Alpine metamorphism of Hercynian hornblende granodiorite beneath the blueschist facies schistes lustrés nappe of NE Corsica

Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle-to-late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca-amphibole + albite + sphene, and biotite to actinolitic Ca-amphibole + albite + phengite + Ti-ore + epidote. Zone 3: Hornblende to crossite + low Ti-biotite + phengite + sphene, and biotite to crossite + low Ti-biotite + phengite + Ti-ore + sphene ± epidote. P-T conditions at the base of the shear zone are estimated to have been 390-490°C at 600-900 M Pa (6-9kbar) and the Corsican basement is therefore deduced to have been buried to 20-30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.     

Isotopic and Chemical Evidence Concerning the Genesis and Contamination of Basaltic and Rhyolitic Magma Beneath the Yellowstone Plateau Volcanic Field

Since 2.2 Ma, the Yellowstone Plateau volcanic field has produced6000 km³ of rhyolite tuffs and lavas in >60 separate eruptions,as well as 100 km³ of tholeiitic basalt from >50 vents peripheralto the silicic focus. Intermediate eruptive products are absent.Large calderas collapsed at 2?0, 1?3, and 0?6 Ma on eruptionof ash-flow sheets representing at least 2500, 280, and 1000km³ of zoned magma. Early postcollapse rhyolites show largeshifts in Nd, Sr, Pb, and O isotopic compositions caused byassimilation of roof rocks and hydrothermal brines during collapseand resurgence. Younger intracaldera rhyolite lavas record partialisotopic recovery toward precaldera ration. Thirteen extracalderarhyolites show none of these effects and have sources independentof the subcaldera magma system. Contributions from the Archaeancrust have extreme values and wide ranges of Nd-, Sr-, and Pb-isotoperatios, but Yellowstone rhyolites have moderate values and limitedranges. This requires their deep-crustal sources to have beenpervasively hybridized (and the Archaean components diluted)by distributed intrusion of Cenozoic basalt, most of which wasprobably contemporaneous with the Pliocene and Qualernary volcanism.In hybrid sources yielding magmas parental to the subcalderarhyolites, half or more of the Nd and Sr may have been contributedby such young basalt. Parents for the extracaldera rhyolites,generated beyond the leading edge of the northeast-propagatingfocus of basaltic intrusion and deep-crustal mobilization, containedsmaller fractions of mantle-derived components. Most Yellowstone basalts had undergone cryptic clinopyroxenefractionation in the lower crust or crust-mantle transitionzone and, having also ascended through or adjacent to crustalzones of silicic-magma generation, most underwent some crustalcontamination. A high fraction of the Pb in most basalts isof crustal derivation. Anomalously low seismic velocities toa depth of 250 km and a high flux of ³He at Yellowstone suggestsublithospheric magma contributions. Elevated baseline Nd- andSr-isotopc ratios suggest additional contributions from oldlithospheric mantle, but this is hard to quantify because ofthe crustal overprint. Foundering of crustally contaminatedmain-stage cumulates into the low-viscosity upper mantle beneaththe principal focus of basaltic intrusion may influence theisotopic compositions of low-K tholeiites and Snake River olivinetholeiites subsequently generated along the Snake River Plainaxis in the wake of the main migrating melting anomaly.