorthitic plagioclase {dollar}to{dollar} clinopyroxene + liquid + aluminous hornblende + calcic hornblende + orthopyroxene + garnet. The liquid fraction varies from {dollar}<{dollar}1% at 750{dollar}spcirc{dollar}C to {dollar}sim{dollar}47% at 1000{dollar}spcirc{dollar}C, with the big increase occurring above 875{dollar}spcirc{dollar}C. Liquids are tonalitic but have very high Al{dollar}sb2{dollar}O{dollar}sb3{dollar} contents (18-21 wt.%). At high liquid fractions ({dollar}sim{dollar}0.5), liquids are high-alumina basaltic. Liquids become more sodic with increasing temperature, but the compositional trends reverse direction, and liquids become more calcic above 975{dollar}spcirc{dollar}C, where garnet is unstable. The water contents of liquids range from over 7 wt.% at low liquid fractions to 2 wt.% at high liquid fractions. In the solid amphibolite runs, liquid interconnectivity may be attained at 875{dollar}spcirc{dollar}C with only 2 vol.% liquid and dihedral angles less than 60{dollar}spcirc{dollar}. The removal of water-rich tonalitic liquids from a substantially melted amphibolitic source could help generate a relatively dry mafic granulite terrane, with densities up to 3.5 gm/cm{dollar}sp3{dollar}. Delamination of this dense lower crust is possible.; Part II. A cogenetic and coeval tonalitic and mafic dike swarm has been identified within a southern fragment (the Owens Mountain area) of the western Foothills terrane (Sierra Nevada, California). The swarm was mylonitized and transposed during emplacement, from 155 to 148 m.y. (U-Pb zircon data), at an estimated depth of 10 km. Steeply SE-plunging fold axes and S-fold geometries indicate a left-lateral sense of shear. The Late Jurassic Nevadan orogeny is a manifestation of dramatic changes in magnitude and direction of North American motion. The Cordilleran dike swarms record a complex pattern of sinistral-sense transtension-transpression that developed during this period of change, at the J2 ({dollar}sim{dollar}150 m.y.) apparent polar wander cusp, and during subsequent, rapid northwestward acceleration of North America.